Carbohydrate Phosphate

The Human being Genome

Robert Thousand. Kliegman MD , in Nelson Textbook of Pediatrics , 2020

Fundamentals of Molecular Genetics

DNA consists of a pair of chains of a carbohydrate-phosphate courage linked past pyrimidine and purine bases to form adouble helix (Fig. 96.1). The sugar in DNA is deoxyribose. The pyrimidines are cytosine (C) and thymine (T); the purines are guanine (Thousand) and adenine (A). The bases are linked by hydrogen bonds such that A always pairs with T and G with C. Each strand of the double helix has polarity, with a free phosphate at i end (5′) and an unbonded hydroxyl on the sugar at the other end (three′). The 2 strands are oriented in opposite polarity in the double helix.

The replication of Deoxyribonucleic acid follows the pairing of bases in the parent DNA strand. The original 2 strands unwind by breaking the hydrogen bonds between base of operations pairs. Free nucleotides, consisting of a base of operations attached to a saccharide-phosphate chain, form new hydrogen bonds with their complementary bases on the parent strand; new phosphodiester bonds are created by enzymes chosenDna polymerases. Replication of chromosomes begins simultaneously at multiple sites, forming replication bubbles that aggrandize bidirectionally until the entire DNA molecule (chromosome) is replicated. Errors in DNA replication, or mutations induced past environmental mutagens such as irradiation or chemicals, are detected and potentially corrected by Deoxyribonucleic acid repair systems.

The central tenet of molecular genetics is that information encoded in Deoxyribonucleic acid, predominantly located in the jail cell nucleus, is transcribed intomessenger ribonucleic acid (mRNA), which is then transported to the cytoplasm, where it is translated into protein. A prototypical gene consists of a regulatory region, segments chosenexons that encode the amino acid sequence of a poly peptide, and intervening segments chosenintrons (Fig. 96.two).

Transcription is initiated by attachment of ribonucleic acid (RNA) polymerase to the promoter site upstream of the start of the coding sequence. Specific proteins bind to the region to repress or activate transcription by opening up thechromatin, which is a complex of DNA and histone proteins. It is the activeness of these regulatory proteins (transcription factors) that determines, in large part, when a gene is turned on or off. Some genes are also turned on and off by methylation of cytosine bases that are adjacent to guanine bases (cytosine-phosphate-guanine bases, CpGs). Methylation is an example of anepigenetic change, pregnant a alter that can affect gene expression, and possibly the characteristics of a cell or organism, but thatdoes not involve a modify in the underlying genetic sequence. Gene regulation is flexible and responsive, with genes being turned on or off during development and in response to internal and external environmental conditions and stimuli.

Transcription gain through the entire length of the gene in a v′ to iii′ direction to form an mRNA transcript whose sequence is complementary to that of 1 of the Dna strands. RNA, like Dna, is a sugar-phosphate chain with pyrimidines and purines. In RNA the carbohydrate is ribose, and uracil replaces the thymine found in DNA. A "cap" consisting of seven-methylguanosine is added to the 5′ end of the RNA in a 5′-five′ bail and, for most transcripts, several hundred adenine bases are enzymatically added to the 3′ end afterward transcription.

Elementary Carbohydrates

N.5. BHAGAVAN , in Medical Biochemistry (Fourth Edition), 2002

Sugar Phosphates

Sugar phosphates, which are phosphoric acid esters of monosaccharides, occur as intermediates in carbohydrate metabolism. Two of these compounds, namely, ribose phosphate and deoxyribose phosphate, are constituents of nucleotides and nucleic acids. Glucose tin can be phosphorylated either at the C 6 main hydroxyl grouping to yield glucose 6-phosphate or at the C1 anomeric hydroxyl grouping to yield glucose ane-phosphate (Figure nine-16). In glucose 1-phosphate, the phosphate grouping can exist in either the α- or β-position. These two forms are non interconverted in solution considering the exchange of the anomeric hydroxyl group by any group prevents the ring opening responsible for the equilibration of anomers. The reducing property is as well lost.

FIGURE 9-16. Sugar phosphates and a nucleoside diphosphate sugar. When the anomeric position of a sugar is not substituted, the configuration at this position is not specified considering the sugar tin exist in either the α- or β-anomeric course.

Some other class of saccharide phosphates consists of nucleoside diphosphate sugars, in which a monosaccharide is fastened through the anomeric hydroxyl group to a nucleoside diphosphate. A nucleoside contains D-ribose, an aldopentose, attached to a purine or a pyrimidine base of operations, as in uridine diphosphate glucose (Figure 9-xvi). Such compounds are important in the synthesis of polysaccharides, the interconversion of sugars, and the synthesis of glycosides.

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Molecular Genetics and Cancer Biology

Alan W. Partin MD, PhD , in Campbell-Walsh-Wein Urology , 2021

DNA

The molecular characteristics of deoxyribonucleic acid (Dna) were first described in 1953 (Watson and Crick, 1953). This molecule serves as the pattern for determination of structure and function of all living organisms.Dna comprises of three basic components: a pyrimidine or purine base, a sugar (2-deoxyribose), and a phosphate (eFig. 62.ane). The DNA construction exists as a double helix in which one strand of bases is ordered in one direction and the other strand is ordered in the opposite direction. The two strands are held together by hydrogen bonds and are organized via complementary base pairing. The 4 bases that primarily make up DNA are adenine, cytosine, guanine, and thymine. Uracil is substituted for thymine in ribonucleic acid (RNA).Hydrogen bonding occurs specifically betwixt the purine adenine (A) and the pyrimidine thymine (T) and between the purine guanine (Grand) and pyrimidine cytosine (C) (eFig. 62.2).In the RNA molecule, adenine base pairs with uracil (U).

eFig. 62.1. The nucleic acrid alphabet consists of iv bases: the purines adenine (A) and guanine (Chiliad) and the pyrimidines thymine (T) and cytosine (C). Uracil (U) is substituted for thymine in the case of RNA. The combination of a base of operations and a sugar (deoxyribose) is referred to equally anucleoside.

eFig. 62.2. The combination of a sugar phosphate group and a base of operations constitutes a nucleotide. The double helix is made from ii polynucleotide chains, each of which consists of a series of v′- to 3′-sugar phosphate links that form a courage from which the bases protrude. The double helix maintains a constant width because purines ever face pyrimidines in complementary A-T and G-C base of operations pairs, respectively.

During the procedure of replication, each strand of the Dna double helix acts every bit a template for generation of the new strand of Dna. The precise ordering of DNA base pairs results in a code that will be processed to ultimately generate proteins responsible for a variety of cellular functions. Each single-stranded Dna molecule has two nonidentical ends referred to as the 5′ (5 prime number) and iii′ (iii prime) termini. The 5′ and 3′ numbering refers to the carbon atoms in the deoxyribose carbohydrate. The five′ carbon of one deoxyribose is linked to the 3′ carbon of some other deoxyribose by a phosphate grouping. Each strand ends with part of the sugar ring exposed based on these connections.

Transcription

Transcription is the first footstep in converting information encoded in DNA into protein. During the process of transcription, linear DNA sequence is converted to linear RNA (messenger RNA; mRNA). mRNA is after translated into a linear gear up of amino acids that form a functional protein (eFig. 62.3). RNA polymerase II is the enzyme that synthesizes mRNA, which is complementary to the Deoxyribonucleic acid template. This master strand of RNA is chosen a pre-mRNA and contains protein coding sequences (exons) and intervening noncoding sequences (introns).

eFig. 62.3. During transcription, a section of 1 DNA strand or the other is used as a template for the synthesis of messenger RNA (mRNA). This synthesis e'er occurs in a v′ to three′ direction.

Deoxyribonucleic acid Repair

C.Chiliad. Cupples , in Encyclopedia of Microbiology (Third Edition), 2009

The sugar phosphate backbone and the bases of Dna are subject to harm by mutagens produced both outside and inside the cell. Dna repair is necessary to remove a multitude of lesions that destroy either the structural or the coding integrity of the molecule. Almost all organisms have a suite of enzymatic pathways that allow them to remove DNA harm straight, to remove damaged bases or nucleotides, and to repair single or double-stranded breaks in the courage. Our nowadays mean solar day cognition of Dna repair is based on over half a century of experimentation with two model microorganisms, Escherichia coli and Saccharomyces cerevisiae. This review focuses on the types of damage that tin can occur and the pathways in these two organisms that opposite the damage.

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Molecular Biology and Genetic Engineering

A. Wesley Burks Dr. , in Middleton'due south Allergy: Principles and Practise , 2020

Anatomy of the Factor

The gene, the basic unit of measurement of heredity, is carried by the chromosome and stored in the nucleus. Genes are made up of deoxyribonucleic acid (Dna), which is the genetic textile for all cellular organisms. DNA has iii chief components: the phosphate (PO4) groups, five-carbon sugars, and nitrogen-containing bases calledpurines, comprising adenine (A) and guanine (G), andpyrimidines, comprising thymine (T) and cytosine (C). These basic subunits are callednucleotides. Nucleic acids are polymers of repeating subunits of nucleotides.

The carbon atoms of the sugar molecules are numbered i′ to v′ proceeding clockwise from the oxygen cantlet (Fig. 10.one). The phosphate group is attached to the v′ carbon cantlet of the carbohydrate, and the base is attached to the 1′ carbon atom. In that location is an additional free hydroxyl group (–OH) attached to the 3′ carbon cantlet. The presence of 5′-phosphate and 3′-hydroxyl groups allows Deoxyribonucleic acid and ribonucleic acid (RNA) to form long chains of nucleotides. The 5′-phosphate of one nucleotide interacts with the 3′-hydroxyl grouping of another nucleotide, and a covalent bond (phosphodiester bond) is formed between the two molecules. This two-unit nucleotide however has a free 5′ phosphate at i end and a iii′-hydroxyl group at the other so that it can interact with other nucleotides at each end. In this manner, a long chain of nucleotides tin be joined together to class a Deoxyribonucleic acid or RNA molecule. All 4 nucleotides are not present in equal amounts, simply the amount of adenine in a DNA molecule is ever equal to the corporeality of thymine, and the corporeality of guanine always equals the amount of cytosine (as expressed in nucleotide symbols, A = T and Chiliad = C).

The three-dimensional construction was non known until early in the 1950s, when British chemist Rosalind Franklin, working in the laboratory of Maurice Wilkins, performed 10-ray crystallography of Dna fibers. The diffraction pattern suggested that the DNA molecule had the shape of a helix or corkscrew, with a bore of two nm and a complete helical plough every 3.4 nm. James Watson and Francis Crick 1 built models of nucleotides and tried to assemble the nucleotides into a molecule. After exploring diverse possibilities, they proposed a "double helix" construction of the DNA molecule, in which the bases of ii strands pointed inward toward one another (base pairing). In this model, the base pairing is betwixt purines (big) pointing toward pyrimidines (modest), thus keeping the bore of the molecule at a constant two nm. The double helix is stabilized by a hydrogen bond between the paired bases: adenine makes double hydrogen bonds with thymine, and guanine forms 3 hydrogen bonds with cytosine (Fig. 10.1).

Dna and Aspects of Molecular Biology

Carlos de los Santos , in Comprehensive Natural Products Chemistry, 1999

7.03.2.one Nomenclature

Nucleic acids are linear biopolymers composed of a sugar–phosphate backbone containing purine and pyrimidine heterocycles (bases) equally side chains. The monomeric units, called nucleotides, are composed of a cyclic saccharide (β-d-ribose in RNA or β-d-2′-deoxyribose in DNA), phosphorylated at the O-5′ position and linked to one of four different bases through a β-glycosyl C-one′—N bond. Purine bases, generically denoted "pur" or R, are guanine (Gua or 1000) and adenine (Ade or A), while pyrimidine bases, denoted "pyr" or Y, are cytosine (Cyt or C), thymine (Thy or T), and uracil (Ura or U, simply found on RNA). In the biopolymer, individual nucleotides are linked via a iii′,5′-phosphodiester bond.

Following IUPAC–IUB recommendations, 9 sugar atoms are differentiated from base of operations atoms past primes, and the concatenation direction goes from C-5′ to C-3′. The courage atoms are counted post-obit the sequence P→O-5′→C-5′→C-four′→C-3′→O-iii′→P and, within the sugar band, the sequence is C-1′→C-2′→C-3′→C-4′→O-4′→C-1′. As shown in Figure 1, the two hydrogen atoms nowadays at the C-2′ position of the deoxyribose are named H-two′1 (H-2′), and H-two′two (H-2″), corresponding to the pro-S and pro-R positions, respectively. In other words, the H-two′ proton and the base are to ane side of the plane determined by the sugar ring and H-ii″ and H-i′ are to the other. The same convention applies to the hydrogen atoms at the C-5′ position, which are labeled H-five′1 (H-v′) and H-5′2 (H-v″), corresponding to the pro-S and pro-R positions, respectively. Looking forth the O-5′—C-5′ bond, a clockwise rotation gives C-iv′, H-5″, H-5′. Effigy 1 and Table 1 evidence the 6 torsion angles needed to define the conformation of the sugar–phosphate backbone. Each angle describes the rotation forth a single bond of the courage and Greek letters, from α through ζ, denote them. It is a mutual practice in X-ray crystallography to specify conformational ranges in which the torsion angles lie using the terms cis, trans, gauche − and gauche +. Their correlation with the Klyne–Prelog note, 10 recommended by IUPAC–IUB, is shown in Figure 2.

Figure 1. IUPAC–IUB recommendation for cantlet nomenclature, chain management, and definition of sugar and courage dihedral angles in nucleic acids. Annotation that δ and νthree define the same torsion along the C-3′—C-iv′ bail.

Table i. Definition of torsion angles in nucleic acids.

Torsion angle Atoms involved Torsion angle Atoms involved
α (due north−1)O-3′—P—O-5′—C-5′ ν0 C-four′—O-4′—C-one′—C-2′
β P—O-5′—C-5′—C-4′ ν1 O-four′—C-1′—C-2′—C-3′
γ O-5′—C-v′—C-four′—C-3′ ν2 C-1′—C-2′—C-3′—C-four′
δ C-5′—C-4′—C-3′—O-3′ ν3 C-2′—C-3′—C-4′—O-4′
ɛ C-4′—C-iii′—O-three′—P(n+one) νiv C-3′—C-four′—O-4′—C-ane′
ζ C-iii′—O-3′—P—O-5′
χ O-4′—C-1′—North-1—C-2 (pyrimidines)
O-4′—C-1′—N-9—C-4 (purines)

Figure ii. Pictorial definition of dihedral angle ranges used in Ten-ray crystallography (cis, trans, gauche +, and gauche −) and their correlation with the IUPAC–IUB recommended Klyne–Perlog note (inner circumvolve) (reproduced by permission of Springer-Velag from Principles of Nucleic Acrid Construction; © Springer-Velag, New York).

Five endocyclic torsion angles (ν0 to νiv), defined in Figure 1 and Table 1, can be used to describe the conformation of the sugar. In general, the ribose ring is nonplanar, a fact known as saccharide pucker. When iv atoms of the sugar ring prevarication in a aeroplane, this plane is chosen as the reference, and the conformation is chosen envelope (E). If not, the reference airplane is that of the three atoms that are closest to the five-atom least-squares airplane, and the conformation is called twist (T). These conformations are depicted in Figure three.

Figure 3. Cycle of pseudorotation for the ribose ring. The virtually frequent conformation found in deoxy- ribonucleic acids is C-2′-endo, on the south part of the bicycle. Envelope and twist forms alternate every 18° (reproduced by permission of the American Chemical Guild from J. Am. Chem. Soc., 1972, 94, 8206; © American Chemic Society).

According to convention, the atoms displaced from the reference aeroplane are called endo or exo, depending on whether they are on the same side every bit C-5′ or the opposite, 11 and are assigned superscripts and subscripts for the endo and exo conformations, respectively. Thus, an envelope crease with the C-three′ cantlet toward the C-v′ side is written equally iiiE, while a twist conformation with the C-two′ atom on the same side as C-5′ and C-3′ on the reverse is denoted 2T3. It is as well very convenient to draw sugar puckers using the concept of bending of pseudorotation, which was start introduced to characterize the construction of cyclopentane, 12 and afterward applied to nucleosides and nucleotides. 13 In this description, endocyclic torsion angles can exist reproduced, with an mistake of less than 0.7°, past a two-parameter equation:

(1) ν j = Φ m cos P + 144 j − 2 where 0 ⩽ j ⩽ 4

Torsion angles are numbered clockwise starting with ν0 (C-4′—O-four′—C-i′—C-ii′) as previously indicated. The maximum aamplitude of pucker, Φm, can be calculated from the previous equation past setting j = 2. The pseudorotation angle, P, takes values from 0° to 360°.

Figure 3 shows the correlation between pseudorotation angle and the envelope and twist conformations. It is a mutual practice to use the terms northward and south conformations when referring to the C-2′-exo/C-iii′-endo and C-iii′-exo/C-2′-endo ranges, respectively. The orientation of the bases with respect to the sugar is defined past χ, the glycosidic torsion angle (O-four′—C-1′—Due north-9—C-4 in purines, and O-4′—C-i′—N-9—C-two in pyrimidines). The preferred conformational range observed in nucleic acids is anti, in which the half-dozen-membered ring of purines, or the O-2 of pyrimidines, is directed away from the sugar. Some other conformational range, ofttimes observed when studying Dna duplexes containing damaged bases, is syn, in which the beefy part of the bases points towards the sugar. Purine and pyrimidine bases tin can interact through formation of hydrogen bonds, defining what is known as Watson–Crick base-pair alignments, a common, but not unique, motif of secondary structure in Dna. With the bases in anti orientation, G pairs with C and A with T in an arrangement that has very similar overall dimensions, every bit shown in Effigy iv.

Figure 4. Watson–Crick base pair alignments in nucleic acids. As the strands run in antiparallel directions, the C-one′ carbons lie on the same border of the base pair.

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Advances in Radiation Biology

William A. Bernhard , in Advances in Radiations Biology, 1981

ane Electron Abstraction

Electron loss volition occur from all four bases and the carbohydrate phosphate courage. Deprotonation of the pristine cations is likely. If damage is transferred, it is more likely to be by stepwise reactions than by pigsty conduction. At high radiation doses, the ratio of base- to sugar-centered complimentary radicals will exist greater than one, mainly because the base radicals are more stable toward thermal or radiation devastation. Water bound to the Dna should destabilize the sugar-centered radicals more than the base-centered radicals. Amid the bases, the more stable the π-cation products, the more visible they should be. Stability seems to decrease in the series Gc(–1), Ac(–half dozen'), Tc(–5'), and Cc(–4'), where the probable sites of deprotonation are given in parentheses. The Tc–5' radical should be especially reactive and the Cc–four' radical specially unstable. The more chop-chop the π-cation deprotonates, the less likely information technology is to recapture an electron. Rapid deprotonation may favor higher concentrations of the guanine π-cation.

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Metabolic Analysis Using Stable Isotopes

Jianhai Du , ... James B. Hurley , in Methods in Enzymology, 2015

2.vii Analysis of Metabolites by LC–MS

Large and/or thermo-unstable organic molecules including sugars, phosphate, and nucleotides are particularly suitable for LC–MS. There are a large selection of methods and instruments bachelor depending on the metabolites of interest. Labeled glucose, glutamine, and glutathione from 13C glucose or xiiiC glutamine in the medium and retina can exist quantified directly past LC–MS. Assay of the mass isotopomer distributions of nucleotides can estimate the rate of ATP production, cGMP degradation, and pathways in nucleotide degradation. Here, nosotros present a protocol to measure out metabolites listed in Table four past LC–MS. We apply a Waters Xevo TQ Tandem mass spectrometer with a Waters ACQUITY arrangement with UPLC. An ACQUITY UPLC BEH Amide analytic column (two.i   ×   50   mm, i.7   μm, Waters) is used for separation. The mobile phase is (A) water with 10   1000M ammonium acetate (pH 8.9) and (B) acetonitrile/water (95/v) with 10   chiliadThou ammonium acetate (pH eight.9) (All solvents are LC–MS Optima class from Fisher Scientific). The gradient elution is (1) 95–61% B in vi   min, (2) 61–44% B at 8   min, (3) 61–27% B at viii.2   min, and 27–95% B at 9   min. The cavalcade is reequilibrated with 95% B at the end of each run. The Flow rate for all gradient is 0.5   ml/min and the total run is 11   min. The injection volume for each sample is 5   μl. Mass spectrometer settings are shown in Table 4. Each transition includes a parent ion and fragmented daughter ion. Transitions for isotopomers tin can be gear up based on the tracers. A priori cognition of the labeled moiety on the parent and/or daughter ions is of import. The formula and fragment design tin exist checked in public databases such equally METLIN (http://metlin.scripps.edu/index.php) and Human Metabolome Database (http://www.hmdb.ca/). For example, the daughter ion of ATP is the purine base. In xviiiO water tracer experiments, the transitions of isotopomers of parent ions tin can exist set at 508 (M0), 510 (M2), 512 (M4), and 514 (M6), while the daughter ion is 136 for all these transitions since purine base will not exist labeled by xviiiO. The chromatograms are analyzed past MassLynx (Waters). Corrections for natural abundance are made as described in the department on GC–MS.

Tabular array 4. Metabolites for LC–MS Analysis

Metabolite Way Parent (m/z) Daughter (thousand/z) Dwell (south) Cone (v) Collision (v)
Glucose Negative 179 89 0.025 18 viii
Glutamine Positive 147 84 0.025 xiv 16
GSSH Positive 613 231 0.04 86 38
GSH Positive 308 84 0.04 22 22
ATP Positive 508 136 0.08 28 30
ADP Negative 426 134 0.025 34 22
AMP Negative 346 79 0.025 34 22
Adenosine Positive 268 136 0.06 22 16
NAD Positive 664 136 0.025 28 52
NADH Positive 666 108 0.06 24 58
GTP Negative 521 159 0.04 40 34
cGMP Positive 346 135 0.06 30 44
Gross domestic product Positive 444 135 0.06 xx sixty
GMP Positive 364 135 0.06 20 46
Guanosine Positive 284 152 0.06 14 xvi
Xanthine Negative 151 108 0.025 32 fourteen
Hypoxanthine Negative 135 92 0.025 32 fourteen

GSH, reduced glutathione; GSSH, oxidized glutathione; cGMP, cyclic GMP.

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Biosynthesis of O-antigen bondage and associates

Peter R. Reeves , Monica One thousand. Cunneen , in Microbial Glycobiology, 2010

4. Initial Transferases that Initiate O-Antigen Synthesis

O-Antigen synthesis is initiated past transfer of a sugar phosphate from an Nucleotide diphosphate (NDP-)sugar to Und-P. The ITs involved fall into two families, the PNPT ( polyisoprenyl-phosphate Northward-acetyl-hexosamine-i-phosphate transferase) family, including WecA and the PHPT (polyisoprenyl-phosphate hexose-1-phosphate transferase) family, including WbaP (Cost and Momany, 2005).

There are relatively few ITs. For East. coli, most O-antigens take the Wzx/Wzy pathway and all just three appear to have GlcNAc or N-acetyl-galactosamine (GalNAc) as starting time sugar, with WecA as the It. There are gene cluster sequences for 2 of the three with known O-antigen structures that lack GlcNAc or GalNAc. The Shigella sonnei O-unit is a dimer of 2-acetamido-4-amino-two, four-dideoxy-d-fucose (FucNAc4N) and 2-acetamido-2-deoxy-l-altruronic acid (AltNAcA) (Kenne et al., 1980), with WbgY, encoded in the cistron cluster, inferred to be the IT for transfer of d-FucNAc4N, the showtime sugar (Xu et al., 2002). The gene cluster is on a plasmid and idea to take been transferred from Plesiomonas shigelloides. The second, E. coli O45, is discussed below with Pseudomonas aeruginosa. To our knowledge, these are the only Due east. coli gene clusters to have an IT gene in the factor cluster, confirming the role of wecA in the others. Nearly S. enterica and Yersinia spp. O-antigens likewise use WecA, which is present in most Enterobacteriaceae genomes, so this pattern may employ throughout this family.

P. aeruginosa also has a single IT gene, wbpL, but WbpL tin can accept a diverseness of Northward-acetyl sugars every bit donor (see below). There are homologues of wbpL in the gene clusters for the Y. enterocolitica O3 outer core (wbcO) and E. coli O45 O-antigen (wbhQ), with both structures having 2-acetamido-ii-deoxy-d-fucose (FucNAc) as the first sugar (Skurnik, 2003; DebRoy et al., 2005).

All PNPTs accept like hydrophobicity plots and WecA has been shown to have 11 transmembrane (TM) segments (Lehrer et al., 2007) (Figure 18.3). At that place is also a proposed reaction mechanism (Price and Momany, 2005). In improver, PHPTs are integral membrane proteins with conserved hydrophobicity plots, but no sequence similarity to PNPT proteins. The WbaP protein has 5 TM segments (encounter Figure eighteen.3), only mutants expressing merely the C-final domain, including the 5th TM segment, can carry out the Gal-phosphate (Gal-P) transferase office (Wang et al., 1996). The mutants were also affected in further processing of the O-unit of measurement and it was proposed that WbaP is a bifunctional protein, with the N-terminal domain involved in further processing of the UndPP-O-units. The office of the C-terminal region was confirmed past Salidas et al. (2008), who also recognized the periplasmic loop betwixt TM segments 4 and v as an additional domain that was proposed to have a role in concatenation length determination. At that place were differences in the observations and proposals for the role of the N-concluding domain, but it is clear that WbaP is not but a Gal-P transferase and more work is needed to define its other roles, that probably involve interaction with proteins such as Wzx, Wzy and Wzz.

Effigy eighteen.3. The TM segment topology models of O-antigen biosynthesis proteins. (A) Initiating transferases: South. enterica WbaP (Saldias et al., 2008) and E. coli WecA (Lehrer et al., 2007), (B) Flippase proteins: S. enterica B Wzx (Cunneen and Reeves, 2008) with that of R. leguminosarum bv. trifolii (also chosen PssL), for exopolysaccharide synthesis, included for comparison (Mazur et al., 2005), (C) O-antigen polymerase: S. flexneri 2a Wzy (Daniels et al., 1998), (D) Chain-length decision: East. coli O86 Wzz (Tang et al., 2007), (E) Ligation: V. cholerae O1 WaaL (Schild et al., 2005). Topology diagrams were drawn using as a guide TOPO2 (http://www.sacs.ucsf.edu/TOPO-run/wtopo.pl) outputs, based on the published topology data for each protein; TOPO2 generates the loops to scale.

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Biophysical, Chemical, and Functional Probes of RNA Structure, Interactions and Folding: Part B

Sebastian Doniach , Jan Lipfert , in Methods in Enzymology, 2009

6 Determining the Thermodynamics of RNA Folding Using Bead Models

As RNA molecules are highly negatively charged due to their sugar–phosphate courage, the conformational country of these molecules is strongly dependent on the concentration of positively charged counterions. In the absenteeism of Mg2+ or other polyvalent counterions, and in low monovalent table salt concentrations, RNA becomes denatured every bit a outcome of ineffective Debye screening of the repulsive forces between different parts of the polymer.

Every bit a result, quantitative representation of the counterion induced free energy changes involved in RNA folding and function is of central importance in agreement RNA folding and the structure–function relationships for RNA molecules. The ability to demark small-molecule ligands or to class protein complexes adds some other dimension to this question for certain functional RNAs.

Poisson–Boltzmann theory is widely used to gauge energy landscapes for counterion induced conformational changes (Chu et al., 2008; Draper, 2008; Draper et al., 2005). Here, one estimates the free energy associated with a cloud of counterions attracted to an RNA (or other charged molecule) by solving cocky consistently Poisson's equation for the electrostatic potential on a spatial filigree embedding the molecule coupled to the Boltzmann factors determining the local concentration of counterions as a function of the electrostatic potential.

To do this (numerically) requires a detailed model of the distribution of stock-still charges for the RNA molecule involved. As a outcome, PB assay has by and large been limited to cases for which high-resolution structures are available from crystallography. However, a complete agreement of RNA folding requires quantitative models for the free energy of both the folded and unfolded conformations, for which no crystal structures tin can exist obtained.

In a contempo piece of work by Lipfert et al. (2007b), it has been shown that the bead models obtained in iii-dimensional reconstruction of the SAXS data can provide a substitute for a detailed diminutive model. Information technology was shown that the composition of the associated ions could be adequately calculated using PB theory in combination with SAXS-derived low-resolution dewdrop models. In add-on, work in progress suggests that information technology is possible to utilise the same strategy to compute the electrostatic contribution to the complimentary energy of RNA folding (J. Lipfert, A. Sim, D. Herschlag, and Due south. Doniach, in preparation).

To illustrate these ideas we show the results of Lipfert et al. on ion bounden to the P4–P6 fragment from the Tetrahymena ribozyme (Lipfert et al., 2007b). Das et al. (2005) measured the number of bound excess Mgtwo+ ions as a function of MgCl2 concentration in a two Grand NaCl background for wild-type P4–P6 and for a mutant that does not bind Mgtwo+ to the "metal ion core." Equally the two specific "metal ion core" Mgtwo+ binding sites are in the interior of the molecules, our PB calculations (which exclude ions from the interior of the molecule) are non expected to capture their contribution to the overall ion binding. We therefore compare the PB simulations with the ion binding data obtained for the P4–P6 mutant that does not exhibit specific ion binding to the "metallic ion cadre." This approach neglects small, just measurable, differences between the mutant and wild-type P4–P6 solution structures in high salt concentrations (Takamoto et al., 2002). The theoretical predictions using the reconstructed bead model (Fig. 11.4, thick, dashed line) hold reasonably well with the Lead calculations for the diminutive resolution coordinates (Fig. 11.4, sparse, solid line). They slightly under-predict the excess number of Mg2+ ions determined experimentally, in item those obtained using the fluorescence indicator HQS (Fig. 11.4, circles). Overall the agreement with the experiment is remarkable, yet, given the fact that the data were obtained in ii Grand NaCl groundwork and that PB theory is generally expected to be valid just in the depression concentration limit.

Figure 11.4. Ion binding to a P4–P6 mutant that does non exhibit specific Mg2+ binding in the "ion core." The number of excess Mg2+ ions was measured using a fluorescence indicator (circles) and atomic emission spectroscopy (squares) by Das et al. (2005) in two G NaCl background. Theoretical predictions were obtained from PB calculations using the PDB coordinates (thin, solid lines) or the reconstructed bead model with uniformly assigned charges (Lipfert et al., 2007b) (thick, dashed lines). Effigy adjusted from reference Bai et al. (2007).

In one case the electrostatic potential, F(ten) is known on the grid, the Coulomb contribution of the deject of counterions to the free energy, ΔG, may be estimated for each new conformation represented past the bead model for partially folded RNAs. However, it should too exist mentioned that another important contribution to the total ΔG is the site-specific bounden of Mg2+ ions. This is a general feature of many crystal structures for folded RNAs. Theoretical estimates of the free energy of specific Mg2+ bounden would need to have into business relationship possible covalent contributions so requires very detailed positional information about the bound Mgii+ and its binding site environment (mostly oxygen). Thus, it is really only possible for states in which high-resolution crystallographic information is available.

However, as a practical matter, once the counterion cloud contributions have been worked out equally discussed above then ΔG values for specific Mgtwo+ binding tin exist deduced as parameters defining their contributions to thermodynamic models of measured folding curves for the transition between different conformational states of the RNA.

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