Serine 3-form

Glycine and serine form two dipeptides, Gly-Ser and Ser-Gly. As the ball-and-stick models show, even dipeptides have distinctive shapes that depend on their primary structure. 

S)-Serine forms tabular crystals with point symmetry 2hn (Figure 22a) the crystals affected by either (/ )- or (S)-thr exhibit reduced morphological symmetry 2 (the mirror plane is lost) and are enantiomorphous (Figure 22b, c). When (R,S)-thr is used as the additive, the morphological symmetry 2/m is left unchanged because the effects induced by each additive separately combine. The crystals turn into rhombs, with a clear increase in the areas of the 011 side faces relative to those of the pure crystals (Figure 22d) (45, 78). 

On surface of platelet membrane phospholipid, phosphatidyl serine forms tenase and prothrombinase complexes... 

As well as remethylation, Hey can be degraded in the trans-sulphuration pathway, which first involves condensation of Hey with serine forming cystathionine, then breakdown of this compound to cysteine and a-oxo-butyrate. These reactions... 

MECHANISM FIGURE 22-18 Tryptophan synthase reaction. This enzyme catalyzes a multistep reaction with several types of chemical rearrangements. An aldol cleavage produces indole and glyceraldehyde 3-phosphate this reaction does not require PLP. Dehydration of serine forms a PLP-aminoacrylate intermediate. In steps and this condenses with indole, and the product is hydrolyzed to release tryptophan. These PLP-facilitated transformations occur at the /3 carbon (C-3) of the amino acid, as opposed to the a-carbon reactions described in Figure 18-6. The /3 carbon of serine is attached to the indole ring system. Tryptophan Synthase Mechanism... 

Cysteine is synthesized by two consecutive reactions in which homocysteine combines with serine, forming cystathionine, which, in turn, is hydrolyzed to a-ketobutyrate and cysteine (see Figure 20.8). Homocysteine is derived from methionine as described on p. 262. Because methionine is an essential amino acid, cysteine synthesis can be sustained only if the dietary intake of methionine is adequate. 

Ketenes are highly reactive electrophiles but not nearly so indiscriminate as carbenes. When the properties of diazoacetyl photoaffinity reagents were evaluated the Wolff rearrangement was found to be a major problem accounting for 30 to 60% of the products arising from O-esters and 100% of the products fromS-esters. For instance, diazoacetyl-chymotrypsin gave rise to O-carboxymethyl serine formed by the attack of water on the ketene (Shafer et al., 1966) (Fig. 3.10). 

The a subunit catalyzes the formation of indole from indole-3-glycerol phosphate, whereas each P subunit has a PLP-containing active site that catalyzes the condensation of indole and serine to form tryptophan. The overall three-dimensional structure of this enzyme is distinct from that of aspartate aminotransferase and the other PLP enzymes already discussed. Serine forms a Schiff base with this PLP, which is then dehydrated to give the Schiffbase of aminoacrylate. This reactive intermediate is attacked by indole to give tryptophan. 

The derivatives of serine formed by haloketones are stable to acid hydrolysis. The resulting ether linkage is apparently not cleaved by the usual hydrolytic conditions so that the loss of a serine residue, if it can be accurately determined, can be used as evidence for the modification of a serine residue (Schroeder and Shaw 1971). 

A. The hydroxyl group of serine forms hydrogen bonds with water. 

Figure 1-7. The catalytic triad in serine proteases. The reactive serine forms an acyl enzyme as a covalent intermediate during the proteolytic cleavage of a peptide bond. During substrate binding a proton is transferred from serine 195 to histidine 57, and the positive charge of the imidazole ring is stabilized by interaction with the carboxylate side chain of aspartic acid 102. The numbering corresponds to the structure of chymotrypsin.

H]serine formed was degraded into ethanol in such a way that C-3 of serine becomes C-1 of ethanol, and the alcohol dehydrogenase reaction was then used to establish the enantiomeric purity and the absolute configuration of the labelled centre. The sample was in fact 72% (S)-[l- H]ethanol and therefore the tritiated serine contained 72% of the label in the (35) and the remainder in the (3R) isomer (see Fig. 17). 

Fig. 8.17 A. Acetylcholinesterase normally catalyzes inactivation of the neurotransmitter acetylcholine in a hydrolysis reaction. The active site serine forms a covalent intermediate with a portion of the substrate during the course of the reaction. B. Diisopropyl phosphofluoridate (DFP), the ancestor of current organophosphorus nerve gases and pesticides, inactivates acetylchohnesterase by forming a covalent complex with the active site serine that cannot be hydrolysed by water. The result is that the enzyme cannot carry out its normal reaction, and acetylcholine accumulates.

In addition to glutamate, a number of amino acids release their nitrogen as NH4 (see Fig. 38.5). Histidine may be directly deaminated to form NH4 and urocanate. The deaminations of serine and threonine are dehydration reactions that require pyridoxal phosphate and are catalyzed by serine dehydratase. Serine forms pyruvate, and threonine forms a-ketobutyrate. In both cases, NH4 is released. 

First serine forms a protonated Schiff base (external aldimine) with pyridoxal-5 -phos-phate. Removal of the serine a-hydrogen leads to a quinonoid intermediate, which then can eliminate the (3-OH to generate the Schiff base of aminoacrylate. The reaction is com-... 

Cysteine possesses a highly reactive sulfhydryl group that forms a disulfide bridge when it is close to another cysteine residue of the same or a different polypeptide chain. Histidine forms coordination complexes and carries an extra proton below pH 6. Serine forms ester bonds with phosphate. Both histidine and serine are part of the catalytic site of many enzymes. 

When glycine is heated with pyridoxal and alum between pH 4 and 6 an insoluble material is formed. The yellow material contains two equivalents of pyridoxal per aluminum, but only one of these is active as a growth factor for yeast the other was found as a condensation product with glycine, liberated by acid from the original complex and identified as S-pyridoxylserine. This is analogous to serine formed from formaldehyde and glycine, with the aldehyde of pyridoxal becoming the jS-carbon of serine. The proposed structure for the complex is shown in (I). 

Show the reaction by which isoleucine and serine form a peptide bond. 

In addition, it was shown that serine could form a complex with pyridoxal phosphate in the presence of nickel nitrate. This complex, with an absorption maximum at 940 m, was taken as evidence for formation of a schiff-base. Again, imder conditions in which serine formed this complex, ethanolamine failed to form a schiff-base. Brady et al. (1958) have suggested that the inactivity of ethanolamine may be due to its failure to form a pyridoxal phosphate intermediate. 

Chargaff et paper chromatograms. From 20 to 100 fig. of each of these compounds in 0.01 ml. was applied to filter paper and developed with several solvent sterns. Choline was located on the developed chromatogram by its conversion to the phosphomolybdate followed by reduction with stannous chloride to molybdenum blue. Amino-ethanol and serine formed characteristic spots with ninhydrin which does... 

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