Amino acyl residues, effect

An enzyme reaction intermediate (Enz—O—C(0)R or Enz—S—C(O)R), formed by a carboxyl group transfer (e.g., from a peptide bond or ester) to a hydroxyl or thiol group of an active-site amino acyl residue of the enzyme. Such intermediates are formed in reactions catalyzed by serine proteases transglutaminase, and formylglyci-namide ribonucleotide amidotransferase . Acyl-enzyme intermediates often can be isolated at low temperatures, low pH, or a combination of both. For acyl-seryl derivatives, deacylation at a pH value of 2 is about 10 -fold slower than at the optimal pH. A primary isotope effect can frequently be observed with a C-labeled substrate. If an amide substrate is used, it is possible that a secondary isotope effect may be observed as welF. See also Active Site Titration Serpins (Inhibitory Mechanism)... 

The conformational effects introduced by A-amino acyl residues in synthetic dehydropeptides have also been extensively studied and were shown to generate and/or to stabilize particular secondary structures in peptides. Much more is known about the conformational preferences of peptides containing Az-residues... 

Racemization of amino acyl residues in food proteins is a reaction that can take place during processing and cooking. This review deals with the occurrence and detection of alkali- and heat-induced racemization in proteins. Differences between calcium hydroxide-and sodium hydroxide-induced racemization and the effects of treatment with these alkalis on protein bioavailability is discussed. 

Recent studies have shown that in addition to the structure of the amino acyl residue, the position of the residue in the peptide (or protein) can have a major effect on racemization (69). Therefore, at the end of an exposure to alkali, and depending on the severity of the treatment, a mixture of the original protein and several D-amino acyl residue-containing proteins is likely to result. The latter are not necessarily Identical, i.e., the D-amlno acyl residues may be located at different positions along the primary structure of the protein, thereby giving rise to a heterogenous mixture of racemized proteins. 

While several laboratories have shown that severe racemiza-tion of proteins can occur during treatment with sodium hydroxide (6,18,22-24,61,62) the effects of other alkalis used in food processing are documented less well. Jenkins, et al. (70) have observed substantial differences in the degree of racemization caused by lime or caustic soda treatment of zein. Lime causes only 50% to 90% of the racemization observed for several amino acyl residues compared to when caustic soda is used. Because a substantial amount of calcium ion remained bound to the protein (approx. 10,000 ppm) compared to l/20th that amount of sodium ion for the caustic soda-treated zein, it is possible that divalent calcium may stabilize the protein making it less susceptible to racemization. Tovar (14) observed increases of 40% to 50% in serine and phenylalanine racemization and a decrease of 30% aspartate racemization for caustic soda-treated fish protein concentrate compared to lime-treated protein (see Table II). These studies indicate that different alkalis have different effects on racemization of proteins specifically, lime may cause less racemization than caustic soda at a similar pH. 

Using the rat liver enzyme they found that the tripeptide Asn.Leu.Thr was a very poor acceptor, while the addition of a further amino-acyl residue at the A- or C-terminus greatly improved its acceptor function. Moreover, when asparagine was free at the A/-terminal the peptides were less effective acceptors than when the A-terminus was substituted by a 2,4-dinitrophenyl residue. It would appear that the essential feature is that there should not be a charged group too close to the asparaginyl group. Provided that this condition is met, further chain extension at the A/-terminal does not further improve acceptor function. Some such effect may also occur near the threonyl residue, for the pentapeptide Tyr. Asn.Leu.Thr.Ser is only one-fifth as efficient as an acceptor as is the hexapeptide Tyr.Asn.Leu.Thr.Ler.Val. 

The other phospholipids can be derived from phosphatidates (residue = phosphatidyl). Their phosphate residues are esterified with the hydroxyl group of an amino alcohol choline, ethanolamine, or serine) or with the cyclohexane derivative myo-inositol. Phosphatidylcholine is shown here as an example of this type of compound. When two phosphatidyl residues are linked with one glycerol, the result is cardiolipin (not shown), a phospholipid that is characteristic of the inner mitochondrial membrane. Lysophospholipids arise from phospholipids by enzymatic cleavage of an acyl residue. The hemolytic effect of bee and snake venoms is due in part to this reaction. 

A way to study the effect of formylation on Met-tRNA structure was opened by specifically labelling the amino acid with C. In the C NMR spectrum of the yeast [ - CH3]Met-tRNAf a single C resonance is observed which corresponds to the methyl group. This indicates a rapid (on the NMR time scale) exchange of the amino acid residue between the 2 and 3 acylation positions (Fig. 19.9 a). A quite similar behaviour is found if E. coli tRNA is aminoacylated with [ - CH3]Met (Fig. 19.9 b). 

Aziridine-2-carboxylic acid residues in peptides are isomerized with Nal and acetone to the corresponding dehydro-amino-acid component. p-Hydroxy-amino-acids are effectively dehydrated by NiV -carbonyldi-imidazole to give the corresponding dehydro-amino-acids. 2-Azidocarboxylic esters (286) are effectively converted into A-acetyldehydro-a-amino-acid esters (287) (Scheme 141) 4oa Esters of A-acyl-2-(diethoxyphosphoryl)glycine have been used in the synthesis of dehydro-amino-acid esters. ... 

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