Serine phosphate Subject

The side chains of proteins can undergo post-translational modification in the course of thermal processes. The reaction can also result in the formation of protein cross-links. A known reaction which mainly proceeds in the absence of carbohydrates, for example, is the formation of dehydroalanine from serine, cysteine or serine phosphate by the elimination of H2O, H2S or phosphate. The dehydroalanine can then lead to protein cross-links with the nucleophilic side chains of lysine or cysteine (cf. 1.4.4.11). In the presence of carbohydrates or their degradation products, especially the side chains of lysine and arginine are subject to modification, which is accompanied by a reduction in the nutritional value of the proteins. The structures of important lysine modifications are summarized in Formula 4.95. The best known compounds are the Amadori product -fructoselysine and furosine, which can be formed from the former compound via the intermediate 4-deoxyosone (Formula 4.96). To detect of the extent of heat treatment, e. g., in the case of heat treated milk products, furosine is released by acid hydrolysis of the proteins and quantitatively determined by amino acid analysis. In this process, all the intermediates which lead to furosine are degraded and an unknown portion of already existing furosine is destroyed. Therefore, the hydrolysis must occur under standardized conditions or preferably by using enzymes. Examples showing the concentrations of furosine in food are presented in Table 4.13. 

Phosphorylation of serine or threonine residues involves an ATP-dependent addition of a phosphate group to a primary (serine) or secondary (threonine) alcohol. Phos-phorylated proteins are often subject to rapid degradation. 

Approximately one-third of cellular proteins contain phosphate and are subject to covalent modification by phosphorylation and dephosphorylation reactions. This reversible phosphorylation of proteins causes conformational changes in the protein that dramatically alters their properties, e.g. from an active to an inactive enzyme, or vice versa. The sites of protein phosphorylation are those amino acid residues that contain hydroxyl groups, most commonly serine but also tyrosine and threonine (Fig. 27.2) (Chapter 31). Phosphorylation uses protein kinase and dephosphorylation uses protein phosphatase. The importance of reversible protein phosphorylation to the living cell is emphasised by the fact that protein kinases and protein phosphatases comprise approximately 5% of the proteins encoded by the human genome. Current research is discovering abnormalities of protein phosphorylation that are associated with diseases, notably type 2 diabetes meUitus (T2DM) and cancer. In the future, the discovery of drugs that modify protein phosphorylation/dephosphorylation promises new therapies for the treatment of these diseases. 

The reactions of amino acids, catalysed by enzymes requiring a derivative of vitamin Bg (Figure 5a) as cofactor, have been the subject of a number of mechanistic studies. Enzymes catalysing decarboxylation, racemisation, dehydration (of serine or threonine) or desulphy-dration (of cysteine) require pyridoxal-5 -phosphate as cofactor (Figure 5b), and are dealt with in later sections. Enzymes catalysing transamination, on the other hand, require either pyridoxal-5 -phosphate or pyridoxamine-5 -phosphate (Figure 5c). Snell and coworkers showed that the reactions of amino acids normally... 

Pyridoxal phosphate was demonstrated to be required as a coenzyme by dissociating it from the apoenzyme. This causes a loss of enzyme activity which can be restored by recombining the apoenzyme with pyridoxal phosphate. The optimum activity is at pH 8.3 and the enzyme requires free sulfhydryl groups for activity. The serine deaminase activity of the enz3rme is strongly inhibited by homocysteine and by cystathionine. No synthetic product appears to be formed with cysteine. The Km of the enzyme for serine is 8.1 X 10 Af. Knowledge of the mechanisms of the reactions catalyzed by pyridoxal phosphate-dependent enzymes has been advanced by model experiments with pyridoxal, metal ions, and amino acids. This subject has been reviewed recently by Snell (61). 

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