Mechanism enzyme cleavage

One of the major reasons for studying the ESR of this series of Co(II)-corrinoids is that this spectral tool may give information on the mechanism of corrin-dependent enzymes. Cleavage of the Co—C—cr-bond has been studied in a few enzymes by using Co(II) ESR, and there is evidence... 

In the catalysis of the lyase from C. perfringens, the participation of lysine residues forming intennediary Schiff bases between enzyme and substrate molecules, and of histidine residues, has been demonstrated with the aid of photooxidation, reagents for histidine modification, and borohydride reduction in the presence of substrate.408-418 Thus, according to Frazi and coworkers,414 the lyase belongs to the class I lyases (aldolases). The catalytic mechanism proposed is outlined in Scheme 3. Evidence has been educed for the existence of a similar mechanism of cleavage of sialic acid by the lyase enriched from pig kidney.411... 

Exercise 25-30 The proteolytic enzyme, papain, differs from chymotrypsin in having cysteine, or a labile derivative thereof, as part of its active site. The enzyme is deactivated by substances that form complexes with, or react with, —SH groups and the activity is restored by reactions expected to regenerate an —SH group. Work out a schematic mechanism for cleavage of a peptide chain with papain that involves acylation of the critical —SH group of papain. 

It has been established that the enzymic cleavage of Maillard-type compounds depends on the concentration of cytochrome P-450 and of the coenzyme NADPH (j3) (Fig. 3). The mechanism suggested for the enzymic cleavage of deoxyfructoserotonin is shown in Figure 4. Therefore, factors interfering with this enzyme system may prevent the detrimental effects of the Maillard reaction in diabetes. 

Indeed both -lactylthiamine pyrophosphate (XX) and a-hydroxyethyl-thiamine pyrophosphate (XXI) have been isolated and identified as products after incubation of pyruvate with a purified carboxylase preparation " . When [2- - C]pyruvate is used, the radioactivity is found in the thiazole part of the molecule after sulfite cleavage of XXL Acetaldehyde is formed from pyruvic acid by yeast carboxylase by enzymic cleavage of intermediate XXI, Uberating thiamine pyTophosphate . XXI has also been identified as intermediate in the formation of acetyl-coenzyme A from pyruvic acid by p3u uvic oxidase . The transketolase reaction has been shown to proceed via a gly-colaldehyde-enzyme intermediate here one may expect to find dihydroxy-ethylthiamine pyrophosphate as active glycol-aldehyde . Such experiments strongly support Breslow s concept of the reaction mechanism. 

Mechanistic studies are extremely valuable in guiding and understanding the catalyses possible with novel catalytic structures. Our study of the preferred mechanism for cleavage of RNA catalyzed by imidazole buffer revealed a detailed mechanism that is not related to the mechanism normally invoked for catalysis by the imidazole groups of ribonuclease A. Based on our studies, we have proposed a novel mechanism for ribonuclease A that is in good agreement with detailed evidence on the enzyme and with subsequent calculations. 

The alkaline process of cleavage of o-fructose to give dihydroxy acetone and o-glyceraldehyde is similar to the important glycolysis reaction catalyzed by the enzyme aldolase (see Chapter 11) which reacts with o-fructose-1,6-bisphosphate at 37°C to give 3-phospho-o-glyceraldehyde and dihydroxy acetone phosphate. The comparison of the alkaline and enzyme cleavage and mechanisms of reaction are shown in Fig. 3.7. 

It is also fitting to consider the exact mechanism for the enzymic cleavage of 17a-hydroxyprogesterone to androgen (Fig. 15), the second... 

Ha T, Ting A Y, Liang J, Caldwell W B, Deniz A A, Chemla D S, Schultz P G and Weiss S 1999 Single-molecule fluorescence spectroscopy of enzyme conformational dynamics and cleavage mechanism Proc. Natl Acad. Sc/. USA 96 893-8... 

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