Enzymes specificity toward substrates

Zinc proteases carboxypeptidase A and thermolysin have been extensively studied in solution and in the crystal (for reviews, see Matthews, 1988 Christianson and Lipscomb, 1989). Both carboxypeptidase A and thermolysin hydrolyze the amide bond of polypeptide substrates, and each enzyme displays specificity toward substrates with large hydrophobic Pi side chains such as phenylalanine or leucine. The exopeptidase carboxypeptidase A has a molecular weight of about 35K and the structure of the native enzyme has been determined at 1.54 A resolution (Rees et ai, 1983). Residues in the active site which are important for catalysis are Glu-270, Arg-127, (liganded by His-69, His-196, and Glu-72 in bidentate fashion), and the zinc-bound water molecule (Fig. 30). 

More-detailed studies of enzyme specificity towards the structure of the nucleoside residue have been performed with the sucrose synthetase of pea seedlings,339,384,501,502 and with wheat-germ arbutin synthetase.339,364,503 Neither enzyme can use the cytidine,504 isocyti-dine, or N3-methyluridine derivatives as substrates. On the other hand, the a-D-glucopyranosyl pyrophosphate esters of 4-thiouri-... 

As modified so far the polyethylenimines, in contrast to enzymes, are weak in structural specificity toward substrates. This need not be a defect, however, for these macromolecular catalysts do not have to operate in a cellular environment and hence need not be subject to constraints designed to maintain the stability of a very complex, integrated biochemical network. Nevertheless, circumstances may arise where substrate specificity may be an essential requirement. We have some ideas on how this might be achieved with these relatively elastic macromolecular frameworks. For example, preliminary experiments show that we can attach —SH groups covalently to the polymer. It should be possible thereafter to add to the polymer solution an inhibitor with a structure analogous to the potential substrate and then to expose the solution to air... 

Enzymes are usually impressively specific in their action. The specificity toward substrate is sometimes almost absolute. For many years urea was believed to be the only substrate for the enzyme urease and succinate the only substrate for succinate dehydrogenase. Even after much searching for other substrates, only... 

Enzymes provide the catalytic entities in biological transformations. The unique characteristics of biocatalysts, namely, activation of substrates and acceleration of reaction rates at ambient temperatures, specificity towards substrates, and stereospecificity and chiroselectivity towards product formation, generate most sophisticated and effective catalysts [101]. Accordingly, extensive efforts of chemists and biochemists are directed towards harnessing chemical transformations by means of technological approaches utilizing enzymes [102, 103]. 

Further light on enzyme action is shed by micelles of amphiphilic molecules, which catalyse many chemical reactions and are often regarded as very simple models for enzymes ( amphiphilic and micelle are defined in Section 14.0). The disposition of hydrophobic (inside) and hydrophilic (outside) groups in a micelle resembles that of enzymes. Like enzymes, too, micelles are denatured by heat or urea, and they show specificity towards substrates (Jencks, 1969). Reactions that liberate anions, e.g. the hydrolysis of esters, are catalysed by cationic micelles, e.g. those of cetyltrimethylammonium bromide 14J6)... 

Enzyme purification and characterisation Acetyl esterases were isolated from a Rapidase C-80 preparation according to the scheme shown in Figure 1. The purified acetyl esterases were devoid of relevant side activities, and showed great differences in their specificity towards the different acetylated substrates. 

A number of rat liver carboxylesterases identified by their pI values are listed in Table 2.6 [73] five nonspecific carboxylesterases were purified from rat liver and were characterized according to their p/ values [61]. They appeared to be isoenzymes, since they had similar substrate specificities toward phenyl and naphthyl esters and monooleylglycerol. Subsequent studies, however, revealed different specificities with respect to their physiological substrates. The pI 5.2 and 5.6 enzymes were shown to be acylcamitine hydrolases (EC 3.1.1.28), and a p/ 6.0 enzyme an octanoylglycerol lipase. The p/... 

This enzyme [EC 2.3.1.21] catalyzes the reversible reaction of pahnitoyl-CoA with carnitine to yield coenzyme A and O-palmitoylcarnitine. The enzyme exhibits a broad specificity toward acyl-CoA substrates, covering a range from Cs to Cis, but optimal activity is observed with pahnitoyl-CoA. 

A measure and/or description of how specific an enzyme is toward a substrate or class of substrates or toward an effector or class of effectors. For effectors (or for ligands binding to macromolecules that are not enzymes), this specificity is readily measured by dissociation (or, association) constants. For enzymes, specificity is best quantitated by the Fmax/.K m ratio. See Specificity Constant. It is crucial, in the complete characterization of an enzyme, that the specificity of the enzyme be known in detail. 

This simple scheme can help us to understand unusual selectivity and high efficiency of such template reactions. The specific character of the enzyme effectiveness towards a particular substrate becomes obvious. The effect of macromolecular template on the reaction rate and particularly on its selectivity suggests that this type of reaction can be regarded as a catalyzed reaction. The template plays a role of a polymeric catalyst. On the other hand, the template polymerization is a particular case of a more general... 

---