Michaelis complex, noncovalent

FIGURE 11.2 Hydrolysis of esters and peptides by serine proteases reaction scheme (a) and mechanism of action (b) (after Polgar15). (a) ES, noncovalent enzyme-substrate complex (Michaelis complex) EA, the acyl-enzyme PI and P2, the products, (b) X = OR or NHR (acylation) X = OH (deacylation). 

A noncovalent complex between two molecules. Binary complex often refers to an enzyme-substrate complex, designated ES in single-substrate reactions or as EA or EB in certain multisubstrate enzyme-catalyzed reactions. See Michaelis Complex... 

The concept of the enzyme-substrate complex is the foundation stone of enzyme kinetics and our understanding of the mechanism of enzyme catalysis. In honor of its introducer, this noncovalently bound complex is often termed the Michaelis complex. 

Typical effects of temperature on catalysis will be illustrated with the reaction of / -galactosidase and o- and p-nitrophenyl galactosides (18). The minimum reaction pathway may be represented by Scheme 1 in which ES is the noncovalent Michaelis complex, and EG represents an enzyme-galactose intermediate (25,26). 

The serine proteases catalyze a nucleophilic displacement reaction of a group attached to a carbonyl carbon—such groups comprising esters, thioesters, and amides. The reaction carried out by this class of enzymes is essentially an acyl transfer in which the substrate binds to the enzyme, forming the noncovalent Michaelis complex, followed by the formation of a covalent bond between the carbonyl carbon of the... 

Because the lipase active site is similar to that of serine proteases, the hydrolytic mechanism may also be analogous [5]. It is suggested that the catalytic reaction begins with the formation of a noncovalent Michaelis complex between the enzyme and the substrate, which then reacts with the nucleophilic oxygen of the serine to form a covalent tettahedral ttansition state. An acyl enzyme intermediate is then formed by cleavage of the substrate ester bond and dissociation of a protonated diacylglyceride. An activated water molecule then attacks the serine ester and forms a second tetrahedral transition state. Collapse of this ttansition state results in fatty acid release and the regeneration of the enzyme. 

Certain enzymes such as DNA polymerases require the binding of multiple substrates. In these multisubstrate enzymes, each substrate often occupies respective subsites within the active center. Although the residues in an active site (or center) are important for catalysis, certain residues are used for the binding of substrates, whereas others are used almost exclusively for chemical catalysis. The noncovalent enzyme-substrate complex that is enzymatically competent is called the Michaelis-Menten complex (see Section I,C,3). 

This mechanism is based on the assumption that the enzyme (E) binds the substrate in a rapid and reversible step to give a noncovalent enzyme—substrate complex (ES) known as the Michaelis-Menten complex. The ES slowly turns over to the product with a first-order rate constant 2- The free enzyme can resume the catalytic cycle. When k i > k2, the rapid equilibrium assumption holds and the ES is in equilibrium with E and S. Under the rapid equilibrium assumption, the rate expression is given by Eq. (1.7)... 

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