Enzymes sequential

In a sequential reaction, all the substrates must be bound to the enzyme before any release of product can occur. Sequential systems can be either ordered or random. In an ordered sequential reaction, substrates must bind to the enzyme in a particular order, whereas in a random sequential system, substrates may bind to the enzyme in any order. In reaction schemes, substrates are usually abbreviated as A, B, C, and D in the order that they bind to the enzyme, whereas products are abbreviated as P, Q, R, and S in the order that they leave the enzyme. Sequential binding of substrates is a consequence of their orientation within the enzyme active site. 

Fig. 76. Enzymes sequentially coupled with GOD for the extension of GOD sensors to other analytes. 

Several methods are available to determine the N-terminal amino acid. In Sanger s method, the polypeptide chain is reacted with l-fluoro-2,4-dinitrobenzene. The dinitrophenyl (DNP) derivative of the N-terminal amino acid can be isolated and identified by ion-exchange chromatography after the polypeptide is hydrolyzed. A group of enzymes called the carboxypeptidases are used to identify the C-terminal residue. Carboxypeptidases A and B, both secreted by the pancreas, hydrolyze peptides one residue at a time from the C-terminal end. Carboxypeptidase A preferentially cleaves peptide bonds when an aromatic amino acid is the C-terminal residue. Carboxypeptidase B prefers basic residues. Because these enzymes sequentially cleave peptide bonds starting at the C-terminal residue, the first amino acid liberated is the C-terminal residue. 

More than 90% of the proteoglycans of sheep lung parenchyma have been solubilized in a non-enzymic sequential procedure. 

The subsequent induction of enzymes which belong to one metabolic pathway by the products of preceding enzymes (sequential gene expression), is a characteristic of catabolic pathways in bacteria. Although sequential formation of secondary products and secondary metabolic enzymes in many plants and microorganisms has been observed the direct influence of secondary products on the expression of enzymes catalyzing their further transformation has been demonstrated in very few organisms. 

In the biosynthesis of steroids lanosterol and cycloartenol, the building blocks DMAPP and IPP are first joined through famesyl diphosphate synthase to form geranyl pyrophosphate (GPP) and then to form FPP, presqualene pyrophosphate, squalene, and (S)-squalene-2,3-epoxide by corresponding enzymes, sequentially. The product (S)-squalene-2,3-epoxide is used to synthesize the sterols lanosterol (in animals) and cycloartenol (in plants) by lanosterol synthase and cycloartenol synthase, respectively [3]. Further, lanosterol and cycloartenol are converted into other steroids through steroidogenesis. 

Steroids are synthetic products of cholesterol [57-88-5]. The chemical stmcture of a steroid hormone is determined by sequential enzymatic processing of the cholesterol molecule. Steroid products differ among steroid-secreting glands because of differences in enzyme processing, eg, the production of estrogen by the ovary requires enzymatic steps that do not occur in the adrenal cortex. 

Figure 4.6 The bifunctional enzyme PRA-isomerase (PRAI) IGP-synthase (IGPS) catalyzes two sequential reactions in the biosynthesis of tryptophan. In the first reaction (top half), which is catalyzed by the C-terminal PRAI domain of the enzyme, the substrate N-(5 -phosphoribosyl) anthranilate (PRA) is converted to l-(o-carboxyphenylamino)-l-deoxyribulose 5-phosphate (CdRP) by a rearrangement reaction. The succeeding step (bottom half), a ring closure reaction from CdRP to indole-3-glycerol phosphate (IGP), is catalyzed by the N-terminal IGPS domain.

Figure 4.7 Two of the enzymatic activities involved in the biosynthesis of tryptophan in E. coli, phosphoribosyl anthranilate (PRA) isomerase and indoleglycerol phosphate (IGP) synthase, are performed by two separate domains in the polypeptide chain of a bifunctional enzyme. Both these domains are a/p-barrel structures, oriented such that their active sites are on opposite sides of the molecule. The two catalytic reactions are therefore independent of each other. The diagram shows the IGP-synthase domain (residues 48-254) with dark colors and the PRA-isomerase domain with light colors. The a helices are sequentially labeled a-h in both barrel domains. Residue 255 (arrow) is the first residue of the second domain. (Adapted from J.P. Priestle et al., Proc.

In this type of sequential reaction, all possible binary enzyme substrate complexes (AE, EB, QE, EP) are formed rapidly and reversibly when the enzyme is added to a reaction mixture containing A, B, P, and Q ... 

FIGURE 18.5 Schematic representation of types of multienzyme systems carrying out a metabolic pathway (a) Physically separate, soluble enzymes with diffusing intermediates, (b) A multienzyme complex. Substrate enters the complex, becomes covalently bound and then sequentially modified by enzymes Ei to E5 before product is released. No intermediates are free to diffuse away, (c) A membrane-bound multienzyme system. 

Srere, P. A., 1987. Complexes of. sequential metabolic enzyme.s. Annual Review of Biochemistry 56 89-124. A review of how enzyme.s in. some metabolic patliways are organized into complexes. 

Vertessy, B. G., Orosz, F., Kovacs, J., and Ovadi, J., 1997. Alternative binding of two sequential glycolytic enzymes to microtnbnles. Molecnlar studies in the phosphofrnctokinase/aldolase/microtnbnle Journal of... 

Ester hydrolysis is common in biological chemistry, particularly in the digestion of dietary fats and oils. We ll save a complete discussion of the mechanistic details of fat hydrolysis until Section 29.2 but will note for now that the reaction is catalyzed by various lipase enzymes and involves two sequential nucleophilic acyl substitution reactions. The first is a trcinsesterificatiori reaction in which an alcohol gioup on the lipase adds to an ester linkage in the tat molecule to give a tetrahedral intermediate that expels alcohol and forms an acyl... 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

The reaction occurs by two sequential nucleophilic acyl substitutions, the first by a cysteine residue in the enzyme, with phosphate as leaving group, and the second by hydride donation fromNADH, with the cysteine residue as leaving group. 

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