Coenzymes II Metallic Coenzymes

The original coenzymes were small organic molecules that activated enzymes and participated directly in catalyzing enzymatic reactions. Most of them were derived from vitamins and were known as biologically activated forms of vitamins such as niacin, riboflavin, thiamine, and pyridoxal. Heme was in a separate category, perhaps because of its widespread biological role as an oxygen carrier, and because it was not a vitamin, it was not widely regarded as a coenzyme. However, heme was clearly an...

Enzymatic Rate Enhancement and Transition State Binding

Accepting transition state theory as a basis for understanding reaction kinetics, it can be shown that an enzyme (or any catalyst) can catalyze a reaction by binding the transition state more tightly than the ground state. This concept made its appearance early in the study of enzymes (Pauling, 1948). It is a straightforward and reasonable concept because of the fact that a binding interaction should be stabilizing therefore, binding the transitionstate should stabilize it, thereby increasing...

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Pyruvoyl-enzymes to be discovered (Recsei and Snell, 1984 van Poelje and Snell, 1990). Pyruvoyl-enzymes catalyze a-decarboxylation of amino acids, similar to many PLP-dependent enzymes, and they include histidine, S-adenosylmethionine, phosphatidyl serine, and aspartate a-decarboxylases. Pyruvoyl-enzymes are found in both prokaryotes and eukaryotes. d-Proline reductase is also a pyruvoyl-enzyme (Hodgins and Abeles, 1967). The pyruvoyl moiety in histidine decarboxylase arises in a self-cleavage...

Ping Pong Mechanisms

Parallel lines in double reciprocal plots usually mean that a substrate reacts with the enzyme and produces a product independently of the presence of another substrate. In the two-substrate case, scheme 2-7 is the ping pong bi bi mechanism. In this mechanism, the leading substrate A reacts with the enzyme, changing it to a chemically different form designated F in scheme 2-7, and this process leads to the formation and release of the product P. The reaction of E F may involve an enzymatic...

Role of Ile16

To comprehend the identification of Ile16 as an ionizing group governing pH dependence, it is necessary to understand the origin of the B chain. Mature a-chymotrypsin consists of chains A, B, and C that arise from proteolytic processing of chymotrypsinogen, the inactive proenzyme form and primary translation product. Activation of chymotrypsinogen in vitro by the action of trypsin and activated chymotrypsin leads to the species depicted in fig. 6-2. Trypsin catalyzes peptide hydrolysis at sites...

Ordered Sequential Mechanisms

As in the case of single-substrate reactions, the substrate-binding steps are often not at equilibrium, and substrate and product dissociation are not much faster than the interconversion of ternary complexes. In these cases, substrate binding must be described in terms of rate constants, as in the Briggs-Haldane formulation, not dissociation constants. Nevertheless, the experimental rate equation often has the same form as eq. 2-10 for the equilibrium random mechanism. The ordered bi bi...

Kinetic and Equilibrium Isotope Effects

Isotope effects can contribute important information in the mechanistic analysis of enzymatic catalysis. An isotope effect is any effect on a reaction that is induced by the substitution of a heavy isotope into a reacting molecule. isotope effects may be induced on either the rate of or the equilibrium constant for a reaction. Effects on the rate are kinetic isotope effects, and effects on the equilibrium constant are equilibrium isotope effects. Most often the kinetic isotope effects provide...

Chemistry of ATP Synthase and the Binding Change Mechanism

As shown in biochemical studies, ATP synthase has preferential binding sites for MgATP and MgADP. Moreover, the kinetic studies of exchange reactions during hydrolysis by F1-ATPase and ATP synthesis by ATP synthase and the effects of varying the free nucleotide concentration on the exchange kinetics, provide valuable clues to site-site interactions within the multimeric enzyme. As shown in early radiochemical experiments, F1-ATPase catalyzes the rapid exchange of 32Pi into unreacted ATP, the...

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Mechanisms for glutaminase action and inactivation by diazonorleucine DON . A The mechanism accounts for the role of the essential cysteine residue in glutaminase action. Cysteine functions as a nucleophilic catalyst, leading to ammonia and a covalent y-glutamyl thioester-enzyme intermediate. Hydrolysis of the y-glutamyl thioester leads to glutamate. B Inactivation of glutaminase by 6-diazo-5-oxo-norleucine DON is a suicide mechanism initiated by the addition of the active site...

Histidine Phosphatases

Histidine Phosphorylation Mechanism

Phosphatases displaying full activity at pH 2.5, with subunit molecular masses of 40 to 60 kDa and dimeric structures, constitute a distinct family. The human lysosomal and prostatic acid phosphatases employ histidine as the nucleophilic catalyst Van Etten, 1982 . Trapping experiments with the substrate p-nitrophenyl 32P phosphate and alkaline denaturation in the steady state leads to a 32P-labeled protein, which on alkaline hydrolysis produces to 81-phosphohistidine Van Etten and Hickey, 1977...

Concerted Acid and Base Catalysis

Because an enzyme brings reacting groups together in the Michaelis complex, the possibilities for concerted general acid and general base catalysis are maximized. An early model of concerted catalysis in solution was observed in the mutarotation of tetramethyl glucose Swain and Brown, 1952 . The bifunctional catalyst 1-pyridone, the dominant tautomer of 1-hydroxypyridine, was 104 times as effective as an equimolar mixture of phenol and pyridine in catalyzing mutarotation, presumably due to...