Enzyme reactions with coenzyme

L-carnitine Infant formulae UV-Vis 10-80 mg L-1 Bioreactor with immobilised enzyme/reaction with carnitine acetyltransferase coupled with acetyl coenzyme A and dithiobenzoate. [105]... 

Dealing with complex systems (two or more coupled enzymatic reactions or reactions with coenzyme regeneration) a complete kinetic investigation and computer simulation of the reaction system is very helpful to achieve the desired selectivity and yield of reaction (e. g. by choosing a sensible substrate and coenzyme concentration, enzyme ratio and reaction time). A case study is available[42, 431 exemplifying the production of L-tert-leucine by reductive animation and simultaneous coenzyme regeneration. 

Chelation is a feature of much research on the development and mechanism of action of catalysts. For example, enzyme chemistry is aided by the study of reactions of simpler chelates that are models of enzyme reactions. Certain enzymes, coenzymes, and vitamins possess chelate stmctures that must be involved in the mechanism of their action. The activation of many enzymes by metal ions most likely involves chelation, probably bridging the enzyme and substrate through the metal atom. Enzyme inhibition may often result from the formation by the inhibitor of a chelate with a greater stabiUty constant than that of the substrate or the enzyme for a necessary metal ion. 

In biological reactions, the situation is different from that in the laboratory. Only one substrate molecule at a time is present in the active site of the enzyme where reaction takes place, and that molecule is held in a precise position, with coenzymes and other necessary reacting groups nearby. As a result, biological radical reactions are both more controlled and more common than laboratory or industrial radical reactions. A particularly impressive example occurs in the biosynthesis of prostaglandins from arachiclonic acid, where a sequence of four radical additions take place. The reaction mechanism was discussed briefly in Section 5.3. 

The retro-Claisen reaction occurs by initial nucleophilic addition of a cysteine -SH group on the enzyme to the keto group of the /3-ketoacyl CoA to yield an alkoxide ion intermediate. Cleavage of the C2-C3 bond then follows, with expulsion of an acetyl CoA enolate ion. Protonation of the enolate ion gives acetyl CoA, and the enzyme-bound acyl group undergoes nucleophilic acyl substitution by reaction with a molecule of coenzyme A. The chain-shortened acyl CoA that results then enters another round of tire /3-oxidation pathway for further degradation. 

To clarify the characteristics of AMDase, the effects of some additives were examined using phenylmalonic acid as the representative substrate. The addihon of ATP and coenzyme A did not enhance the rate of the reaction, different from the case of malonyl-CoA decarboxylase and others in those, ATP and substrate acid form a mixed anhydride, which in turn reacts with coenzyme A to form a thiol ester of the substrate. In the present case, as both ATP and CoA-SH had no effect, the mechanism of the reaction will be totally different from the ordinary one described above. It is well estabhshed that avidin is a potent inhibitor of the formation of the biotin-enzyme complex. In the case of AMDase, addition of avidin has no influence on the enzyme activity, indicating that AMDase is not a biotin enzyme. 

Methyl coenzyme M reductase plays a key role in the production of methane in archaea. It catalyzes the reduction of methyl-coenzyme M with coenzyme B to produce methane and the heterodisulfide (Figure 3.35). The enzyme is an a2P2Y2 hexamer, embedded between two molecules of the nickel-porphinoid F jg and the reaction sequence has been delineated (Ermler et al. 1997). The heterodisulfide is reduced to the sulfides HS-CoB and HS-CoM by a reductase that has been characterized in Methanosarcina thermoph-ila, and involves low-potential hemes, [Fe4S4] clusters, and a membrane-bound metha-nophenazine that contains an isoprenoid chain linked by an ether bond to phenazine (Murakami et al. 2001). 

More than one hundred enzymes, and many of their coenzymes, have been recognized in animal mitochondria. Whether or not such an organization of enzymes is present in such a cell unit for performing a sequence of reactions with carbohydrates remains to be determined (see the Section on vitamin C). 

A few years later, in 1953, the versatility of pyridoxal phosphate was illustrated by Snell and his collaborators who found many of the enzyme reactions in which pyridoxal phosphate is a coenzyme could be catalyzed non-enzymically if the substrates were gently heated with pyridoxal phosphate (or free pydridoxal) in the presence of di- or tri-valent metal ions, including Cu2+, Fe3+, and Al3+. Most transaminases however are not metal proteins and a rather different complex is formed in the presence of the apoprotein. 

Hydride Transfer in NAD+- and NADP -Dependent Enzymes. The transfer of the hydride ion in redox reaction of NAD+- and NADP+-dependent enzymes can occur either to the re- or the xi-face of the pyridine ring of the coenzyme . Such stereochemistry is crucial in the characterization of these enzymes. The same enzymes from different sources can express different stereospecificities. For example, E. coli NAD(P)+ transhydrogenase expressed one form of stereospecificity whereas the Pseudomonas aeruginosa enzyme catalyzes the identical reaction with the other NAD form . 

Yang and Schulz also formulated a treatment of coupled enzyme reaction kinetics that does not assume an irreversible first reaction. The validity of their theory is confirmed by a model system consisting of enoyl-CoA hydratase (EC 4.2.1.17) and 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) with 2,4-decadienoyl coenzyme A as a substrate. Unlike the conventional theory, their approach was found to be indispensible for coupled enzyme systems characterized by a first reaction with a small equilibrium constant and/or wherein the coupling enzyme concentration is higher than that of the intermediate. Equations based on their theory can allow one to calculate steady-state velocities of coupled enzyme reactions and to predict the time course of coupled enzyme reactions during the pre-steady state. 

This enzyme [EC 2.7.8.7] catalyzes the reaction of coenzyme A with the apo-[acyl-carrier protein] to generate adenosine 3, 5 -bisphosphate and the holo-[acyl-carrier protein]. 

This enzyme [EC 4.1.3.21] catalyzes the reaction of 2-hydroxybutane 1,2,4-tricarboxylate with coenzyme A to produce acetyl-CoA, water, and a-ketoglutarate (or, 2-oxoglutarate). 

This enzyme [EC 1.2.7.3], also called 2-oxoglutarate synthase, catalyzes the reversible reaction of a-ketoglutarate (or, 2-oxoglutarate) with coenzyme A and oxidized ferredoxin to produce succinyl-CoA, carbon dioxide, and reduced ferredoxin. 

This enzyme [EC 1.2.1.27] catalyzes the reaction of 2-methyl-3-oxopropanoate with coenzyme A and NAD+ to produce propanoyl-CoA, carbon dioxide, and NADH. The enzyme will also catalyze the conversion of propanal to propanoyl-CoA. 

This cobalamin-dependent enzyme catalyzes the reaction of methyltetrahydromethanopterin with coenzyme M to produce methyl-coenzyme M and tetrahydrometha-nopterin. 

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