Reaction nonenzyme-catalyzed

Most reactions in secondary metabolic pathways are catalyzed by specific enzymes. Spontaneous, i.e., nonenzyme catalyzed reactions are significant in only a few cases, e.g., in the formation of some high molecular compounds like humic acids (D 3.3.1), melanins (D 22.1.3) and lignins (D 22.2.3), and in some cycliza-tions. There is also no evidence that enzymes of primary metabolism play a role in secondary metabolic pathways. 

Nonenzyme catalyzed reactions have been studied in conaderable detail and have been reviewed most recently by Snell (273). 

Since the reaction has been reviewed recently (12) only a few additional facts will be mentioned. Many optically active cyanohydrins can be prepared (33) with e.e. s of 84 to 100% by the use of the flavopnotein D-oxynitrilase adsorbed on special (34) cellulose ion-exchange resins. Although the enzyme is stable, permitting the use of a continuously operating column, naturally only one enantiomer, usually the R isomer, is produced in excess. This (reversible) enzyme-catalyzed reaction is very rapid (34). Nonenzymic catalysts, such as the cinchona alkaloids, permit either enantiomer to be prepared in excess. 

Enzyme-catalyzed reactions involve specific, rapid combination of substrate and enzyme to form a complex that is rapidly converted to products through transition states that are controlled by the enzyme s environment. Since enzymes are homogeneous chemical catalysts, we expect them to operate by routes that parallel some of the same processes in reactions that do not involve enzymes. The relative magnitude of enzymic and nonenzymic catalytic parameters has been called catalytic proficiency by Wolfenden6,17 24 and this has been a subject of intense current interest.7,25 32 Wolfenden noted that while nonenzymic reactions have diverse rates, enzyme-catalyzed processes are highly evolved to be comparable in rate, no matter how slow their nonenzymic counterparts. 

As shown in Figure 8.4, the synthesis of NAD from tryptophan involves the nonenzymic cyclization of aminocarhoxymuconic semialdehyde to quinolinic acid. The alternative metahoUc fate of aminocarhoxymuconic semialdehyde is decarboxylation, catalyzed hy picolinate carboxylase, leading into the oxidative branch of the pathway, and catabolism via acetyl coenzyme A. There is thus competition between an enzyme-catalyzed reaction that has hyperbolic, saturable kinetics, and a nonenzymic reaction thathas linear, first-order kinetics. 

The ring nitrogen of pyridoxal phosphate exerts a strong electron withdrawing effect on the aldimine, and this leads to weakening of all three bonds about the a-carbon of the substrate. In nonenzymic reactions, all the possible pyridoxal-catalyzed reactions are observed - a-decarboxylation, aminotrans-fer, racemization and side-chain elimination, and replacement reactions. By contrast, enzymes show specificity for the reaction pathway followed which bond is cleaved will depend on the orientation of the Schiff base relative to reactive groups of the catalytic site. As discussed in Section 9.3.1.5, reaction specificity is not complete, and a number of decarboxylases also undergo transamination. 

Compared to linoleic acid linolenic acid has one more double allylically activated CH2 group, therefore 4 regioisomeric hydroperoxides (9-HPOTE, 12-HPOTE, 13-HPOTE and 16-HPOTE) are generated in form of enantiomeric pairs in nonenzymic catalyzed LPO reactions. 

The absolute amounts of aniline covalently bonded to the soil fulvic acid in the presence and absence of the peroxidase were not measured. However, the relative signal to noise ratios obtained in the NMR spectra indicate that significantly more aniline was taken up by the fulvic acid in the enzyme catalyzed reaction. It should also be pointed out that, in the execution of the peroxidase experiment, the solution containing the fulvic acid, aniline, and peroxidase darkened instantaneously upon addition of the hydrogen peroxide, indicating significantly faster kinetics than in the nonenzyme reaction. 

Figure 8-8 Schematic diagram showing effect of temperature on the rate of nonenzyme-catalyzed and enzyme-catalyzed reactions.

The nature of the action of the mutarotase from P. notatum has been investigated extensively by Bentley and Bhate,140 and compared with the acid-, base-, and solvent-catalyzed reactions (see also, p. 31). Through use of lsO on C-l, it was shown that dehydrogenations do not occur on C-l. In addition, no dehydrogenation occurred at carbon-bound hydrogen atoms a single-displacement mechanism was thus eliminated. The enzyme probably transfers a proton, in a process similar to that usually involved in nonenzymically catalyzed muta-rotations. 

As noted in Section I, the mechanism of an Sn2(P) reaction can involve the formation of a transiently stable pentacovalent, trigonal bipyramidal intermediate that, depending on the exact reaction, can undergo pseudorotation before breakdown to product can be achieved. No evidence for pseudorotation has yet been obtained for enzyme-catalyzed reactions however, many nonenzymic displacement reactions have been demonstrated to proceed by retention of configuration, and this is best explained by pseudorotation of a pentacovalent intermediate. 

Imidazolone propionate hydrolase catalyzes the enzymatic cleavage of the imidazole ring to yield formi-minoglutamate. The rat liver enzyme has been partially purified. In addition to the enzymic conversion, two nonenzymic spontaneous reactions yield N-formyl-isoglutamine and 4-oxoglutamic acid. In addition to the oxidative pathways for histidine, there exist three other pathways for its use protein synthesis, decarboxylation to yield histamine (see Inflammation), and transaminase. The activity of histidine pyruvic transamination in rat liver is three times that of histidase. The product of the transaminase reaction is imidazole pyruvic acid, which in turn is converted to imidazole acetic acid. 

A major advantage that nonenzymic chiral catalysts might have over enzymes, then, is their potential ability to accept substrates of different structures by contrast, an enzyme will select only its substrate from a mixture. Striking examples are the chiral phosphine-rhodium catalysts, which catalyze die hydrogenation of double bonds to produce chiral amino acids (10-12), and the titanium isopropoxide-tartrate complex of Sharpless (11,13,14), which catalyzes the epoxidation of numerous allylic alcohols. Since the enantiomeric purities of the products from these reactions are exceedingly high (>90%), we might conclude... 

Enzyme and Nonenzyme Catalysts By nature, enzymes themselves are chiral and they catalyze a variety of chemical reactions with stereoselectivity. These reactions include oxidation, reduction, and hydration. Examples of enzymes are oxidases, dehydrogenases, lipases, and proteases. Metoprolol, an adrenoceptor-blocking drug, is produced using an enzyme-catalyzed method. 

This enzyme [EC 1.4.1.12] catalyzes the reaction of 2,4-diaminopentanoate with NAD(P)+ and water to produce 2-amino-4-oxopentanoate, ammonia, and NAD(P)H. The enzyme can also utilize 2,5-diaminohexanoate as a substrate (although not as effectively as the substrate mentioned above) forming 2-amino-5-oxohexanoate, which then cyclizes nonenzymically to form 1-pyrroline-2-methyl-5-carboxylate. 

A quantitative expression developed by Albery and Knowles to describe the effectiveness of a catalyst in accelerating a chemical reaction. The function, which depends on magnitude of the rate constants describing individual steps in the reaction, reaches a limiting value of unity when the reaction rate is controlled by diffusion. For the interconversion of dihydroxacetone phosphate and glyceraldehyde 3-phosphate, the efficiency function equals 2.5 x 10 for a simple carboxylate catalyst in a nonenzymic process and 0.6 for the enzyme-catalyzed process. Albery and Knowles suggest that evolution has produced a nearly perfect catalyst in the form of triose-phosphate isomerase. See Reaction Coordinate Diagram... 

This FAD-dependent enzyme [EC 1.4.3.10] catalyzes the reaction of putrescine with dioxygen and water to produce 4-ammobutanal, ammonia, and hydrogen peroxide. 4-Aminobutanal then condenses nonenzymically to 1-pyrroline. 

Until 1968, not a single nonenzymic catalytic asymmetric synthesis had been achieved with an enantiomeric excess above 50%. Now, the intramolecular aldol cyclisation, catalyzed by chiral amino acids has proven to be a very useful synthetic tool. This reaction was extensively covered by two reviews 23,68). Two more papers 72 published recently, should also be cited. 

The transition state-stabilizing power of the rapid enzymic process can be held up for comparison with a particularly slow nonenzymic reaction that is brought about through addition of a large amount of heating. In particular, comparisons of the rates of enzyme-catalyzed process and spontaneous nonenzymic reaction produce dramatically large ratios. 

Nonenzymic transphosphorylation catalyzed by divalent metal ions may have been the prototype reaction for the biochemical evolution of those enzymes which utilise the free energy of hydrolysis of ATP to drive chemical transformation. 

Another early success in biomimetic chemistry concerns reactions promoted by thiamin. In 1943, more than 35 years ago, Ukai, Tanaka, and Dokowa (12) reported that thiamin will catalyze a benzoin-type condensation of acetaldehyde to yield acetoin. This reaction parallels a similar enzymic reaction where pyruvate is decarboxylated to yield acetoin and acetolactic acid. Although the yields of the nonenzymic process are low, it is clearly a biomimetic process further investigation by Breslow, stimulated by the early discovery of Ugai et al., led to an understanding of the mechanism of action of thiamin as a coenzyme. 

LIP catalyzes the one-electron oxidation of a variety of substrates to generate aryl cation radicals. These radicals undergo subsequent nonenzymic reactions to yield a multiplicity of final products. MnP oxidizes Mnll to Mnlll which acts as a unique redox couple, and in turn oxidizes phenolic substrates to phenoxy radicals, which undergo subsequent reactions to yield the final products. 

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