Acetate-forming enzymes

When subjected to drought stress, excised wheat Triticum aestivum L.) leaves increase ethylene production as a result of an increased synthesis of ACC 71 and an increased activity of the ethylene-forming enzyme (EFE) which catalyzes the conversion of ACC 71 to ethylene. Rehydratation to relieve water stress reduces EFE activity to levels similar to those in non-stressed tissue. Pretreatment of the leaves with N-benzyladenine (BA) 75 or indole-3-acetic acid lAA 76 prior to drought stress caused further increase in ethylene production. Conversely, pretreatment of wheat leaves with abscisic acid ABA 77 reduced ethylene production to levels of non-stressed leaves, accompanied by a decrease in ACC 71 content, Eq. (29). 

Peptides formed enzymically or by mineral acid hydrolysis or thermal degradation of higher molecular veight protein can also serve as flavor precursors in thermally induced reactions. The reactivity of peptides is evidenced by their behavior during pyrolysis/GC (9), heating in air (10), reactions vith mono- (11), and dicarbonyl (12, 13) compounds and reactions in hot acetic acid (1A). The types of reactions observed for peptides include side-chain thermolysis, fragmentation into amino acids, DKP formation and Halliard reaction vith ambient carbohydrates. 

Further studies indicate that an ADP-forming acetyl-CoA synthetase is also operative in other extremely thermophilic archaea Pyrococcus woesei, Thermococcus celer, Hyperthermus butylicus, Desulfurococcus amylolyticus), which form acetate as end product of their fermentation [305]. In contrast, in acetate forming (eu)bacteria, acetate formation from acetyl-CoA and the synthesis of ATP from ADP and Pj are catalyzed by two enzymes phosphate acetyltransferase and acetate kinase. This holds true for the extremely thermophilic (eu)bacterium. Thermotoga maritima[3Q5], which ferments... 

The XANES of the Mn catalase provided the first definitive proof that this enzyme cycles between the MnJ and Mn n oxidation levels (137, 352) The extent of catalase activity correlated with the proportion of MnJ1 or Mn enzyme however, samples with Mnlv quantitatively showed reduced activity. The EXAFS of the Mn catalases have been less informative because of the Mn-Mn separations in the reduced, active enzymes (135). Nevertheless, EXAFS of the superoxi-dized enzyme demonstrated that the Mn,uMnlv enzyme has a Mn-Mn separation of —2.7 A, which is consistent with a di-yu.2-oxo core (135). Subsequent spectroscopic analysis confirmed that a diamond core with a bridging syn,syn acetate formed the enzyme active site (9). 

Dihydroxyacetone phosphate is formed enzymically from fructose 1,6-diphosphate or by phosphorylation of dihydroxyacetone with ATP . Synthesis has been effected by phosphorylation of monoacetyldihydroxy-acetonedimethyl acetal . ... 

This enzyme, sometimes also called the Schardinger enzyme, occurs in milk. It is capable of " oxidising" acetaldehyde to acetic acid, and also the purine bases xanthine and hypoxanthine to uric acid. The former reaction is not a simple direct oxidation and is assumed to take place as follows. The enzyme activates the hydrated form of the aldehyde so that it readily parts w ith two hydrogen atoms in the presence of a suitable hydrogen acceptor such as methylene-blue the latter being reduced to the colourless leuco-compound. The oxidation of certain substrates will not take place in the absence of such a hydrogen acceptor. 

Enzyme-Catalyzed Reactions Enzymes are highly specific catalysts for biochemical reactions, with each enzyme showing a selectivity for a single reactant, or substrate. For example, acetylcholinesterase is an enzyme that catalyzes the decomposition of the neurotransmitter acetylcholine to choline and acetic acid. Many enzyme-substrate reactions follow a simple mechanism consisting of the initial formation of an enzyme-substrate complex, ES, which subsequently decomposes to form product, releasing the enzyme to react again. 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

In the chymotrypsiii mechanism, the nitrophenylacetate combines with the enzyme to form an ES complex. This is followed by a rapid second step in which an acyl-enzyme intermediate is formed, with the acetyl group covalently bound to the very reactive Ser . The nitrophenyl moiety is released as nitrophenolate (Figure 16.22), accounting for the burst of nitrophenolate product. Attack of a water molecule on the acyl-enzyme intermediate yields acetate as the second product in a subsequent, slower step. The enzyme is now free to bind another molecule of nitrophenylacetate, and the nitrophenolate product produced at this point corresponds to the slower, steady-state formation of product in the upper right portion of Figure 16.21. In this mechanism, the release of acetate is the rate-llmitmg step, and accounts for the observation of burst kinetics—the pattern shown in Figure 16.21. 

Rittenberg and Bloch showed in the late 1940s that acetate units are the building blocks of fatty acids. Their work, together with the discovery by Salih Wakil that bicarbonate is required for fatty acid biosynthesis, eventually made clear that this pathway involves synthesis of malonyl-CoA. The carboxylation of acetyl-CoA to form malonyl-CoA is essentially irreversible and is the committed step in the synthesis of fatty acids (Figure 25.2). The reaction is catalyzed by acetyl-CoA carboxylase, which contains a biotin prosthetic group. This carboxylase is the only enzyme of fatty acid synthesis in animals that is not part of the multienzyme complex called fatty acid synthase. 

Cyclic dithioketals and acetals represent another important class of sulfur containing chiral auxiliaries, which are available in chiral form by biooxidation. Biotransformations were performed on a preparative scale using whole-cells (wild type and recombinant) and isolated enzyme. Again, enantiocomplementary oxidation of unsubstituted dithianes (linear and cyclic, R = H) was observed when using and CPMOcomo (Scheme 9.28) [211,212]. Oxygenation of functionalized substrates (R = substituted alkyl) with gave preferably trans... 

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