Activation in enzyme reactions

Several classes of vitamins are related to, or are precursors of, coenzymes that contain adenine nucleotides as part of their structure. These coenzymes include the flavin dinucleotides, the pyridine dinucleotides, and coenzyme A. The adenine nucleotide portion of these coenzymes does not participate actively in the reactions of these coenzymes rather, it enables the proper enzymes to recognize the coenzyme. Specifically, the adenine nucleotide greatly increases both the affinity and the speeifieity of the coenzyme for its site on the enzyme, owing to its numerous sites for hydrogen bonding, and also the hydrophobic and ionic bonding possibilities it brings to the coenzyme structure. 

The importance of hydrophobic binding interactions in facilitating catalysis in enzyme reactions is well known. The impact of this phenomenon in the action of synthetic polymer catalysts for reactions such as described above is significant. A full investigation of a variety of monomeric and polymeric catalysts with nucleophilic sites is currently underway. They are being used to study the effect of polymer structure and morphology on catalytic activity in transacylation and other reactions. 

A coenzyme is an organic compound that activates the primary enzyme to a catalytically active form. A coenzyme may act as a cofactor (see footnote 2), but the converse is not necessarily true. For example, the coenzyme nicotinamide adenine dinucleotide, in either its oxidized or reduced forms (NAD+ or NADH), often participates as a cofactor in enzyme reactions. 

Heme alone can reportedly elicit a catalatic reaction (the reaction mediated by catalase) but at a much reduced, almost negligible, rate compared to the catalatic proteins containing heme, and this may explain the observation of catalase activity in enzymes not normally associated with catalatic activity (4, 5). Other enz5unes have evolved that can catalyze a similar reaction in the absence of heme, but this review will limit itself to a consideration of heme containing proteins with catalatic activity. 

The description of single enzyme activity in chemical reactions, together with the activity of other biomolecules, is typical for biochemical models of alkaloid biogenesis. There is no contradiction between chemical and biochemical, which serve to enrich one another. In many cases, typical chemical and biochemical models are unified in papers today ". ... 

In enzymic reactions the central ES<= EP transformation is very fast, and the value of kcat is very high. In addition to correctly oriented binding of the substrate at the active center of the enzyme, an effective decrease in activation energy of this reaction step might also be provided by stabilization of the transition state of the substrate molecule in the ES complex. 

Esters are widespread in fruits and especially those with a relatively low molecular weight usually impart a characteristic fruity note to many foods, e.g. fermented beverages [49]. From the industrial viewpoint, esterases and lipases play an important role in synthetic chemistry, especially for stereoselective ester formations and kinetic resolutions of racemic alcohols [78]. These enzymes are very often easily available as cheap bulk reagents and usually remain active in organic reaction media. Therefore they are the preferred biocatalysts for the production of natural flavour esters, e.g. from short-chain aliphatic and terpenyl alcohols [7, 8], but also to provide enantiopure novel flavour and fragrance compounds for analytical and sensory evaluation purposes [12]. Enantioselectivity is an impor-... 

A major source of acceleration in enzymic reactions is approximation, that is to say, the bringing together of two or more reactants in the active site. Once the reagents are in contact, the subsequent reaction is intra- rather than intermolecular. Comparisons of the rates of intermolecular and intramolecular reactions are, however, difficult because the rate constants for bimolecular reactions have the units of M "1 s-1, whereas rate constants for unimolecular reactions have the units of s l. The best one can do in comparing them is to state the molarity at which the reactants would have to be present in the bimolecular reaction to achieve the rate of the unimolecular process when the effective molarity is large-say 1000 M or more-one has some measure of the power of approximation to accelerate chemical reaction. 

Carboxyl groups are activated in a reaction that splits ATP and joins C02 to enzyme-bound biotin. This activated C02 is then passed to an acceptor (pyruvate in this case) in a carboxylation reaction. 

Most, if not all, milks contain sufficient amounts of lipase to cause rancidity. However, in practice, lipolysis does not occur in milk because the substrate (triglycerides) and enzymes are well partitioned and a multiplicity of factors affect enzyme activity. Unlike most enzymatic reactions, lipolysis takes place at an oil-water interface. This rather unique situation gives rise to variables not ordinarily encountered in enzyme reactions. Factors such as the amount of surface area available, the permeability of the emulsion, the type of glyceride employed, the physical state of the substrate (complete solid, complete liquid, or liquid-solid), and the degree of agitation of the reaction medium must be taken into account for the results to be meaningful. Other variables common to all enzymatic reactions—such as pH, temperature, the presence of inhibitors and activators, the concentration of the enzyme and substrate, light, and the duration of the incubation period—will affect the activity and the subsequent interpretation of the results. 

It is frequently found in enzyme reactions that activity depends on two groups,... 

To date, the composition of active sites is known for many enzymes, the most probable action mechanisms are suggested, and comparison data exist on catalytic group properties in enzymes and free molecules in solutions. Note also that the chemical composition of catalytic active sites of enzymes is independent of the presence of any specific compounds. Moreover, the majority of them are the well-known compounds for homogeneous catalysis histidine imidazole, carboxylic groups of aspartic and aminoglutaric acids, flavins, hemins, etc. However, as homogeneous catalysts, they possess rather moderate or even poor catalytic activity in appropriate reactions. 

The first chemical synthesis of 7,8-dihydroneopterin-3 -triphosphate (477), the biosynthetic precursor of all naturally occurring pterin derivatives, has been achieved from neopterin-3 -monophos-phate (474) by formylation of the two hydroxy groups (475) followed by coupling with pyrophosphate to (476). Chemical or catalytic reduction resulted in the 7,8-dihydro derivative (477), which was fully active as a substrate in enzymic reactions <88BBR(152)1193>. 

Another motif recurs in this activation reaction. The enzyme-hound acyl-adenylate intermediate is not unique to the synthesis of acyl CoA. Acyl adenylates are frequently formed when carboxyl groups are activated in biochemical reactions. Amino acids are activated for protein synthesis hy a similar mechanism (Section 29.2.1). although the enzymes that catalyze this process are not homologous to acyl CoA synthetase. Thus, activation by adenylation recurs in part because of convergent evolution. 

Some nitrite reductases contain iron and copper other enzymes active in these reactions contain manganese. Reactions catalyzed by copper and iron enzymes with NO, N2O, and N2 as products have also been reported. 

O-Phosphoserine and 0-phosphothreonine 0-Phosphoserine has been reported in a number of proteins in which it appears to function as an intermediate in enzymic reactions (e.g. phosphoglucomutase), as a regulator of enzymic activity (e.g. phospho-rylase) or protein function (e.g. histones), and as a nutritive component (e.g. casein). O-Phosphothreonine has been reported in some of these proteins, but generally to a much more limited extent and only in those which also contain O-phosphoserine. 

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