Co-enzymes

Enzyme action is frequently accelerated or retarded by the presence of other substances both organic and inorganic. Such substances have been divided into three categories (a) co-enzymes, without which certain enzymes are unable to function (i) activators, and (c) inhibitors. 

Glucose [50-99-7] urea [57-13-6] (qv), and cholesterol [57-88-5] (see Steroids) are the substrates most frequentiy measured, although there are many more substrates or metaboUtes that are determined in clinical laboratories using enzymes. Co-enzymes such as adenosine triphosphate [56-65-5] (ATP) and nicotinamide adenine dinucleotide [53-84-9] in its oxidized (NAD" ) or reduced (NADH) [58-68-4] form can be considered substrates. Enzymatic analysis is covered in detail elsewhere (9). 

Assays using equiUbrium (end point) methods are easy to do but the time requited to reach the end point must be considered. Substrate(s) to be measured reacts with co-enzyme or co-reactant (C) to produce products (P and Q) in an enzyme-catalyzed reaction. The greater the consumption of S, the more accurate the results. The consumption of S depends on the initial concentration of C relative to S and the equiUbrium constant of the reaction. A change in absorbance is usually monitored. Changes in pH and temperature may alter the equiUbrium constant but no serious errors are introduced unless the equihbrium constant is small. In order to complete an assay in a reasonable time, for example several minutes, the amount and therefore the cost of the enzyme and co-factor maybe relatively high. Sophisticated equipment is not requited, however. 

Oxidative substitutions at ring junction positions in various tetrahydro-5-deaza-pterins (79JA6068) and -flavins (77JA6721) have been studied, e.g. to give (13), and the oxidation-reduction reactions of 5-deazaflavins (e.g. 78CL1177, 80CPB3514) across the 1,5-positions, e.g. (19) (20), are involved in their co-enzymic role in enzymic oxidations (see Section... 

So far the economic feasibility of co-enzyme dependent biocatalyses is confined to relatively small market niches comprising products with high added value. 

Ammonium chloride source of nitrogen (required for synthesis of proteins, nudeic adds and co-enzymes). 

Phosphate constituent of nudeic adds, phospholipids and co-enzymes. 

Co-enzyme obtained from cultures of various strains of Streptococcus haemolyticus and capable of changing plasminogen into plasmin (complex enzyme mixture of streptokinase, streptodornase and streptolysin 0"). From fermentation liquors of hemolytic streptococci species Streptococcus haemolyticus), e. g. H 46 A. 

GOT (AST is the more recent abbreviation) catalyzes the transamination of 1-aspartic acid in the presence of a-ketoglut-aric acid, with pyridoxal phosphate being a required co-enzyme. The reaction is ... 

Figure 4.35 Reduced synthesis tree for Khorana-Todd synthesis of co-enzyme A. Step counts are shown in parentheses.

ABA-l-GAT Arsanilic acid conjugated with the synthetic polypeptide l-GAT AC Adenylate cyclase ACAT Acyl-co-enzyme-A acyltransferase... 

An enzyme, the most typical biocatalyst, is a protein (or peptide molecular chain), which can be made from living cells and promote, direct or facilitate the occurrence of a specific chemical reaction, without being consumed during the course of such reaction. The term enzyme is mostly used to describe proteinaeceous catalysts. However, in some instances it also includes co-enzymes or co-factors as they are supposed to be required to bring about the desired reaction. 

In the present paper we are concerned principally with the initiation step. The new observations and suggestions regarding this step put forward by Polanyi and his collaborators [17-21] marked the first real practical and theoretical advance since the work of Whitmore. They found that in the polymerization of isobutene by TiCl4 or BF3, both at room temperature and at very low temperatures, the metal halide alone was inactive, and that a third component, the co-catalyst, was required to initiate the polymerization. The word co-catalyst was chosen for the substances concerned, by analogy with co-enzyme . It is to be preferred to the term promoter , often used by American workers, as this indicates a substance which speeds up a reaction which would also take place in its absence, and since the characteristic of co-catalysts is that they are essential to the reaction. The first co-catalyst to be discovered was water, but shortly afterwards certain alcohols and acids were found to act in a similar manner. 

Many enzymes require the participation of dissociable coenzymes such as NAD+, NADP+ or ATP for their catalytic activities. The use of coenzymes to activate immobilized enzymes on a large scale is hampered by their relatively low stability and high cost. Attempts are therefore being made to stabilize the coenzymes and to find suitable means for their continuous regeneration. The principal approach has been to covalently attach a co-enzyme to a polymeric water-soluble matrix, thus making the co-enzyme, like the enzyme, potentially reusable (9,10). 

Aconitase, an unstable enzyme,4 is concerned with the reversible conversion of cis-aconitate to either citric acid or isocitric acid. It may be noted that the entire system of tricarboxylic cycle enzymes are present in the mitochondria separated from cells, and, furthermore, it has been found that the mitochondrial enzymes differ from the isolated enzymes in that the former require no addition of D.P.N. (co-enzyme I) or T.P.N. (co-enzyme II) for activity. Peters suggests that the citrate accumulation is caused by the competitive reaction of the fluorocitrate with aconitase required for the conversion of citrate to isocitrate. This interference with the tricarboxylic acid... 

Reiativeiy recentiy, the gases nitric oxide (NO) and carbon monoxide (CO) have been found to act as neurotransmitters in the nervous system. Nitric oxide is synthesized from L-arginine via nitric oxide synthase, requiring NADPH as a co-enzyme and tetrahydrobiopterin as a cofactor. Unlike other neurotransmitters, NO is a smaii, very soiubie moiecuie and cannot be stored in synaptic vesicles. Rather, it is synthesized on demand and freeiy diffuses through membranes. It is not broken down enzymaticaiiy because it is unstabie and degrades rapidiy. NO may have several actions, one of which is to increase the production of cGMP by guanyiyi... 

One class of agents which inhibits LT production in stimulated cell systems, but probably not by direct 5-LO inhibition, is the anti-inflammatory corticosteroids, represented by dexamethasone (10) [30-34]. The well-known inhibition of PG production seen with eorticosteroids is not due to direct CO inhibition, but has been attributed to the inhibition of arachidon-ic acid mobilization by phospholipase A2, caused by enhanced biosynthesis of one or more proteins called lipocortins [35] (although this hypothesis is now being seriously questioned [36]). More recent evidence indicates the possibility of down-regulation of CO enzyme levels [37-39] similar mechanisms involving altered gene regulation could be involved in the observed effects on LT production as well. 

Biocatalytk decarboxylation is a imique reaction, in the sense that it can be considered to be a protonation reaction to a carbanion equivalent intermediate in aqueous medimn. Thus, if optically active compoimds can be prepared via this type of reaction, it would be a very characteristic biotransformation, as compared to ordinary organic reactions. An enzyme isolated from a specific strain of Alcaligenes bronchisepticus catalyzes the asymmetric decarboxylation of a-aryl-a-methyhnalonic acid to give optically active a-arylpropionic acids. The effect of additives revealed that this enzyme requires no biotin, no co-enzyme A, and no ATP, as ordinary decarboxylases and transcarboxylases do. Studies on inhibitors of this enzyme and spectroscopic analysis made it clear that the Cys residue plays an essential role in the present reaction. The imique reaction mechanism based on these results and kinetic data in its support are presented. 

Plant Part FAD Co-enzyme Molecular Native Weight (kDa) SDS-page Natural substrate (as glycoside) Ref. 

Coenzymes are densely functionalized organic cofactors capable of catalyzing numerous diverse chemical reactions. Nature exploits the intrinsic chemical reactivity of these molecules to extend the chemical fimctionaUty of enzymes well beyond the reactivity of the coded amino acids. When these constituents are incorporated via covalent or non-covalent interactions into coenzyme-depen-dent enzymes, the inherent reactivity of the co enzyme is augmented and directed to effect chemical transformations with substrate and product selectivities, rates, and yields that are unachievable by either the protein or coenzyme alone. Thus, coenzymes play a critical role in the execution of a large number of essential metabolic processes. 

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