Enzyme carbonyl containing

The abundance of diverse oximes as naturally occurring products is attributed, in general, to complicated metabolic processes that include enzyme-induced oxidation of either amino acids or biogenic amines, as well as to the relatively low reactivity of oximes compared to HA. The co-occurrence of HA and a variety of carbonyl-containing molecules in plants and animals may also contribute to the formation of the oxime bond in nature. 

Aldose reductase (ALR2 EC 1.1.1.21) is an 36 kDa enzyme that catalyzes the reduction of a wide range of carbonyl-containing compounds to their corresponding alcohols. It is a member of an extensive aldo-keto oxidoreductase enzyme family, a collection of structurally similar proteins expressed in both animals and plants. Most members of the enzyme family possess similarities in molecular mass, pH optimum, coenzyme dependence, and demonstrate overlapping specificity for many substrates and inhibitors. 

Formation of Secondary Products and Lipohydroperoxide Destruction. As early as 1945 Holman and Burr (132) found that crude soybean lipoxygenase acting on a number of substrates produced carbonyl-containing material in addition to diene. Holman, as noted above (107), used his crystalline enzyme and found that it was difficult to establish a correspondence between O2 consumption and diene conjugation. The diene concentration always tended to be too low. Privett et al. 123) found that the reaction products varied with enzyme concentration and method of addition. Vioque and Holman 133) identified 9-keto-ll,13- and 13-keto-9,ll-octadecadienoate with the usual hydroperoxides in a reaction carried out with linoleic acid and a relatively large amount of crude soybean lipoxygenase at pH 9. 

Thus, saponins are apparently able to stimulate or suppress the immune system by two different mechanisms. In one mechanism, saponins interact with cell membrane components to alter cell permeability, membrane-associated enzymes, cell-surface receptors, and other components, and thus result in a nonspecific stimulation (or suppression) of phagocytosis, phagocytic chemiluminescence, and other functions of phagocytic lymphocytes. These effects can take place in vivo or in vitro. In the other mechanism, formation of imines (Schiff bases) by carbonyl-containing saponins can provide a co-stimulatory signal necessary for specific immunopotentiation of T cells that leads to a CMI response. This second mechanism, which is known as an adjuvant effect, takes place only in vivo, and is discussed in more detail in subsequent sections of this article. From previous work [62,73], it is expected that, for saponins with appropriate structures, both mechanisms would take place. Perhaps, the best-known case where both mechanisms can take place is that of the saponins derived from Q. saponaria Molina, which are next discussed in more detail. 

The aldolases are a diverse class of enzymes that catalyse the coupling of a carbonyl-containing compound (nucleophile), containing one, two or three carbons, with an aldehyde (electrophile). In most cases the nucleophile is either pyruvic acid or dihydroxyacetone phosphate, whereas the electrophilic aldehyde is much more variable in structure. In many cases the reaction generates two new stereogenic centres in the product. In general, only one isomer is obtained from the four possible stereoisomeric products (Scheme 5.1). 

The mechanism for NADH reduction of the carbonyl group of an aldehyde or a ketone follows. First, the carbonyl-containing compound and NADH are positioned on the surface of the enzyme catalyst in a highly specific relationship to each other. Then follows a redistribution of valence electrons, one part of which is the transfer of a hydrogen atom with its pair... 

Ribulosebisphosphate carboxylase (isolated by a published procedure ) (250 mg, 3.57 /imoles of protomeric unit > ) in 50 ml of metal-free 0.1 Af Bicine/60 inM potassium bicarbonate/0.1 mAf EDTA (pH 8.0) was treated at 25° with four successive 0.25-ml additions, at 20-min intervals, of 20 mAf Br-butanone- P2- Twenty minutes after the fourth addition, less than 10% of the initial activity (as determined by the method of Racker ) remained, and the reaction was terminated by the addition of 2-mercaptoethanol (10 mAf). A duplicate enzyme solution containing ribulosebisphosphate (1 mAf) was treated with reagent in an identical manner 95% of the initial enzymic activity was retained. A third enzyme solution under the same conditions but lacking both the reagent and substrate served as control. The three protein solutions were dialyzed against 0.1 M sodium chloride at 4° after dialysis the protein samples were made 0.1 M in sodium bicarbonate and 0.01 Af in sodium [ H]borohydride in order to reduce the carbonyl of the protein-bound reagent. The mixtures were maintianed in an ice bath for 30 min and then dialyzed exhaustively at 4° against 50 mAf sodium chloride. The samples then were assayed for protein concentration, radioactivity ( H and P), and sulfhydryl content (see the table). 

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers... 

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

Carboxypeptidases are zinc-containing enzymes that catalyze the hydrolysis of polypeptides at the C-terminal peptide bond. The bovine enzyme form A is a monomeric protein comprising 307 amino acid residues. The structure was determined in the laboratory of William Lipscomb, Harvard University, in 1970 and later refined to 1.5 A resolution. Biochemical and x-ray studies have shown that the zinc atom is essential for catalysis by binding to the carbonyl oxygen of the substrate. This binding weakens the C =0 bond by... 

Two classes of aldolase enzymes are found in nature. Animal tissues produce a Class I aldolase, characterized by the formation of a covalent Schiff base intermediate between an active-site lysine and the carbonyl group of the substrate. Class I aldolases do not require a divalent metal ion (and thus are not inhibited by EDTA) but are inhibited by sodium borohydride, NaBH4, in the presence of substrate (see A Deeper Look, page 622). Class II aldolases are produced mainly in bacteria and fungi and are not inhibited by borohydride, but do contain an active-site metal (normally zinc, Zn ) and are inhibited by EDTA. Cyanobacteria and some other simple organisms possess both classes of aldolase. 

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