Interconversion enzymes

M3. Maxwell, E. S., The enzymic interconversion of uridine diphosphogalactose and undine diphosphoglucose. J. Biol. Chem. 229, 139-151 (1957). 

A protocoF for assessing the behavior of enzymes having one or more isomerization steps. The method facilitates determination of Kup, a kinetic parameter associated with the enzyme interconversion. See Iso... 

Figure 2. A multistep enzyme interconversion cascade illustrating the sequential modification and conversion of target enzymes from one state of activity to another. Although written here as a cascade of increasing catalytic activity, enzyme-catalyzed covalent modification can either activate or inhibit target enzymes, depending on the particular system under study.

Three types of activated sugar intermediate are known to play an important role in the biosynthesis of carbohydrate chains their structures and enzymic interconversions are shown in Scheme 1 for D-glucopyranose... 

Terminology to deal with this type of situation has not yet been developed or has not come into widespread use [87]. Conversion of one molecular form of an enzyme to another more active form is referred to as activation of that enzyme. Effects such as those of 5 -AMP on phosphorylase b where the activator must be present continuously for activity to be maintained and where the formation of a new molecular species is not involved, is referred to as allosteric stimulation or, where less is known about the mechanism, simply as stimulation of the enzyme. Interconversion allows metabolic reactions to be switched on and off without changes in the intracellular concentration of metabolites. 

Enzymic interconversion of sugars has also been exploited as an additional fresh milk processing step to convert the normal lactose milk sugar, to which some people react unfavorably, into glucose and galactose, which are much better tolerated [82]. Products from this milk can now be enjoyed by those who could only tolerate milk substitutes previously. 

S. A. Barker, Enzymic interconversion of D-glucose and D-fmctose, and of D-xylose and D-xylulose, Methods Carbohydr. Chem., 10 (1994) 241-248. 

The several known enzymic mechanisms for the synthesis of sugar nucleotides have been previously reviewed. It was pointed out that the mechanism of epimerization, whereby the glycosyl moiety of the glycosyl ester of a nucleotide is transformed into one having a different group-configuration, was obscure. This process may be exemplified by one of the first such enzymic interconversions to be discovered ... 

Covalent modHication of enzymes, enzyme modulation, enzyme interconversion Oligomeric (i.e. multichain) enzymes may exist in two or more forms, which are interconvertible by enzyme-catalysed covalent modifications, and which differ in their catalytic properties, e.g. activity, substrate affinity and dependence on effectors. Usually the difference in activity is such that one form is active and the other inactive. The activities of the conversion enzymes are in turn regulated by other enzymes, metabolites and/ or effectors. Covalent modifications are therefore important in physiological regulation, in addition to Allostery (see). Whereas allostery provides fine adjustment of metablic rates, C provides an on/off switching of cellular functions, which is very sensitive to environmental influences. 

An important extended example of enzyme interconversion cycles is the reciprocal control of glycogen metabolism involving glycogen synthase and glycogen phosphorylase (Section 11.5). The activities of both enzymes are regulated in concert by phosphorylation and dephosphorylation reactions so that when the synthetic pathway is in operation, the degradative pathway is reciprocally r uced. 

While the examples considered above of enzymic interconversions in extracellular fluids are irreversible, covalent modification of intracellular enzymes is usually reversible. Two major mechanisms of reversible covalent modification are known phosphorylation-dephosphorylation, which we will consider further, and adenylation-deadenylation, which is a microbial system. Holtzer (1969) and Segal (1973) have reviewed this topic. 

Fig. 11.37 Free energy profile for the nucleophilic attack of water on CO2 (a) in aqueous solution and (b) in the enzyme carbonic anhydrase. (Graphs redrawn from Aqvist J, M Fothergill and A Warshel 1993. Computer Simulai of the COj/HCOf Interconversion Step in Human Carbonic Anhydrase I. Journal of the American Chemical Society 115 631-635.)...

The chemical reaction catalyzed by triosephosphate isomerase (TIM) was the first application of the QM-MM method in CHARMM to the smdy of enzyme catalysis [26]. The study calculated an energy pathway for the reaction in the enzyme and decomposed the energetics into specific contributions from each of the residues of the enzyme. TIM catalyzes the interconversion of dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde 3-phosphate (GAP) as part of the glycolytic pathway. Extensive experimental studies have been performed on TIM, and it has been proposed that Glu-165 acts as a base for deprotonation of DHAP and that His-95 acts as an acid to protonate the carbonyl oxygen of DHAP, forming an enediolate (see Fig. 3) [58]. 

The enzyme-catalyzed interconversion of acetaldehyde and ethanol serves to illustrate a second important feature of prochiral relationships, that ofprochiral faces. Addition of a fourth ligand, different from the three already present, to the carbonyl carbon of acetaldehyde will produce a chiral molecule. The original molecule presents to the approaching reagent two faces which bear a mirror-image relationship to one another and are therefore enantiotopic. The two faces may be classified as re (from rectus) or si (from sinister), according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re if coimter-clockwise, then si. The re and si faces of acetaldehyde are shown below. 

In this chapter, we have examined the use of cells and enzymes to chemically transform lipids. We have had to be selective and have predominantly focused attention on the transformation of sterols and steroids. We first explained why these compounds were commercially important and why they only occur in low concentrations in natural systems. We pointed out that a very large number of reaction types are possible, but those which have found greatest use include stereospedfic hydroxylations, alcohol/ketone interconversion, hydrolysis, conjugation and isomerisation. 

Further examples of the utility of the allylic sulfoxide-sulfenate interconversion in the construction of various biologically active natural products include intermediates such as the /Miydroxy-a-methylene-y-butyrolactones (e.g. 63)128 and tetrahydrochromanone derivative 64129. Interestingly, the facility and efficiency of this rearrangement has also attracted attention beyond the conventional boundaries of organic chemistry. Thus, a study on mechanism-based enzyme inactivation using an allyl sulfoxide-sulfenate rearrangement has also been published130 131. 

A more general access to biologically important and structurally more diverse aldose isomers makes use of ketol isomerases for the enzymatic interconversion of ketoses to aldoses. For a full realization of the concept of enzymatic stereodivergent carbohydrate synthesis, the stereochemically complementary i-rhamnose (Rhal EC 5.3.1.14) and i-fucose isomerases (Fuel EC 5.3.1.3) from E. coli have been shown to display a relaxed substrate tolerance [16,99,113,131]. Both enzymes convert sugars and their derivatives that have a common (3 J )-OH configuration, but may deviate in... 

Iron-sulfur centers can participate in regulation mechanisms either directly, when they control the activity of an enzyme, or at a more integrated level, when they modulate the expression of some genes. The regulation mechanisms that have been elucidated so far involve either a change in the redox state or the interconversion of iron—sulfur centers. 

A typical mammafian cell possesses over 1000 phos-phorylated proteins and several hundred protein kinases and protein phosphatases that catalyze their interconversion. The ease of interconversion of enzymes between their phospho- and dephospho- forms in part... 

The citric acid cycle is the final common pathway for the aerobic oxidation of carbohydrate, lipid, and protein because glucose, fatty acids, and most amino acids are metabolized to acetyl-CoA or intermediates of the cycle. It also has a central role in gluconeogenesis, lipogenesis, and interconversion of amino acids. Many of these processes occur in most tissues, but the hver is the only tissue in which all occur to a significant extent. The repercussions are therefore profound when, for example, large numbers of hepatic cells are damaged as in acute hepatitis or replaced by connective tissue (as in cirrhosis). Very few, if any, genetic abnormalities of citric acid cycle enzymes have been reported such ab-normahties would be incompatible with life or normal development. 

Enzyme Reference Serums. Several companies sell lyophilized or stabilized reference serums for the calibration of instruments and for quality control. The label values given for the enzymatic activity of these serums should never be taken at face value, as at times they may be quite erroneous (19,33). Also, these values should only be used for the assay with which they were standardized, as interconversion of activity from one method to another for the same enzyme may often lead to marked errors. For instance, it is not recommended that alkaline phosphatase expressed in Bodansky units be multiplied by a factor to convert it to the units of the Ring-Armstrong method, or any other method for that matter. 

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