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The stereochemical course of phosphoryl transfer catalysed by glucose 6-phosphatase.

ABSTRACT: Rat liver microsomal glucose 6-phosphatase catalyses phosphoryl transfer between D-glucose 6-[(R)-16O,17O,18O]phosphate and D-glucose with retention of configuration at the phosphorus atom. Since individual phosphoryl-transfer steps appear in general to occur with inversion of configuration, this observation is most simply interpreted in terms of a double-displacement mechanism with a phosphoryl-enzyme intermediate. Such an intermediate has been proposed previously from kinetic and 32P-labelling experiments.

SUBMITTER: Lowe G

PROVIDER: S-EPMC1163697 | biostudies-other | 1982 Mar

REPOSITORIES: biostudies-other

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The stereochemical course of phosphoryl transfer catalysed by glucokinase.

Project description:Adenosine 5'-[gamma(S)-16O,17O,18O]triphosphate has been used to determine the stereo-chemical course of phosphoryl transfer catalysed by rat liver glucokinase. The chirality of the product, D-glucose 6-[16O,17O,18O]phosphate was analysed by 31P n.m.r. spectroscopy. The reaction proceeds with inversion of configuration at phosphorus. The simplest interpretation of this result, which is the same as that observed with yeast hexokinase [Lowe & Potter (1981) Biochem. J. 199, 277-233], is that the phosphoryl group is transferred between MgATP2- and glucose in the ternary complex by an 'in-line' mechanism. It accords with the veiw that the kinetic differences between glucokinase and the other hexokinases arise from differences in rate constants and not from any fundamental differences in chemical mechanism.

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The stereochemical course of reactions catalysed by the cellobiohydrolases produced by Talaromyces emersonii.

Project description:Three forms of exocellobiohydrolase (EC 3.2.1.91), CBH IA, CBH IB and CBH II, were isolated to apparent homogeneity from culture filtrates of the aerobic fungus Talaromyces emersonii. CBH IA and CBH II appear to be native forms of these enzymes, while CBH IB may represent a proteolytic degradation product of the CBH IA enzyme. The hydrolysis of beta-cellobiosyl fluoride by each form was monitored by 1H-n.m.r. spectroscopy. The reactions catalysed by CBH IA and CBH IB proceed with retention of the anomeric configuration, whereas that catalysed by CBH II is one of inversion. Thus one may deduce that CBH IA (or CBH IB) and CBH II operate double and single displacement reactions respectively during catalysis of substrate. On the basis of these findings and the observed substrate specificities of the various forms, one may conclude that CBH IA (and CBH IB) is a family C enzyme, while CBH II belongs to family B [Henrissat, Claeyssens, Tomme, Lemesle & Mornon (1989) Gene 81, 83-95].

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