Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Types of reactions catalyzed by enzymes
Enolic intermediates in isomerization reactions
Other sugar phosphate isomerases

The enzyme mannose-6-phosphate isomerase, which catalyzes The conversion of mannose-6-phosphate into fructose-6-phosphate, is a Zn2+-containing monomeric protein with a Molecular Weight of ~ 45,000. Dimeric Triosephosphate isomerase (with a molecular weight of ≈52,000) catalyzes the interconversion of glyceraldehyde- and dihydroxyacetone phosphates and is considered the most active among the Enzymes involved in Glycolysis (Sec. A,5). Its molecular activity is ~ 2,800 s-1 for the reaction directed to the right in equation (7-55) and 250 s-1 for the reverse reaction (the predominant direction in METABOLISM). Although the high catalytic activity of this enzyme may facilitate the intramolecular transfer of the proton abstracted by the enzyme, such a transfer has rarely been observed. This indicates that a relatively weak base, such as a carboxylate ion, can serve as a proton acceptor at the C-3 position of dihydroxyacetone phosphate [135]. Various experiments with covalently attached labels also point to the involvement of a carboxyl group belonging to the Glu-165 residue [130].

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FIG. 7-9. Ring-opening and isomerization Reactions Catalyzed by glucose-6-phosphate isomerase (after Noltmann [130]).

Recently, the crystal Structure of chicken Muscle triosephosphate isomerase was determined by X-Ray Diffraction Analysis [136]. Approximately 22% of the 247 amino acid residues in each monomer form eight-stranded parallel ß-sheet regions. These sheets twist to form a barrel-like structure (Ch. 2, Sec. B,4). The ß-structures alternate with four a-helical regions comprising 55% of all amino acid residues. The structure of this enzyme somewhat resembles that of glyceraldehyde-3-phosphate dehydrogenase (Fig. 2-10).

Another important isomerase is the enzyme that interconverts ribose-5-phosphate and ribulose-5-phosphate.



Last update: 06/08/2026

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