Catalytic groups in relation to the structure of hexokinase.

Hexokinase offers considerable promise for the study of phosphoryl transfer, since the three-dimensional structure of this enzyme (from yeast) is available from the work of T. A. Steitz and co-workers (Steitz et al., 1977; Anderson et al., 1978a.b; Bennett & Steitz, 1978; Anderson et al., 1979), there is a considerable body of kinetic and chemical evidence on the catalytic pathway, and a large conformational change is known to be involved to bring the substrate ligand, glucose, into the required relationship with the phosphorylating ligand, ATP (Bennett & Steitz, 1978. 1980; McDonald et al., 1979; Anderson et al., 1979). Yeast hexokinase offers advantages for these studies since from this source the enzyme is abundant and is readily purified in bulk (Rustum et al., 1971; Barnard, 1975). It is purified thus in chemically homogeneous form as two related isoenzymes, A and B, which differ in amino acid sequence and in quantitative specificity (Barnard. 1975). Each can exist in an association equilibrium as a dimer or monomer of a 5 1 OOO molecular weight subunit (Derechin et al., 1972, 1977). Hexokinase B is the more active of the two isoenzymes and was used in our studies, as an electrophoretically pure preparation, with specific activity on glucose in standard conditions (Barnard, 1975) at 25OC of 750units/mg.