Abstract
Two isoforms of phosphoglucomutase (PGM, EC 2.7.5.1) designated as PGM-I and PGM-II were purified from developing seeds of Brassica campestris L to their electrophoretic homogeneity. Both the forms had molecular mass of 59 kD each and were monomeric. PGM-I exhibited maximum activity at pH 7.5, while PGM-II evinced pH optima at 8.25. Both the forms exhibited hyperbolic response towards increasing concentrations of the substrate with Km values of 0.10 mM for PGM-I and 0.12 mM for PGM-II and had absolute requirement for glucose-1,6-P2. Fructose-1,6-P2 and 2,3-diphosphoglyceric acid inhibited the two forms non-competitively, whereas, ribulose-1,5-P2 inhibited only PGM-II, with Ki value of 0.8 mM. ATP inhibited the enzyme uncompetitively with Ki values for 0.26 mM (PGM-I) and 0.12 mM (PGM-11). Use of group specific protease inhibitors indicated Ser, His and Cys to play significant role in catalysis. On the basis of their differential behaviour and kinetic properties, PGM-I and PGM-II may be the forms present in cytosol and leucoplasts, respectively.
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