Characterization and Kinetics of Isoenzymes of Pyruvate Kinase from Developing Castor Bean Endosperm 1

AUTOR(ES)

Ireland, Robert J.

RESUMO

Isozymes of pyruvate kinase (PK) have been isolated from developing castor bean endosperm. One isozyme, PKc, is localized in the cytosol, and the other, PKp, is in the plastid. Both isozymes need monovalent and divalent cations for activity, requirements which can be filled by K+ and Mg2+. Both isozymes are inhibited by citrate, pyruvate, and ATP. PKc has a much broader pH profile than PKp and is also more stable. Both have the same Km (0.05 millimolar) for PEP, but PKp has a 10-fold higher Km (0.3 millimolar) for ADP than PKc (0.03 millimolar). PKc also has a higher affinity for alternate nucleotide substrates than PKp. The two isozymes have different kinetic mechanisms. Both have an ordered sequential mechanism and bind phosphoenolpyruvate before ADP. However, the plastid isozyme releases ATP first, whereas pyruvate is the first product released from the cytosolic enzyme. The properties of the two isozymes are similar to those of their counterparts in green tissue.

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