STRUCTURE AND FUNCTION OF CHLOROPLAST PROTEINS .6. FURTHER STUDIES ON PCMB-TREATMENT OF SPINACH LEAF RUDP CARBOXYLASE

The inhibitory effect of PCMB on spinach leaf RuDP carboxylase was studied by both electron microscope examination and kinetic analyses. The prior treatment of the enzyme protein with RuDP as well as NaHCO3 plus Mg++ exerted a potent protective effect against the PCMB inhibition. From the results of the electron microscope studies, it was shown that the partial loss of the enzyme activity up to 50% was not definable at the ultrastructural level. But the complete loss of the enzyme activity paralleled the disorganization of the oligomeric structure of the enzyme molecule, and a prolonged contact of the enzyme protein with PCMB at high concentration produced aggregated particles to a marked degree. A simultaneous kinetic analysis of the PCMB inhibition of RuDP carboxylase showed that in the lower concentration range the inhibitory effect was of the uncompetitive type. By elevating the concentration of PCMB, however, the inhibition ceases to be uncompetitive and tends to become mixed type. The overall results suggest that SH-groups are primarily engaged in the structural organization of the RuDP carboxylase molecule. © 1968.