Partial reduction in ribulose 1,5-bisphosphate carboxylase/oxygenase activity by carboxypeptidase A

Treatment with carboxypeptidase A of ribulose bisphosphate carboxylase/oxygenase (rubisco) from spinach and Chlamydomonas, but not tobacco, reduced activity by 60-70%. Further studies with the spinach enzyme indicated that only one amino acid from each of the large (valine) and small (tyrosine) subunits was removed and the loss of activity was correlated with modification of the large subunit. The modified enzyme also had a two-fold greater Km for RuBP but CO2/O2 specificity was only 5% lower and may not be significantly different. The relative rates of release of valine and tyrosine also depended on the presence or absence of RuBP or CO2 plus Mg during treatment. The results indicate that the C-terminal amino acid in the large subunit of spinach, which is not located near the active site region, plays a previously unrecognized role in determining the catalytic activity of the enzyme.