106 

 The Vraax/Km ratio reflects catalytic efficiency of enzymes 

 that catalyze the same reaction in cells. SP-I and SP-II 

 have the same Vmax, but different Kms, therefore, the 

 Vmax/Km ratio suggests that SP-II with the lower Km , is 

 catalytically more efficient than SP-I. 



PCMB, a potent inhibitor of SDH activity in plant 

 species (48, 53) inhibited both bifunctional proteins, 

 implying that the presence of sulfhydryl groups are critical 

 for catalytic activity of the proteins. This inhibition can 

 be prevented by the addition of thiol reagents, such as DTT, 

 which did, in fact, protect the enzyme from inhibition by 

 PCMB. Most interesting is that PCMB activated activity of 

 the DQT functional domain. My interpretation is that this 

 may reflect physical overlapping of catalytic sites on the 

 protein. Protocatechuic acid, also a known inhibitor of 

 SDH activity (53) inhibited SDH activity of both proteins, 

 but had no effect on the DQT functional domain. PCA is the 

 product of the quinate catabolic system, and its 

 physiological significance as an inhibitor of SDH is 

 unclear. 



The pH and temperature optima did not indicate any 

 differences between the two S-protein, but were different 

 for the functional domains of the two proteins. The SDH 

 functional domain of the bifunctional proteins had the 

 higher pH and temperature optima. Thermal inactivation 

 studies of the two bifunctional protein showed that the SDH 



