The use of specific lysine modifications to locate the reaction site of cytochrome c with sulfite oxidase

Abstract

The reduction of cytochrome c by beef liver sulfite oxidase was found to be strongly inhibited by high ionic strength, indicating the importance of electrostatic interactions to the reaction. The reaction rates of sulfite oxidase with singly trifluoroacetylated or trifluoromethylphenylcarbamylated cytochrome c derivatives were studied to determine the role of individual lysines in the reaction. The reaction rate was decreased by modification of the lysines immediately surrounding the heme crevice, the decreases following the order: Lys 13 greater than Lys 25 congruent to Lys 79 approximately equal to Lys 87 greater than Lys 8 approximately equal to Lys 27 approximately equal to Lys 72. Modification of lysines 22, 55, 88, 99, and 100 had no effect on the reaction rate. These results indicate that the interaction site on cytochrome c for sulfite oxidase is at the heme crevice region, and overlaps considerable with that for cytochrome oxidase.