FIGURE 6.

Hypothetical catalytic mechanism of HI. A, His-mediated isomerization from (Z)-3-hexenal to (E)-2-hexenal catalyzed by HI. In this scheme γ-nitrogen of His is depicted as a representative example of the catalyst. B, inhibition of rCaHI activity by suicidal substrate, 3-hexyn-1-al. Numbers in parentheses indicate pmol of 3-hexyn-1-al and enzyme used for analysis, respectively. Activity without 3-hexyn-1-al was defined as 100% activity. Theoretically stoichiometric relationship between enzyme and suicidal substrate is indicated by a dotted line. C, suicidal substrate inhibitory mechanism of HI. Also in this scheme, γ-nitrogen of His is depicted as a representative example.