Abstract
1. The substrate specificity of the enzyme protochlorophyllide reductase in barley (Hordeum vulgare) etioplasts was investigated. 2. It was shown that naturally occurring esterified protochlorophyllide and chemically prepared protochlorophyllide methyl ester are not substrates for the enzyme, suggesting an important role for the C-7 carboxylic acid group in binding of the porphyrin to the enzyme. 3. Removal of magnesium from the protochlorophyllide leads to inactivity of the compound as a substrate for the enzyme. However, activity can be restored by replacing the magnesium with zinc, whereas nickel, copper or cobalt failed to restore substrate activity. 4. Binding of the second substrate, NADPH, to the enzyme probably occurs through the 2'-phosphate group in the coenzyme.