Biosynthesis of porphyrins and related macrocycles. Part 21. The interaction of deaminase and its product (hydroxymethylbilane) and the relationship between deaminase and cosynthetase

Abstract

Experiments are described which show that (a) deaminase is inhibited by its own product, the linear hydroxymethylbilane; (b) this inhibition of uptake of porphobilinogen (PBG) is competitive, affecting K_M but not V_max.; (c) addition of synthetic hydroxymethylbilane to deaminase sets up the covalently bound linear tetrapyrrolic system on the enzyme; (d) it is the binding of the next PBG unit after construction of the linear tetrapyrrole which causes release of hydroxymethylbilane from deaminase; (e) the linear tetrapyrrole bound to deaminase is only slowly removed in the presence of a large excess of cosynthetase; (f) when the first PBG unit is taken up by deaminase it is covalently bound to the enzyme with release of one mole of ammonia; (g) the tentative conclusion is reached that this covalent binding occurs via the ε-amino group of lysine; (h) deaminase has no effect on the rate of formation of uro'gen-III from hydroxymethylbilane by cosynthetase.