FIGURE 1: Feedback inhibition of enzymes.

(A) Classical regulation: An end-product (Z) of a biosynthesis pathway inhibits activity of an enzyme (a) catalyzing the first irreversible step (commitment step).

(B) The polyamines spermidine (C) and spermine (Z) regulate the activity of ornithine decarboxylase (ODC; a) by inducing the biosynthesis of the ODC-inhibitor antizyme (i) and preventing its degradation. Binding of antizyme to ODC (symbolized by ia) inhibits ODC activity and induces its degradation; a novel finding is that polyamines directly promote antizyme-mediated degradation of ODC (highlighted by red color).