Briefly describe the biological rationale for each of the following allosteric phenomena:
(a) Activation of pyruvate carboxylase by acetyl-CoA;
(b) Activation of pyruvate dehydrogenase kinase by NADH;
(c) Inhibition of isocitrate dehydrogenase by NADH;
(d) Activation of isocitrate dehydrogenase by ADP;
(e) Inhibition of a-ketoglutarate dehydrogenase by succinyl-CoA;
(f) Activation of pyruvate dehydrogenase phosphatase by Ca2+.