Regulation in macromolecules as illustrated by haemoglobin

Abstract

Mηδέν ἄγαν—nothing in excess. These words of the old Greek saying might well find a place in modern Biology. The maintenance of order and balance—homeostasis, as the physiologist would have it—is an attribute of all living things. At a molecular level the study of control and regulation, involving the conjugate concepts of structure and function is a central theme of Biophysics. How is it that an enzyme, by the braking effect of its end products, is prevented from going too far, or, alternatively, by the accumulation of other metabolic substances in its environment, is brought into play? How is it that a working respiratory protein like haemoglobin, by the reciprocal action of its metabolic opposite, carbon dioxide, is aided in the taking up of oxygen in the lungs and the giving it off in the tissues? These are problems of linkage. From one point of view they may be thought of in terms of information— the communication between ligand and macromolecule and between one part of the macromolecule and another. From a different point of view they may be thought of in terms of the storage and transfer of energy, of the way in which the work of introducing ligand, the same or different, at another. The analysis may be pitched either at the phenomenological, or at the mechanistic, level; but whatever the approach it profits cosiderably from the introduction of the concept of the binding potential.