CONFORMATIONAL FLUCTUATIONS IN DEOXY HEMOGLOBIN REVEALED AS A MAJOR CONTRIBUTOR TO ANIONIC MODULATION OF FUNCTION THROUGH STUDIES OF THE OXYGENATION AND OXIDATION OF HEMOGLOBINS A(0) AND DEER LODGE BETA-2(NA2)HIS-]ARG

zOrganisms rely on regulation at the molecular level, such as the allo steric regulation of hemoglobin (Hb) function by anions, to meet chall enges presented by changing environmental and physiological conditions . A comparison of the effects of anions on oxygenation, oxidation, and sulfhydryl reactivity of Hb leads us to suggest that a large and sign ificant part of the shift in oxygen affinity brought about by anion bi nding occurs as a result of increased conformational rigidity of the T state of deoxy Hb. As conformational rigidity increases, it becomes i ncreasingly difficult for subunits in the deoxygenated T-state tetrame r to assume higher oxygen affinity forms (T', T '', T'''...) with less steric hindrance. The oxygen affinity reflects the average of the rap idly equilibrating conformations within the T state and is correspondi ngly decreased when anion levels are increased. The initial stage of t he oxidation of Mb is relatively insensitive to steric alterations and thus reflects, primarily, the electronic aspects of the quaternary (T , T', T '', T'''...) <-> R equilibrium, We show that the reactivity of the sterically obscured sulfhydryl of beta 93 Cys in deoxy Hb is much greater in chloride-free buffers than in buffers with added chloride. Anion-induced decreases in the extent and frequency of conformational fluctuations of subunits within the T-quaternary state thus reduce su lfhydryl reactivity as well as oxygen affinity. This parallel in anion ic control of function allowed us to test. and disprove, the possibili ty that uncompensated positive charges in the central cavity of Hb Dee r Lodge increase the frequency and extent of conformational fluctuatio ns in its deoxy structure. This Hb variant exhibits increased anion se nsitivity, increased oxygen affinity, and increased ease of oxidation, but without increased reactivity of its sulfhydryl groups, indicating that active-site alterations in deoxy Hb Deer Lodge are primarily ele ctronic and not associated with increased conformational fluctuations within its T state. The restoration of normal properties in Hb Deer Lo dge by addition of anions reinforces our conclusion that anionic contr ol can be exerted through both steric and electronic alterations. The anionic control of fluctuations within the T state of Hb, illustrates an important principle of macromolecular structure-function relationsh ips: that functional regulation can be achieved by alterations in conf ormational rigidity.