ABSTRACT
A set of variant human hemoglobins, each with an Ala or Gly substitution at a single residue, has been prepared, and the kinetics of their reactions with carbon monoxide have been measured. This reaction is rate-limited by the binding of the first CO to the deoxygenated T state of the protein. The magnitudes of the effects of the mutations on CO combination vary widely, and, with the exception of beta Y145, the residues with the most significant effects on these kinetics are found in the hinge region of the alpha 1 beta 2 interface. Mixed-metal hybrids, with zinc protoporphyrin IX in place of heme on both alpha or both beta subunits, were prepared for beta W37E, beta W37A, alpha Y140G, and alpha Y140A, hinge region variants causing large kinetic changes, and for beta Y145G. Such hybrids permit measurements of the kinetics of CO binding to only the heme-containing alpha or beta subunits within the unliganded hemoglobin tetramer. Mutations at beta 37 and alpha 140 have global effects on the T state, increasing the rates of CO binding to both types of subunits. Mutation of beta Y145 has a large effect on the beta subunits in the deoxygenated T state, but very little effect on the alpha subunits. Oxygen equilibria measurements on the crystalline T state of beta W37E also indicate large affinity increases in both subunits of this variant. The overall oxygen equilibria of the variant hemoglobins in solution are sensitive to numerous variables besides the properties of the deoxygenated T state. In contrast to CO combination kinetics, the residues whose alterations cause the largest changes in overall oxygen equilibria in solution are scattered seemingly randomly within the alpha 1 beta 2 interface.
Subject(s)
Hemoglobins/chemistry , Hemoglobins/genetics , Mutation , Amino Acid Substitution , Carbon Monoxide/metabolism , Dimerization , Globins/chemistry , Globins/genetics , Globins/metabolism , Hemoglobin A/chemistry , Hemoglobin A/genetics , Hemoglobin A/metabolism , Hemoglobins/metabolism , Humans , In Vitro Techniques , Iron/chemistry , Kinetics , Models, Molecular , Mutagenesis, Site-Directed , Oxygen/metabolism , Protein Structure, Quaternary , Protein Subunits , Protoporphyrins/chemistry , Recombinant Proteins/chemistry , Recombinant Proteins/genetics , Recombinant Proteins/metabolism , Zinc/chemistryABSTRACT
Unlike the situation with the vas deferens of rats, guinea-pigs, rabbits and dogs, neither phentolamine nor yohimbine enhanced electrically-evoked release of [3H]norepinephrine from the in vitro human vas deferens. However, clonidine produced, by a phentolamine-sensitive mechanism, a concentration-related inhibition of release in the human vas deferens. The results indicate that, even though presynaptic alpha-adrenoceptors exist, a negative feedback regulation of release by norepinephrine, which occurs in the vas deferens of the other species, may not be functionally important in the vas deferens of the human.
