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J Wyman

Publications and source records attributed to J Wyman.

At least 73 records · Page 4Linked to original sources
PubMed1974

A probabilistic approach to cooperativity of ligand binding by a polyvalent molecule.

Cooperativity of ligand binding is defined operationally in terms of the Hill parameter n, whose values uniquely define the shape of the binding curve. When n is everywhere equal to 1 the system behaves as if the sites were all the same and independent. If at any point n is unequal to 1 the system is either cooperative (n > 1) or anticooperative (n < 1) at that point. This means that the binding of ligand at any site is influenced by the presence or absence of ligand at other sites. In the present paper this influence is formulated in terms of probabilities of the various binding events and of conditional probabilities which relate them. The result is a network of linked probabilities which determine the cooperativity or anticooperativity of the system. This probabilistic formulation supplements the more usual one in terms of free energies and places the subject in a broader perspective.

Binding Sites↗
PubMed1973

Heme proteins: quantum yield determined by the pulse method.

We report results of the application of the "pulse method" to the study of photodissociation of various ligands from several heme proteins. By use of this technique, which allows the determination of the quantum yield of photodissociation accurately and rapidly, several ligands (CO, O(2), isocyanides) have been investigated for sperm-whale myoglobin and trout hemoglobin I. In agreement with previous results, the new data lead to the conclusion that no simple relationship exists between the quantum yield and the affinity constant in the ground state. For trout hemoglobin I, the experiments were extended to measure the quantum yield of the CO photodissociation as a function of the initial degree of saturation, from fully saturated down to the initial values of about 1.5%. The results yield additional information to that obtained by the "steady-state" method, and in particular exclude the idea that the photochemical yield is in any way dependent on the fractional saturation of the molecule with carbon monoxide.

Animals↗
PubMed1973

Heme proteins: effect of an intermediate on photochemical behavior.

This paper analyzes, in terms of a triangular kinetic scheme, the possible effects of an intermediate in the photochemical behavior of simple heme proteins. Both steady-state and transient phenomena are analyzed. The magnitude of the quantum yield for a given system is determined by a competition between quenching and ligand detachment, measured by the two rate constants, kappa(-1) and kappa(2), respectively; as a first approximation, it can be described by a single parameter, namely the ratio kappa(-1)/kappa(2). The individual values of kappa(-1) and kappa(2) can only be determined at light intensities so high that the measured quantum yield becomes itself a function of light intensity. Under these conditions the relaxation time for transient approach to the steady-state can be complex, corresponding to heavily damped chemical oscillations. It is pointed out that when the model is extended to include more than one intermediate, multiple oscillations are possible. In this more general case the system in the steady-state may also show pseudocooperativity or anticooperativity.

Binding Sites↗
PubMed1972

Carbon monoxide binding by hemoglobin and myoglobin under photodissociating conditions.

Carbon monoxide binding by myoglobin and hemoglobin has been studied under conditions of constant illumination. For hemoglobin, the homotropic heme-heme interaction (cooperativity) and the heterotropic Bohr effect are invariant with light intensity over a 1000-fold change of c((1/2)). The dissociation constant, measured as c((1/2)), increases linearly with light intensity, indicating that photodissociation is a one-quantum process. At sufficiently high illumination the apparent enthalpy of ligand binding becomes positive, although in the absence of light it is known to be negative. This finding indicates that light acts primarily by increasing the "off" constants by an additive factor. The invariance of both cooperativity and Bohr effect raises a perplexing issue. It would appear to demand either that the "off" constants for the various elementary steps are all alike (which is contrary to current ideas) or that the additive factor is in each case proportional to the particular "off" constant to which it is added (a seemingly improbable alternative).

Animals↗
PubMed1972

Partial restoration of normal functional properties in carboxypeptidase A-digested hemoglobin.

In the absence of organic phosphates human hemoglobin A digested with carboxypeptidase A (des His, Tyr beta) has high ligand affinity, a greatly reduced Bohr effect, and no heme-heme interaction. Under these conditions, it shows the simple, homogeneous ligand-binding kinetics characteristic of noncooperative heme proteins in which the high combination velocity for both O(2) and CO accounts, to a larger extent, for the increased affinity for both these ligands. Addition of inositol hexaphosphate dramatically alters the functional properties of this digested hemoglobin. The Bohr effect is greatly increased, and at neutral pH the protein shows significant, though still reduced, heme-heme interaction, together with a 5-fold decrease in affinity. In the presence of saturating amounts of the organic phosphate, the value of n is pH dependent, dropping from 1.9 at pH 5.8 to 1.3 at pH 8.6. After inositol hexaphosphate addition, the combination of the deoxy form of the digested hemoglobin with CO is 10-times slower than that observed in the absence of the inorganic phosphate; also the combination with CO after flash photolysis is biphasic and is similar, in many respects, to that observed for unmodified hemoglobin. Besides these functional changes, addition of inositol hexaphosphate to the modified deoxyhemoglobin results in an increase in the extinction coefficient at 430 nm similar to that observed on mixing the isolated alpha and beta chains of normal hemoglobin. The results are consistent with the idea that inositol hexaphosphate shifts an equilibrium between high- and low-affinity forms of the protein.

Carbon Monoxide↗