PubMed Health⌕ Search

Biomedical subjects

D Thusius

Publications and source records attributed to D Thusius.

11 recordsLinked to original sources
PubMed1977

On the analysis of chemical relaxation amplitudes.

The calculation of chemical relaxation amplitudes for a reaction mechanism involving an arbitrary number of thermodynamically independent steps is discussed. A formalism is developed which considerably reduces the computational labor required to derive expressions relating experimental amplitudes to thermodynamic functions and specific signal changes of elementary steps. The treatment includes the case of strong kinetic coupling between all reactions. The results are of particular interest in regard to the numerical fitting of amplitude data to an assumed mechanism.

Kinetics↗
PubMed1976

Mechanism of yeast cytochrome b2 action. I. Thermodynamics and relaxation kinetics of the interaction between cytochrome b2 and oxalate.

Oxalate is the strongest known inhibitor of yeast cytochrome b2 activity. We have used spectrophotometric titration, temperature-jump relaxation, and calorimetry in an investigation of the interaction between enzyme and inhibitor. The titration data are consistent with noncooperative binding to one site per subunit. This conclusion is corroborated by temperature-jump results which reveal a single relaxation phenomenon which obeys second-order kinetics. Further evidence for a simple binding reaction enthalpy estimated from relaxation amplitudes is in good agreement with the value obtained directly with batch calorimetry. The forward and reverse rate constants evaluated from the temperature-jump experiments are, respectively, 1 x 10(4) M-1 sec-1 and 15 sec-1. Although considerably smaller than a diffusion-controlled value, the forward rate constant is characterized by an unusually small activation energy of approximately 3 kcal/mol. This, together with a large unfavorable association activation entropy of -30 eu, suggests that oxalate diffuses freely to the active site, but only a small fraction of the collisions are productive due to severe steric requirements.

Binding Sites↗
PubMed1976

Mechanism of yeast cytochrome b2 action. II. Steady-state kinetics of oxalate inhibition.

From a careful steady-state kinetic study it is shown that the inhibition of L-lactate oxidation by cytochrome b2 with ferricyanide as acceptor is of the mixed competitive-noncompetitive type, indicating the formation of an active ternary complex between enzyme, substrate, and inhibitor. With a large excess of acceptor, the simplest formal mechanism consistent with all available data is: E + L equilibrium EL; E + S equilibrium ES leads to EP leads to E + P; ES + L equilibrium ESL leads to EPL leads to EL + P, where L is oxalate, S is L-lactate, P is pyruvate, and E is enzyme. The inhibition kinetics together with the rate constants for oxalate binding to free enzyme (Thusius, D., Blazy, B., and Baudras, A. (1976), Biochemistry, preceding paper in this issue) and recent steady-state experiments on L-lactate deuterated at C-2 (Lederer, F. (1974), Eur. J. Biochem, 46, 393) lead to estimates of some of the elementary rate parameters in the above scheme. As in the case of oxalate (see Thusius et al. reference above), the association rate constant for substrate binding (1.1 x 10(5) M-1 sec-1) is much smaller than a diffusion-controlled value. Our results also imply that dissociation of complex EP to free enzyme and pyruvate is partially rate limiting for the overall reaction.

Binding Sites↗
PubMed1975

The temperature jump relaxation technique.

The temperature jump relaxation technique is a convenient and general means of studying rapid reversible reactions of biological macromolecules. Recent advances in automatic data acquisition and the introduction of different optical detection systems will soon allow us to exploit the full potential of kinetic measurements near equilibrium. On the other hand, the kinetic approach can be quite limited if not accompanied by detailed structural and thermodynamic studies. Finally, we must accept the fact that one can rarely demonstrate a reaction mechanism to the exclusion of all plausible alternative models.

Electricity↗