Ivan G. Darvey

Transient phase kinetics of enzyme reactions where more than one species of enzyme is present at the start of the reaction

Abstract This paper presents an extension of the analysis given by Darvey (1968) of the transient phase kinetics of a general enzyme mechanism to situations where more than one species of enzyme are present at the start of the reaction. Solutions are presented for the time course of the changes in concentration of all the enzyme and ligand (substrate, product, activator or inhibitor) species participating in the reaction during the transient phase. The conclusions which were reached for the restrictive initial condition (only one species of enzyme is present at the start of the reaction) are shown to also hold for the more general initial condition considered in this paper.

The relationship between steady-state kinetic parameters and equilibrium parameters for one substrate/one product enzymic reactions

Abstract This paper discusses two mechanisms for one substrate/one product enzymic reactions: Although these mechanisms can be distinguished by the use of product inhibition studies (Darvey, 1972), Haldane relationships cannot be used as a further criterion for discriminating between the two mechanisms. Equilibrium parameters are derived for mechanisms I and II in systems where the concentration of substrate and product bound to the enzyme is not negligible in comparison with the concentration of free substrate and product. Relationships between these parameters and the conventional steady-state kinetic parameters provide additional means for distinguishing the two mechanisms.

What factors are responsible for the greater yield of ATP per carbon atom when fatty acids are completely oxidised to CO2 and water compared with glucose

Some current textbooks of biochemistry present calculations which indicate that when palmitic or stearic acid is completely oxidised to CO2 and water, more molecules of ATP are produced per carbon atom than when glucose is similarly oxidised. This greater yield of ATP is not due to ATP produced as a result of the β-oxidative process, but rather due to the increased yield of acetyl-CoA molecules produced from fatty acids (3 acetyl-CoA molecules per 6 carbon atoms) compared with that produced from glucose (2 acetyl-CoA molecules per 6 carbon atoms).

Kinetics of isotope exchange at equilibrium for a one substrate-one product enzyme mechanism

It is shown that the general solution of Morales, Horovitz and Botts STAArchs. Biochem. Biophys.; 99: 258 (1962)! describing isotope exchange kinetics at equilibrium for the one substrate-one product enzyme mechanism: E + S in equilibrium X in equilibrium E + P, reduces to the steady-state solution normally applied to experimental data, under usual experimental conditions. The analysis provides a firmer theoretical basis for the equations generally used in analyzing kinetic data from isotope exchange experiments. (auth)

Is free ethanolamine required as a catalyst for the de novo biosynthesis of phosphatidylethanolamine and ethanolamine in mammals

Most authors of current textbooks of biochemistry avoid discussing the problem of the primary source of ethanolamine or phosphatidylethanolamine in mammals. In Chinese hamster ovary cells, serine can not only be exchanged for ethanolamine in phosphatidylethanolamine, but it can also be exchanged for choline in phosphatidylcholine. This exchange of serine for choline in phosphatidylcholine thus provides a means of synthesizing ethanolamine or phosphatidylethanolamine without requiring the initial presence of ethanolamine or phosphatidylethanolamine before any molecules of ethanolamine or phosphatidylethanolamine can be synthesized.

Amino acid titration curves - misshapen or mislabeled?

Titration curves are frequently used to illustrate the change in charge of amino acids with pH, and as such are a fundamental part of most biochemistry courses. We have noticed that the graphs of titration curves appearing in many biochemistry textbooks are drawn incorrectly near the origin of the titration. This article demonstrates that this curve is misshapen, or, alternatively, that the axis of the graph is incorrectly labelled.

The use of alternative substrates in steady-state enzyme kinetics: A re-examination of the criteria for distinguishing between sequential bi bi mechanisms

SummaryA new set of criteria based on the use of alternative substrates, is proposed for distinguishing between sequential bi bi enzyme mechanisms. Some of the criteria previously advocated are shown to be invalid.

One-substrate-one-product enzymic reactions: The relationship between isotope-exchange kinetic, steady-state kinetic and equilibrium parameters

Expressions are derived for the parameters that can be obtained from (1) steady-state kinetics, (2) isotope-exchange kinetics at equilibrium, and (3) equilibrium binding experiments for the following two one-substrate-one-product enzymic mechanisms: E + S ⇌ X 1 ⇌ X 2 ⇌ … X n ⇌ E + P ( mechanism I ) E 1 + S ⇌ X 1 ⇌ X 2 ⇌ … X n ⇌ E m + P E m ⇌ E m − 1 ⇌ X m − 2 ⇌ … E 2 ⇌ E 1 } ( mechanism II ) It is shown that the exchange constants for both mechanisms are identical with the reciprocals of the equilibrium parameters ( Darvey, 1973 ) that can be obtained from binding experiments. Therefore, expressions relating isotope exchange constants and steady-state kinetic parameters are analogous to those relating equilibrium parameters and steady-state kinetic parameters. The paper also presents, for mechanisms I and II, relationships between the maximum rate of isotope-exchange at equilibrium and the steady-state kinetic parameters. These relationships provide a new criterion for distinguishing experimentally between these two mechanisms.

How does the ratio of ATP yield from the complete oxidation of palmitic acid to that of glucose compare with the relative energy contents of fat and carbohydrate

The authors of many recent popular textbooks of biochemistry quote values for the ‘energy content’ of triacylglcyerol and dry carbohydrate on a weight basis as well as presenting calculations for the yield of ATP obtained when a molecule of glucose, or palmitic acid, is completely oxidised to CO2 and H2O. By extending these calculations and computing the yield of ATP in terms of the weight of glucose or palmitic acid oxidised to CO2 and H2O, it can be shown that the value for the ratio of the energy content of fat to that of carbohydrate is almost identical to the ratio of the yield of ATP per gram of palmitic acid oxidised, compared with that of glucose. Therefore, the efficiency (on a per gram basis) by which energy is made available as ATP is comparable for both the oxidation of fat and carbohydrate, thus underscoring the fact that the catabolic pathways for both fat and carbohydrate ultimately use the same means of generating ATP, namely, oxidative phosphorylation.

One substrate-one product enzymic reactions The determination of the velocity constants for an enzyme mechanism involving two conformational forms of enzyme and one enzyme intermediate

Abstract This paper examines the information that can be obtained about the magnitude of the velocity constants for the following enzyme mechanism from measurements of its steady-state kinetic parameters: Equations are presented which express the velocity constants for this mechanism in terms of steady-state kinetic parameters alone.