David R. Goddard

The Mechanism of Enzymatic Oxidations and Reductions

E investigations of the last two decades have added greatly to our knowledge of the chemical nature of the enzymes of oxidation-reduction and their physiological roles in cellular metabolism. This subject has been reviewed elsewhere by Oppenheimer and Stern (1939), Green (1940), Sumner and Somers (1947), Kalckar (1941), and Goddard (1945), and will be only incidentally dealt with in this article. This paper is an attempt to present a unified treatment of the mechanism of a diversity of enzymatic oxidation-reductions and the pertinent kinetic relationships. A few basic assumptions are used as the foundation for the treatment: 1. The enzymes of oxidation-reduction are conjugated proteins with a prosthetic group or simple proteins acting with organic coenzymes. 2. Every enzyme of oxidation-reduction has two substrates, an electron donor and an electron acceptor. 3. The enzymes are active partners in the mechanism of the reactions, but the cyclical nature of the reactions returns the enzyme to its original state. That is, they are true catalysts. 4. An enzyme may itself be a substrate for another enzyme. 5. A trimolecular complex is formed between the enzyme, the acceptor, and the donor, in two bimolecular reactions. 6. Electron transfer in univalent steps constitutes the oxidation, and hydrogen transfer occurs through the solvent by ionic association and dissociation. 7. The catalysis occurs within the acceptorenzyme-donor ternary complex by univalent electron transfer. This results in the formation of free radicals (or semiquinones) of enzyme-donor, enzyme-acceptor, or acceptorenzyme-donor. We consider it improbable that free radicals in the solvent play an appreciable role in cellular metabolism. 8. The concepts of oxygen activation and chain reactions are not only unnecessary but are impossible, if the high specificity which enzymes show is to be retained. 9. The kinetics of enzymatic reactions can be explained in terms of the rates of formation of the binary complexes and perhaps of the ternary complex. Many of these assumptions are not original with us, but we do not know of any treatment where they have all been made and where the conclusions have been drawn which follow from these assumptions. After attempting to justify these assumptions, we shall use them as a basis for a kinetic treatment

Studies on the Metabolism of Plant Neoplasms I. Oxygen Uptake of Tomato Crown-Gall Tissues

1. A comparative manometric time-course study was made, from the seedling stage until flowering, of the oxygen uptake of the hypocotyl of the normal tomato plant (Lycopersicon esculentum) and of the same organ inoculated with Agrobacterium tumefaciens. 2. The oxygen uptake of the normal and inoculated hypocotyl of tomato plants grown at two levels of nitrate-nitrogen nutrition also was measured manometrically, and the total nitrogen contents of these organs, sampled at the time of one of the manometric tests, were determined. 3. Calculated on a basis of fresh weight, the rates of oxygen uptake of slices of crown-gall tissues were at all times greater than those of slices of the control tissues. 4. The rates of oxygen uptake by both control and tumorous tissues, especially of the latter, appear to be in part functions of the stage of development of the tissues at time of slicing. 5. It is indicated that in comparative studies of the oxygen uptake of crown-gall and normal tissues it is not permissible to use the apparently non-affected parts of the tumorous organ as a control. 6. Differences in the total nitrogen percentage of tumorous and control hypocotyl tissues grown with adequate exogenous supplies of nitrogen compared with the nitrogen content of equivalent tissues grown under -N conditions are not reflected in the rate of oxygen uptake calculated on a fresh-weight base. 7. Total nitrogen does not appear to be a more adequate basis than fresh weight for calculating the rates of oxygen uptake of crown-gall and control tissues.