ENZYMES AND BIOLOGICAL ORGANIZATION DAVID E. GREEN, Ph.D. (Cantab.), and JOiIAN JÄRNEFELT, M.D., Ph.D. (Helsinki)* I. Individual Enzymes as Units ofEnzymatic Action One ofthe cornerstones of contemporary biochemical thinking is the concept of the individual enzyme as the unit of enzymatic action. According to this view, every biochemical reaction is catalyzed by a specific enzyme; and all multistep processes can be accounted for by the collaboration ofa group ofdiscrete, specific enzymes. Furthermore, the characteristics of any enzyme-catalyzed sequence can be predicted on the basis ofa detailed knowledge ofthe properties ofthe individual enzymes which are concerned in this process. Any number ofexamples appear to vindicate fully these working assumptions. The successful reconstruction of highly complex processes, such as glycolysis, fatty acid oxidation, purine synthesis, and the pentose and urea cycles, with combinations ofisolated enzymes can be evaluated as impressive support for the thesis that certain multienzyme processes are merely the sum of the individual enzymatic reactions. By contrast, this approach has utterly failed when applied to enzymatic processes which are intimately bound up with subcellular structure. Processes such as electron transport, oxidative phosphorylation, and photosynthesishave been found to be wedded to mitochondrial or chloroplast structure , and it has not been possible to deal with these processes by classical methods. The dichotomy between the systems which are readily capable of solubilization and reconstruction and the structure-bound systems for which such a procedure is not possible has been a source of considerable concern to biochemists. Ifthe concept of biological organization at the level ofenzymes is accepted as a general rule, this disturbing dichotomy automatically disap- * The Institute for Enzyme Research, University of Wisconsin, Madison, Wisconsin. I63 pears. The greater the stability and strength ofthe structure, the greater the difficulty ofconverting enzymes to their soluble forms. The properties ofthe structure determine why some enzymes are more readily brought into soluble form than others and why some multienzyme sequences are more difficult co reconstruct than others. II.Association ofOxidative Activity with Structure Otto Warburg as far back as 1913 (1) recognized that the oxygenreacting mechanism of aerobic cells was associated with subcellular particles . In the late 1920's, Keilin defined this relationship more precisely (2, 3). He provided evidence that aerobic cells contain an electron-transfer mechanism, the central part ofwhich consists ofa group ofparticle-bound cytochromes that transfer electrons originating from various metabolites to molecular oxygen. The entire mechanism—including the dehydrogenases which catalyze theprimarydehydrogenation ofthe substrate molecules as well as cytochrome oxidase, which catalyzes the terminal reaction with molecular oxygen—was found to be particle-bound, although parts ofthe chain, such as cytochrome c, could be extracted from the particles in soluble form. The full implications ofthe association ofa fundamental enzyme sequence with particles were certainly not grasped by Keilin's contemporaries. The one-by-one isolation ofsoluble enzymes was by then well under way, and no need was seen for invoking the concept of organization at the enzyme level. III.The Classical Approach to Enzyme Organization It may seem odd to the biologically oriented reader that such a negative attitude toward the concept oforganization at the enzyme level has been taken by many biochemists. The views expressed by M. Dixon and E. Webb in their monograph Enzymes (4) are typical ofthe views ofa not inconsiderable group of biochemists who regard the concept of enzyme organization as largely unnecessary. Referring to the mitochondrion, Dixon and Webb state (4): "We do not consider the idea of a rigid arrangement ofthe enzymes in sequence is necessary to explain the published experimental observations. It seems to us that all that is necessary for efficient working of the system is that the components should be held sufficiently near together to make the transit time small, either by confining them within a membrane, by adsorption on a surface, by association under the influence of electrostatic or other forces, or by combina164 D. E. Green andj.järnefelt · Enzymes and Biological Organization Perspectives in Biology and Medicine · Winter 1939 tion with a colloidal particle." The implication of this statement is that the enzymes ofthe mitochondrion are not organized; they are constrained within some sort ofnetwork or cage so that their interaction time is...