ENZYMES 



While enzymes must be differentiated from noncatalytic proteins, 

 nonetheless a broader classification of proteins is conceivable in which 

 enzymes represent a special case of what we may call the functional 

 type of protein. We may define the functional protein as one which 

 performs a specific physiological function. Thus, catalase decom- 

 poses hydrogen peroxide; hemoglobin combines reversibly with mo- 

 lecular o.xygen; cytochrome C is reduced by the reduced forms of certain 

 enzymes and in turn its reduced form is oxidized by cytochrome 

 oxidase; prothrombin plays a specific role in blood clotting; visual 

 purple acts as a photoreceptor, etc. Limitation of space precludes 

 further development of the concept of functional proteins. Suffice 

 to say that, in the author's opinion, processes like those of blood co- 

 agulation, complement fixation, and antibody formation, are phe- 

 nomena which have much in common with enzymic phenomena, and 

 that the highly specific functional proteins responsible for these processes 

 have more in common with enzymes than with purely structural 

 proteins. The concept of functional proteins has the virtue of opening 

 new horizons in the form of novel types of proteins. Just as myosin is 

 a protein of muscle specialized to convert chemical energy to mechani- 

 cal energy or as visual purple is a protein in the retina specialized to 

 convert light energy, presumably, ultimately to the electrical energy 

 of nerve conduction, so there may be analogous proteins in nerve, 

 cellular membranes, etc. specialized to carry out the particular physio- 

 logical functions of these organs. The trend in biochemistry would 

 appear to be toward the inclusion of more and more proteins in the 

 category of catalysts. In fact, it is conceivable that eventually all 

 proteins apart from purely structural proteins will be found to perform 

 in a highly specific way some physiological catalysis, and the currently 

 prevalent idea of storage and inert proteins will soon be as outmoded 

 as the so-called endogenous nitrogen metabolism. 



There is another aspect of myosin and visual purple that well 

 merits consideration. Biochemists have long exercised themselves 

 over the problem of the means by which the organism converts energy 

 from one form to another. In one or two instances, the curtain sur- 

 rounding these interconversions has been pierced. Myosin and visual 

 purple may well be considered as examples of energy transformers. 

 Thus, myosin in effect converts the chemical energy of hydrolysis of 

 adenosine triphosphate into mechanical energy. The protein itself 



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