108 - The Cell 



properly localized in the protoplasmic struc- 

 ture—in the ribosomes or other parts of the 

 endoplasmic reticulum, in the Golgi appara- 

 tus, in the mitochondria, or other specialized 

 structures. Each endothermic reaction de- 

 pends upon a simultaneous exothermic reac- 

 tion, and these reactions must be coupled in 

 such a way that energy is transferred from 

 the exothermic to the endothermic process. 

 Such an energy coupling between metabolic 

 reactions depends in turn upon the integrity 

 of the protoplasmic structure. The coupled 

 reactions must occur synchronously and in 

 close proximity to each other; and these con- 

 ditions are usually realized only when the 

 whole team of enzymes is suitably arranged 

 in some specialized part of the intracellular 

 structure. In muscle, for example, a con- 

 tinued resynthesis of adenosine triphosphate, 

 an important organic component of the 

 tissue, must go on if the muscle is to retain 

 its contractile power; and this resynthesis is 

 coupled with the decomposition of phospho- 

 creatine, another component of the fibers (see 

 Chap. 24). Likewise the resynthesis of phos- 

 phocreatine is coupled with the breakdown 

 of glycogen in the muscle; and in turn, the 

 restoration of glycogen is coupled with the 

 oxidation of other substrates. In general, the 

 exothermic processes of the muscle, such as 

 glycolysis and oxidation, proceed readily after 

 the protoplasmic structure has been dis- 

 rupted, as by grinding the muscle; but the 

 synthetic reactions are fully achieved only in 

 the intact muscle. Despite the theoretical 

 capacity of enzymes to act reversibly, the con- 

 ditions necessary for reversal are not always 

 available, and many enzymes exert their in- 

 fluence in one direction only. Moreover, even 

 when a certain reaction is reversed in the 

 protoplasm, the reversal does not necessarily 

 involve the same enzyme that was utilized 

 initially. 



The importance of the fine structure of the 

 cell in determining the behavior of associated 

 enzymes is further emphasized by recent 

 studies on mitochondria. Each mitochondrion 

 represents a complex of six enzymes. This 



group as a whole accomplishes many reac- 

 tions that cannot be duplicated after the 

 mitochondria have been broken up com- 

 pletely, although some reactions may con- 

 tinue when the mitochondria are partially dis- 

 rupted. As will be explained later, the mito- 

 chondrial enzymes, acting as a team, appear 

 to play an important part in the oxidative 

 metabolism of a variety of compounds de- 

 rived not only from sugars, but also from 

 amino acids and fatty acids (see Krebs cycle, 

 p. 153). 



OTHER CHARACTERISTIC OF ENZYMES 



Enzymes tend to display a greater specific- 

 ity than their inorganic counterparts. For 

 example, three specifically different enzymes 

 are necessary to hydrolyze the three common 

 disaccharide sugars— sucrose, maltose, and 

 lactose (Table 5-1). Frequently an enzyme 

 acts only upon a single substrate, and even 

 less specific enzymes act only on substrates 

 with similar chemical structures. 



Undoubtedly the specificity of enzyme ac- 

 tion indicates that a molecular fit between 

 the substrate and the active site of the en- 

 zyme is an essential prerequisite lor catalysis. 

 Several such cases already have been cited 

 (aminopeptidase, carboxypeptidase, trypsin, 

 ami chymotrypsin) and two other examples 

 will suffice: (1) lactic dehydrogenase, which 

 acts upon only one type of molecular con- 

 figuration ( H — C — OH ) ; and (2) succinic 



dehydrogenase, which requires a different 



H H 



, I I 

 chemical structure! C C- 



I I 

 H H 

 shown in Table 5-i. 



The catalytic potency of enzymes is ex- 

 ceedingly great. A single molecule or cata- 

 lase, for example, proves able to handle the 

 decomposition of about 5 million molecules 

 of HoOo per minute even at a low tempera- 

 ture (0° C), which is required to make such 



