152 F. Dickens, G. E. Clock and P. McLean 



wider choice of carbohydrate pathways (Gunsalus, Horecker 

 and Wood, 1955), will not be considered in this account. A 

 brief introductory sketch of the main relevant points in each 

 of these pathways will first be given before proceeding to 

 details. As shown in Fig. 1, besides the entry of glucose into 

 the cell, its initial phosphorylation is a primary requirement; 

 the possible pathway via glucose dehydrogenase and further 

 oxidation of the unphosphorylated gluconic acid so formed 

 does not seem to be important in the animal organism 

 (Stetten and Stetten, 1950; Salmony and Whitehead, 1954). 

 The phosphorylation of glucose by the hexokinase reaction 

 requires magnesium ions and consumes 1 equivalent of ATP. 

 The two hexose monophosphate stages which follow thus 

 represent the first key-points for divergence of metabolic 

 pathways, and these stages are therefore "pacemakers" in 

 the nomenclature of Krebs and Kornberg (1957). 



The glycolytic and Krebs cycle pathways 



The main glycolytic or Embden-Meyerhof (E-M) route is 

 too well known to need comment, except to note that adeno- 

 sine triphosphate (1 ATP) and magnesium ions are required 

 for the stage of departure from the phosphogluconate path- 

 way at the phosphofructokinase step and that 2 ATP per 

 mole of hexose are regenerated in each of the "substrate- 

 linked" phosphorylations: (a) D-GA3P dehydrogenase (which 

 requires 2 moles of inorganic phosphate and in animal tissues 

 is quite specific for DPN), and (b) 2 ATP produced from the 

 PEP which results from enolase action on 2-phosphoglycerate. 

 The reduced DPN formed in (a) must be reoxidized, either 

 by lactic dehydrogenase plus pyruvate or, aerobically, via the 

 cytochrome system, the latter involving the flavoprotein 

 DPNH-cytochrome c reductase and cytochrome oxidase 

 systems. The E-M pathway, besides being historically the 

 first, is also the major pathway of carbohydrate breakdown 

 and the main route leading to energy production in almost all 

 animal tissues. It also provides a route via its reversible 



