376 2. ANALOGS OF ENZYME KEACTION COMPONENTS 



HEXOKINASES 



We shall now turn to various aspects of carbohydrate metabolism and 

 begin with those enzymes responsible for the initial phosphorylation of 

 sugars. Hexokinases catalyze the reaction between two types of substrate 

 — hexoses and ATP — and analogs of either may inhibit. Discussion of cer- 

 tain particularly important glucose analogs (2-deoxy-D-glucose, 6-deoxy-6- 

 fluoro-D-glucose, and related compounds), whose actions may involve not 

 only hexokinases but other early steps in carbohydrate metabolism, will 

 be postponed to the following section. The remaining analog inhibitors may 

 be classed as (A) hexoses, (B) hexose phosphates, (C) glucosamine and deri- 

 vatives, and (D) nucleotides and polyphosphates. It should be remembered 

 that the values of K„, and K^ may often be composite in that the sugar or 

 its derivative may occur in solution in a variety of forms (for example, a- 

 or /^-isomers, or pyranose or furanose rings). These forms may have quite 

 different activities and different individual constants. 



Inhibition by Hexoses 



Most hexokinases are not specific for the phosphorylation of one sugar 

 but act at varying rates with different hexoses. Thus yeast hexokinase 

 phosphorylates glucose {K„i = 0.16 mM), fructose {K„j = 1.7 mM), and 

 mannose (K,,, = 0.1 mM); brain hexokinase is very similar (Slein et al., 

 1950). If one active site on the enzyme is responsible, competition between 

 the substrates should be observed. At equimolar concentrations, glucose 

 and mannose almost completely inhibit the phosphorylation of fructose 

 (88-98%), whereas fructose inhibits the phosphorylation of the former 

 hexoses very little (15-18%), which would be expected on the basis of their 

 relative iiC,„'s. The calculated K/s are essentially the same as the J^,„'s. 

 The inhibition of Schistosoma fructokinase* by glucose and mannose is also 

 quite marked, whereas galactose is much less potent (Bueding and Mac- 

 Kinnon, 1955). The fructokinase of rat intestinal mucosa shows similar 

 specificity, the following inhibitions being observed with 6 niM inhibitor 

 when fructose is 10 mM: glucose (90%), mannose (80%), mannoheptulose 

 (65%), xylose (50%), allose (0%), and galactose (0%) (Sols, 1956). Ascites 

 cell glucokinase is inhibited rather well by talose (jK", = 3.2 mM) and altrose 

 {K^ = 6 mM), but not by allose, gulose, or idose (Lange and Kohn, 1961 b). 

 Yeast glucokinase is competitively inhibited by mannose {K^ = 0.11 mM) 

 (Fromm and Zewe, 1962). Mannoheptulose induces a temporary diabetic 

 state by blocking the uptake of glucose into the tissues through inhibition 

 of glucokinase (Coore and Randle, 1964). 



* The terms "glucokinase," "fructokinase," etc., will be used to designate kexo- 

 kinases when the corresponding substrates are used without implying that the en- 

 zymes are specific for these substrates. 



