Hexokinases I—III

Four isoenzymes of hexokinases (types I-IV) constitute a family of enzymes that probably arose from a common ancestral gene by gene duplication and fusion events. Hexokinases I-III (M.W. 100,000) have a Km of about 0.1 mM and are allosterically inhibited by glucose-6-phosphate. Due to their low K, hexokinases I-III are saturated with the substrate glucose at... 

Third, hexokinase IV is not inhibited by glucose 6-phosphate, and it can therefore continue to operate when the accumulation of glucose 6-phosphate completely inhibits hexoldnases I III. 

Figure 5.3 Major control points of glycolysis and the TCA cycle. Enzymes I, hexokinase II, phosphofructokinase III, pyruvate kinase IV, pyruvate dehydrogenase V, citrate synthase VI, aconitase VII, isocitrate dehydrogenase VIII, a-oxoglutarate dehydrogenase.

Fig. 2. Galactosylation with galactosyltransferase and integrated cofactor regeneration. (Enzymes i phosphoglucomutase, ii UDP-glucose-pyrophosphorylase, iii inorganic pyrophosphorylase, ivUDP-galactose-4-epimerase, V galactosyltransferase, vi pyruvate kinase, vii hexokinase) [31]...

Enzymes i, hexokinase ii, pyruvate iii, transketolase iv, DAHP synthetase... 

Reagents i, Hexokinase ii, 2-deoxy-scyllo-inosose synthase iii, HI, HOAc... 

Mammalian hexokinase exists in four major isoen-zymic forms that can be separated by electrophoresis. The distribution pattern is the same in different tissues of rat, hamster, rabbit, monkey, and cow for types I, II, and III. Type IV has been found only in liver [33]. 

Tissues vary, however, in the relative amounts of the various isoenzymes present, and isoenzymes have been named after the tissues in which they predominate. Thus, type I is called brain hexokinase, type II muscle, and type IV liver. Type III seems to be... 

---