Glycogen metabolism
Glycogen metabolism
The metabolic path for Glycogen starts with alfa-D-Glucose that isphosphorylated by a set of enzymes: Hexokinase 1 ( HXK1 ) [1], [2], [3], Hexokinase 2 ( HXK2 ) [4], [5],[6], Hexokinase 3 ( HXK3 ) [7], [6], [8], Glucokinase (hexokinase 4) ( HXK4 ) [9], [10],[11], [12], [13] andalpha-(D)-Glucose-6-phosphate is formed. Also, alpha-D-Glucose isisomerically transformed into beta-D-Glucose by Galactose mutarotase (aldose1-epimerase) ( GALM ) [14], [15], [16]. Beta-D-Glucose undergoes the same transformations that alpha-D-glucose underthe action of the same enzymes to form beta-(D)-Glucose-6-phosphate whichsubsequently takes another stereoisomerisation step toalpha-(D)-Glucose-6-phosphate. This step is catalyzed by Glucose phosphateisomerase ( GPI ) [17]. Phosphoglucomutase 1 ( PGMU ) catalyzesthe next isomerization of alpha-(D)-Glucose-6-phosphate leading to formation ofalpha-(D)-Glucose 1-phosphate [18], [19]. UDP isthen attached to alpha-(D)-Glucose 1-phosphate by UDP-glucose pyrophosphorylase 1( UGPA1 ) or UDP-glucose pyrophosphorylase 2 ( UGPA2 ) to formUDP-D-glucose [20], [21], [22], [23].
UDP-D-glucose polymerizes with (alpha-D-glucosyl-(1-4))n to formGlycogen. This step is catalyzed by Glucan (1,4-alpha-), branching enzyme 1 (GLGB ) [24], [25], [26], [27]. Onthe other hand, Glycogen synthase 1 (muscle) ( GYS1 ) and Glycogen synthase 2(liver) ( GYS2 ) can cleave UDP-D-glucose from Glycogen to release(alpha-D-glucosyl-(1-4))n [28], [29], [30],[31], [32]. A number of glycogen phosphorylases (Phosphorylase,glycogen, liver ( PYGL ), Phosphorylase, glycogen, brain ( PYGB ) andPhosphorylase, glycogen, muscle ( PYGM )) mediate phosphorolysis ofGlycogen, which is regarded as transfer of one sugar residue (alpha-D-glucose1-phosphate) from Glycogen to inorganic phosphate. As a result Glycogenchain decreased for one sugar moiety with formation of a reduced product(alpha-D-glucosyl-(1-4))(,n=3)-alpha-D-glucosyl-(1-6)*(glycogen). [33], [34], [35], [36], [37], [38]. Thisproduct is a substrate for Amylo-1, 6-glucosidase, 4-alpha-glucanotransferase - glycogendebranching enzyme ( GDE ) that fractionates the polymeric structure to form twometabolites: the main chain - alpha-D-glucosyl-(1-6)*(glycogen) and branch chainof Glycogen - (alpha-D-glucosyl-(1-4))(,n=3) [39], [40],[41], [42], [43]. GDE also affects both productsof the latter reaction. Alpha-D-glucosyl-(1-4))(,n=3) can be directlyrepolymerized to Glycogen; alpha-D-glucosyl-(1-6)*(glycogen) can befurther hydrolyzed giving alpha-D-Glucose as a product.[40], [41], [43].
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