Chapter 25 Flashcards
what is UDP-glucose?
(uridine diphosphate glucose)
activated form of glucose
the glucose donor in glycogen synthesis
what synthesizes UDP-glucose?
by UDP-glucose pyrophosphorylase
what renders the synthesis of UDP-glucose irreversible?
the hydrolysis of pyrophosphate
how is UDP-glucose synthesized?
UDP-glycose pyrophosphorylase hydrolyzes UTP and puts it in glucose
what drives the synthesis of UDP-glucose reaction to the right?
the cleavage of PPi
what does glycogen synthase do in the reaction of Glucose to UDP-glucose?
glycogen synthase catalyzes the transfer of glucose from UDP-glucose to a growing chain
what does glycogen synthase do in the synthesis of glycogen?
it transfers a glucose moiety from UDP-glucose to the C-4 terminal residue of a glycogen chain to form an α-1,4-glycosidic bond
what does glycogen synthase require as a primer?
an oligosaccharide of glucose residues
what synthesizes the primer needed by glycogen synthase?
glycogenin
what is glycogenin?
a dimer of two identical subunits
what does each subunit of glycogenin do?
each subunit generates an oligosaccharide of glucose residues 10-20 molecules long
what does glycogen synthase do to the primer?
it extends the primer
glycogen synthase can only synthesize what kind of linkages?
α-1,4-linkages
what does a branching enzyme do?
it generates branches allowing glycogen synthase to extend the branch polymer
how does a branching enzyme create branches?
it generates branches by cleaving an α-1,4-linkage and taking a block of approximately 7 glucoses and synthesizing an α-1,6-linkage,
what does the branching of an enzyme allow?
increase insolubility
increases ends for synthesis and degradation
when is glycogen synthase inactive?
when in the phosphorylated b form
when is glycogen synthase active?
in the unphosphorylated a form
what is the key regulatory process of glycogen synthesis?
the conversion of the b form in the T state to the active R state by binding glucose 6-phosphate
what is the difference between phosphorylation of glycogen synthase and glycogen phosphorylase?
it has opposite effects
glycogen phosphorylase is phosphorylated and active in a form and unphopshorylated in b form
glycogen synthase is phosphorylated in b form and unphosphorylated in a form
Quick quiz: why is the fact that phosphorylation has opposite effects on glycogen synthesis and breakdown advantageous?