glycogen and pentose phosphate pathway Flashcards
fates of glucose
glycolysis, glyogen synthesis/storage or pentose phosphate pathway to generate NADPH and ribose sugars. The later 2 are active when glucose is present in excess of what is needed for glycolysis
Describe the structure of glycogen and why this is important
highly branched polymer of glucose monomers. Found primarily in liver and muscle
what controls glycogen breakdown
insulin and glucagon
major sites of glycogen storage
liver and skeletal muscle- glycogen in muscle must be released as lactate and synthesized into glucose by liver (cori cycle) b/c muscle lacks glucose-6-phosphatase
what is the building block of glycogen and how is it synthesized
uridine diphosphate (UDP)-glucose - glucose-6-phosphate > glucose-1-phosphate > UDP-glucose.
glycogen synthase function
transfers UDP-glucose to growing glycogen chain- UDP-glucose is transferred to hydroxyl group of glycogen to form alpha 1,4 glycosidic linkage. UDP is displaced and released. Only adds glucose residues if polysaccharide chain haas more than 4 glucose residues.
What forms he initiating site for glycogen synthesis
glycogenin
glycogen branching enzyme function
After glycogen synthase has added 11 glucose residues, branching enzyme transfers 6 or 7 glucose residues from a chain to an internal site to form α-1,6-linkages that produces a branch point in the growing polymer of glucose molecules. The new branch point must be at least 4 residues away from a pre-existing branch point
Result of abnormal glycogen branching
glycogen breakdown is slower which can result in hypoglycemia during fasting or reduced exercise tolerance.
steps in glycogen degradation
- release of glucose-1-phosphate from glycogen. 2. remodeling of the remaining glycogen to permit further degradation. 3. conversion of glucose-1-phosphate into glucose-6-phosphate for further metabolism or export from the cell.
glycogen phosphorylase function
catalyzes the cleavage of glycogen to glucose 1-phosphate. Key regulated enzyme in glycogenolysis. Stops when it reaches 4 residues away from alpha-1,6-glycosidic bond branch point
debranching enzyme function
shifts a block of 3 glycosyl residues from one outer branch to the other, converting the branched structure into linear structure so that glycogen phosphorylase can continue
Function of glucagon or epinephrine in glycogen synthesis/breakdown
glucagon or epi activate protein kinase A > phosphorylates phosphorylase kinase b converting it to phosphorylase kinase a (active) > this phosphorylates glycogen phosphorylase b (inactive) to the a form (active) > glycogen phosphorylase a begins glycogen breakdown
function of insulin in glycogen regulation
insulin causes de-phosphorylation of glycogen phosphorylase (via phosphoprotein phosphatase 1, PP1) and of phosphorylase kinase (via protein phosphatase), ultimately causing glycogen synthesis
liver and skeletal muscles role in glycogen synthesis and degradation
In muscle, glycolysis and the TCA cycle/electron transport supplies ATP, and the rate of these pathways increase as the muscle works harder. The liver’s role is to maintain a constant level of glucose by producing glucose when other tissues demand it, and storing it when it is provided in excess by the diet.
