glycogenesis & glycogenolysis

what is glycogen?

a homopolysaccharide that is a storage form in animals stored in the liver (6-8%) and muscle (1-2%) of glucose. the quantity is more in the muscle (250g) than liver (75g) due to higher muscle mass. it is store as granules in cytosol

how does glycogen differ from fat as a source of energy?

fat cannot be rapidly metabolized like glycogen. fat cannot generate energy in the absence of oxygen, such as anerobic glycolysis). brain requires a continuous supply of glucose, which comes from glycogen. fat cannot produce glucose like glycogen can

what is glycogenesis?

an anabolic metabolic process where excess glucose is converted glycogen for storage, primarily in liver and muscle, to provide an energy reserve for future use. it is the intracellular synthesis of glycogen from glucose

what is glycogenolysis?

a catabolic metabolic process where stored glycogen is broken down into glucose, it is glycogen degradation. it degrades glycogen to glucose-6-phosphate & glucose in the liver and muscles respectively

are glycogenolysis and glycogenesis reversal pathways of each other?

no, as each metabolic process has its own pathways and purposes not related to each other

where is the main site of glycogenesis?

the cytosol of the liver, forming 8-10% of its wet weight, and the muscle, forming 1-2% of its weight. most other cells may store minute amounts (very small quantities)

what are the sites of glycogenolysis?

liver, skeletal muscles, and its subcellular cells are cytosol

what is the substrate of glycogenolysis?

glycogen

what occurs during step 1 of glycogenesis?

glucose is converted to glucose-6-phosphate by enzyme glucokinase (in the liver) or hexokinase (in the muscles). it is an irreversible process, considered the preparatory step. it also consumes 1 ATP, and traps glucose in the cell, similar to the first step of glycolysis

what occurs during step 2 of glycogenesis?

glucose-6-phosphate (G6P) is converted to glucose-1-phosphate (G1P) by enzyme phosphoglucomutase. it is considered a reversible isomerization step

what occurs during step 3 of glycogenesis?

G1P, combined with UTP, is converted to UDP-glucose by UDP-glucose pyro phosphorylase. it releases pyrophosphate (PPi), which is hydrolyzed to drive the reaction forward. it is the activation step, and 1 UTP (equal to 1 ATP) is consumed

what occurs during step 4 of glycogenesis?

UDP-glucose combined with glycogen primer are catalyzed by glycogen synthase to form glycogen (elongated chain). it adds glucose via α-1,4 bonds. it is the regulatory step, and is a rate-limiting step

what occurs during step 5 of glycogenesis?

the formation of branches in glycogen occurs via the branching enzyme. amylo-1,4 → 1,6 transglucosylase. it creates α-1,6 linkages.

what are the steps for glycogen degradation?

1. action of glycogen phosphorylase

2. action of debranching e

what occurs during step 1 of glycogenolysis?

glycogen (n residues) is catalyzed by glycogen phosphorylase to form glucose-1-phosphate (G1P) and glycogen (n - 1 residues). it cleaves α-1,4 glycosidic bonds and requires Pi. it is the rate limiting step and is irreversible

what occurs during step 2 of glycogenolysis?

the transfer of 3 glucose units from a branch to a nearby chain by debranching enzyme 4-α-glucanotransferase. it prepares branch for cleavage by phosphorylase. it is also irreversible

what occurs during step 3 of glycogenolysis?

hydrolysis of α-1,6 branch point, which forms free glucose, catalyzed by debranching enzyme α-1,6-glucosidase. it produces one free glucose per branch and is the only step to release free glucose. it is also an irreversible step

what occurs during step 4 of glycogenolysis?

the reversible conversion of glucose-1-phosphate to glucose-6-phosphate. it is the only reversible step. G6P can enter glycolysis or be converted to glucose in the liver by G6Pase

what occurs during step 5 of glycogenolysis?

G6P is converted to glucose by enzyme glucose-6-phosphatase, ONLY in the liver and kidneys. it is irreversible and allows glucose to leave the cell

what is absent in the muscle?

glucose-6-phosphatase

what is the key enzyme of glycogenesis?

glycogen synthase, which is in step four

what is the key enzyme of glycogenolysis?

glycogen phosphorylase, which is in step one

what are the 3 regulatory mechanisms of glycogenesis and glycogenolysis?

1. allosteric regulation

2. hormonal regulation

3. influence regulation

what is the allosteric and influence / metabolic regulation of glycogenesis and glycogenolysis?

when substrate availability and energy levels are high, glycogenesis increases and subsequently, glycogenolysis decreases. when glycogen concentration is low and energy levels are low, glycogenolysis increases and glycogenesis decreases. when the body is in a well-fed state, G6P allosterically activates glycogen, and simultaneously, it allosterically inhibits glycogen phosphorylase, which is also allosterically inhibit by free glucose in the liver

what is the hormonal regulation of glycogenesis and glycogenolysis?

hormones control glycogen synthesis and degradation by covalent modification, i.e, by phosphorylation & dephosphorylation. cAMP acts as a second messenger, activating protein kinase, which causes phosphorylation of an enzyme, either activating (such as glycogen phosphorylase) or deactivating (such as glycogen synthase) it

what does insulin promote?

dephosphorylation, activating glycogen synthase and inhibiting glycogen phosphorylase

how does calcium affect glycogen metabolism?

when the muscle contracts, Ca2+ ions are released from sarcoplasmic reticulum, where Ca2+ ions bind to calmodulin (calcium binding protein), which directly activates protein kinase without the involvement of cAMP

when glycogenolysis increases and glucogenesis decreases, what are the subsequent reactions?

a decrease in glucose and insulin and an increase in Ca2+, glucagon and epinephrine (adrenaline)

when glycogenolysis decreases and glucogenesis increases, what are the subsequent reactions?

an increase in glucose and insulin, with a decrease in Ca2+, glucagon and epinephrine (adrenaline)

what is glycogenesis stimulated and inhibited by?

regulated by insulin and inhibited by glucagon and epinephrine

what is glycogenolysis stimulated and inhibited by?

stimulated by glucagon and epinephrine and inhibited by insulin

what are the differences between glycogenesis and glycogenolysis?

• glycogenesis stores glucose as glycogen, is stimulated by insulin, has a main enzyme of glycogen synthase and uses UTP

• glycogenolysis is the degradation of glycogen into glucose, stimulated by glucagon and epinephrine, has a main enzyme of glycogen phosphorylase and does not directly use ATP