Glycogen Metabolism Study Material - Key Terms and Definitions

Debranching enzyme has two activities:

One transfers trisaccharide groups

Another one cleaves the remaining single glucose units from the main chain

Glycogeneolysis

Breakdown of glycogen to glucose or glucose 6-phosphate

Glycogenesis

Synthesis of glycogen

Where are glycogenolysis and glycogenesis most important?

Muscle and liver

Gluconeogenesis

Formation of glucose from nonhexose precursors

Where does Gluconeogenesis occur?

All animals, plants, fungi, and microorganisms

In a well fed state, the glucose is provided by...

Dietary carbohydrate

In the well-fed state, what accumulates in the liver?

Glycogen

Once exogenous glucose is exhausted, what maintains blood glucose levels?

Hepatic glycogenolysis

The store of liver glycogen lasts how long during fasting?

12-24 hours

What allows blood glucose to be maintained long after all dietary glucose has been absorbed and completely oxidized?

Gluconeogenesis

Glycogen granules are abundant where during a well fed state? Are they seen in 24-hour fasted animals?

Abundant in the liver in well fed animal, virtually absent in 24-hr fasted animals

During heavy exercise, are glycogen granules present in muscle?

No

In a well-fed human, liver glycogen content can account for how much of the wet weight of the organ?

10%

The muscle stores _______ glycogen than the liver when glycogen is expressed on the same basis. Muscle stores _____% of its wet weight from glycogen

Less, 1-2%

Does the average person have more muscle or more liver? Is there more muscle glycogen or more liver glycogen in the average person?

You have more muscle

There is about twice as much total muscle glycogen as liver glycogen

Glycogen is composed of ________ residues, which are linked in what way?

Glucosyl residues linked by a-1,4-glycosidic linkages

Branches in glycogen are attached through what linkages?

A-1,6-glycosidic linkages

Glycogen's branched structure composed of only one real beginning (reducing end) and many branches terminating with no reducing glucosyl units gives numerous sites of attack for _____________. The multiple sites also function as ________ for the addition of __________ by ___________

Glycogen phosphorylase, primers, glucosyl units, glycogen synthase

To initiate glycogen synthesis, _________ is converted to _________ by _____________

Glucose 6-phosphate, glucose 1-phosphate, phosphoglucomutase

Does phosphoglucomutase function in only glycogen synthesis?

NO

It catalyzes a near equilibrium reaction under intracellular conditions, so it can function in both glycogen degradation and synthesis

What must be present for this reaction to occur?

A bisphosphate compound as an intermediate

Glucose 1,6-bisphosphate

Glucose 6-phosphate to glucose 1,6-bisphosphate is catalyzed by

Phosphoglucokinase

UDP-glucose is called...

Activated glucose - it has a large negative free energy of hydrolysis, used to build the glycogen molecule

What is UDP-glucose formed by?

Glucose 1-phosphate uridylyltransferase

Formation of UDP glucose is...

Energetically favorable and irreversible

What does glycogen synthase do?

Transfers the activated glucosyl moiety of UDP-glucose to a glycogen molecule to form a new glycosidic bond between C1 of the activated sugar and C4 of the glucosyl residue of the growing chain

The reducing end of glucose is always added to what end of the glycogen chain?

Nonreducing

UDP formed as a product of glycogen synthase is converted back to _______ by ____________

UTP, nucleoside diphosphate kinase

__________ functions as a primer for glycogen synthesis

Glycogenin

Glycogenin is what type of enzyme?

Self-glucosylating

Glycogenin uses ________ to link glucose to one of its own __________ residues

UDP-glucose, tyrosine

Where is Glycogenin found in glycogen?

Buried within the molecule, covalently attached to the single reducing end of the glycogen molecule

Does glycogen synthase create the chains of a1-4 glycosidic linkages in glycogen molecules or the a1-6 glycosidic branches found in glycogen?

Only the a1-4 chains, not the branches

What enzyme catalyzes the branching of glycogen?

1-4 a-glucan branching enzyme

How does (1-4)a-Glucan branching enzyme work?

It removes a block of about 7 glucosyl residues from a growing chain in glycogen and transfers it to another chain to produce a1-6 linkage

The creation of a highly branched structure of glycogen requires the concerted efforts of...

Glycogen synthase and branching enzyme

What does glycogen phosphorylase do?

Causes phosphorolysis of glycogen

Pi is used in the cleavage of an a1-4 glycosidic linkage to yield glucose 1-phosphate

Where does phosphorolysis of glycogen occur?

Nonreducing end of glycogen molecule

Amylase

Degrades glycogen and starch in the gut

How does a-amylase work?

Simple hydrolysis

How does the way a-amylase works differ from glycogen phosphorylase?

A-amylase uses simple hydrolysis, using water

Glycogen phosphorylase uses an inorganic phosphate to cleave a1-4 glycosidic bonds

Glycogen phosphorylase stops attacking a1-4 glycosidic linkages how many residues from an a1-6 branch point?

4

A glycogen molecule that has been degraded by phosphorylase to the limit caused by the branches is called...

Phosphorylase-limit dextrin

What is needed to continue degrading glycogen with glycogen phosphorylase?

Debranching enzyme

What does the debranching enzyme do?

1. cleaves 3/4 glucosyl residues of a branch and transfers them to the end of a glucosyl residue on the glycogen chain

2. hydrolyzes the remaining single glucosyl residue

Glycogen serves as a fuel reserve for the synthesis of ________ within ________

ATP, muscle

Liver glycogen functions as a ___________ reserve

Glucose

When are liver glycogen levels high and low?

High- after intaking food

Decreases slowly as it is mobilized to help maintain a nearly constant blood glucose level

Glycogen synthase and glycogen phosphorylase are ________ regulated

Reciprocally

An increase in cAMP signals activation of _____________ and an inactivation of _____________

Glycogen phosphorylase, glycogen synthase

Glycogen phosphorylase occurs in two forms:

a: catalytically active form

b: catalytically inactive form

How is phosphorylase b converted to a (active)?

phosphorylase kinase

What does adenylate cyclase cause formation of?

cAMP

cAMP activates

PKA

PKA turns on

phosphorylase kinase

Phosphorylase kinase

Converts inactive phosphorylase to active phosphorylase

Phosphoprotein phosphatase 1 (PP1)

removes phosphoryl groups from phosphorylase a, converting it to the less active form, phosphorylase b