Glycogen Metabolism

Purpose of Starch + Glycogen degradation:

Obtaining glucose from storage (or diet)

α-Amylase is:

an endoglycosidase.

α-Amylase is present:

in saliva and pancreatic secretions which hydrolyzes the α1,4 linkage

α-Amylase's Function:

Cleaves dietary amylopectin or glycogen to maltose (disaccharide), maltotriose (trisaccharide) and other small oligosaccharides.

α-Amylase is active on either side of a branch point, but activity is reduced near the branch points (T/F).

True.

amylo-1,6-glucosidase:

debranching enzyme that breaks down α1,4 linkage

Debranching enzyme:

cleaves "limit dextrins"

What are the 2 activities of the debranching enzyme?

It transfers trisaccharide groups. And cleaves the remaining single glucose units from the main chain.

α-Amylase digestion leaves:

highly branched limit dextrins.

Debranching enzyme ____________ activity transfers:

glucanotransferase activity transfers a trisaccharide unit from one branch to the end of another.

Is Digestive breakdown of starch (and dietary glycogen) regulated or unregulated?

Digestive breakdown of starch (and dietary glycogen) is unregulated. Nearly 100% of ingested food is absorbed and metabolized.

Tissue glycogen

an important energy reservoir.

Are synthesis and degradation of storage glycogen regulated or unregulated?

Tightly regulated.

Glycogen consists:

of "granules" of high MW.

Glycogen phosphorylase cleaves:

one sugar unit from the end of a glycogen chain, and uses inorganic phosphate to phosphorylate the glucose. This avoids the use of ATP to phosphorylate glucose.

Limit dextrins are then degraded:

by the debranching enzyme. The catabolic and anabolic enzymes are present in glycogen granules

Glucose-1-phosphate can be converted:

to glucose-6-phosphate by phosphoglucomutase

This reaction is a phosphorolysis:

the glycosidic bond is split by phosphate and not H2O

ΔGo' is close to zero, but ΔG in vivo is -6 kJ/mol because:

of the high ratio of [Pi] to [glucose-1-phosphate].

In muscle:

enters glycolysis

In liver:

glucose-6-P is hydrolyzed to glucose for transport to other tissues.

Glycogen phosphorylase is a dimer of identical 842 residue subunits (T/F).

True.

Each subunit contains:

an active site (at the center of the subunit) and an allosteric effector site near the subunit interface

A regulatory phosphorylation site is located:

at Ser14 on each subunit

A glycogen-binding site exerts:

regulatory control

Each subunit contributes:

a "tower helix" (residues 262 to 278) to the subunit-subunit interface

In the dimer, the tower helices:

extend from their respective subunits and pack against each other

Muscle glycogen phosphorylase shows cooperativity:

in substrate binding

What are the allosteric inhibitors of glycogen phosphorylase?

ATP and Glucose-6-P

AMP is an ______ ______ of glycogen phosphorylase.

allosteric activator

When ATP and glucose-6-P are abundant,

glycogen breakdown is inhibited.

When cellular energy reserves are low:

(i.e., high [AMP] and low [ATP] and [G-6-P]) glycogen catabolism is stimulated

high energy status →

glycogen breakdown is inhibited

low energy status →

glycogen breakdown is stimulated

R State

An active form of the Glycogen Phosphorylase enzyme.

T State

The inactive form of the Glycogen Phosphorylase enzyme.

What promotes conversion to the active state?

AMP

What promotes conversion to the inactive state?

ATP, glucose-6-P, and caffeine

A significant conformation change occurs:

at the subunit interface between the T and R state

This conformational change at the interface:

is linked to a structural change at the active site that affects catalysis

Phosphorylation of serine-14:

converts the less active enzyme phosphorylase b to the more active phosphorylase a

Phosphorylase b

Inactive Version

Phosphorylase a

Active Version

What type of conformational change does Phosphorylation cause? And how?

Phosphorylation causes a large conformational change and converts the enzyme to a form in which it is much less sensitive to allosteric regulation.

The ____ form is less sensitive to allosteric regulation than the ____ form.

a form; b form

Phosphorylation converts the enzyme from:

a form that is allosterically regulated, to a form that is persistently active.

With Phosphorylase a, the equilibrium is:

shifted towards the R state. Thus the phosphorylated enzyme is more active, with no requirement for an allosteric activator.

Phosphorylation reduces:

the value of L ([T0]/[R0]) in the MWC model.

Phosphorylase Kinase that:

Phosphorylates glycogen phosphorylase is itself regulated by phosphorylation.

Glucagon secretion is:

inhibited by insulin.

In diabetics, glycogen is:

degraded, even when [glucose] is high.

Cyclic AMP:

Second messenger. Transduces the message of the hormone.

Adenylyl Cyclase:

is membrane associated. This mechanism amplifies the signal, because one hormone-receptor complex can activate many G proteins, and many cAMP molecules can be synthesized before the G protein dissociates.

cAMP activates:

cAMP-dependent protein kinase.

cAMP binding:

causes dissociation of the C subunit, which is the active kinase, which phosphorylates phosphorylase kinase. This is an example of enzyme regulation by binding to a regulatory protein.

Glycogen synthesis pathway is (same/different) than degradation.

different

Glucose is activated for glycogen synthesis by:

attachment to uridine diphosphate, to form the sugar nucleotide UDP-glucose.

UDP-glucose is formed from

glucose-1-phosphate.

UTP is catalyzed by

UDP-glucose pyrophosphorylase.

The mechanism of the UDP-glucose pyrophosphorylase reaction involves:

the attack by a phosphate oxygen of glucose-1-P on the α-phosphorus of UTP, followed by release of the pyrophosphate anion.

Hydrolysis of PPi provides:

the driving force for the UDP-glucose pyrophosphorylase reaction.

Glycogen Synthase:

catalyzes Formation of α(1→4) glycosidic bonds in Glycogen

Glycogenin

Single protein at the core which the large glycogen particle is built around.

The first glucose is linked to a _______ on the protein.

tyrosine -OH

Sugar units are then added by the action of:

glycogen synthase.

Glycogen synthase transfers:

glucosyl units from UDP-glucose to C-4 hydroxyl at a nonreducing end of a glycogen strand.

During the Glycogen Synthase step, what intermediate is formed?

An oxonium ion intermediate is formed.

Formation of glycogen branches is catalyzed:

by the branching enzyme.

How many residue segments are transferred? And where?

Six- or seven-residue segments of a growing glycogen chain are transferred to the C-6 hydroxyl group of a glucose residue on the same or a nearby chain.

Glycogen Metabolism regulation?

Glycogen metabolism is a highly regulated process, involving reciprocal control of glycogen phosphorylase and glycogen synthase.

GP allosterically activated by _______ and inhibited by _______, _______, and _______.

AMP; ATP, glucose-6-P and caffeine

Glycogen Synthase is stimulated by:

Glucose-6-P

Glycogen Synthase and Glycogen Phosphorylase are regulated:

covalent modification - phosphorylation.

Glycogen synthase is phosphorylated at:

multiple sites by protein kinases (including cAMP dependent protein kinase)

Phosphorylated glycogen synthase has a _______ activity and is _________ activated by _______ concentrations of glucose-6-phosphate.

Phosphorylated glycogen synthase has a (lower) activity and is (allosterically) activated by (high) concentrations of glucose-6-phosphate.

How many serine residues and protein kinases are involved in Glycogen Synthase regulation?

At least ___ serine residues are phosphorylated and ____ different protein kinases are involved. * 9;4

Phosphoprotein Phosphatase-1 (PP1)

Carry out Dephosphorylation.

What inactivates glycogen phosphorylase?

PP1

What activates Glycogen Synthase?

PP1

Dephosphorylated enzyme

Have a high activity and do not require glucose-6-phosphate for activity.

Phosphorylation has (similar/opposite) effects on glycogen phosphorylase (catabolic enzyme/anabolic enzyme) and glycogen synthase (catabolic enzyme/anabolic enzyme).

Phosphorylation has opposite effects on glycogen phosphorylase (the catabolic enzyme) and glycogen synthase (the anabolic enzyme).

Storage and utilization of tissue glycogen and other aspects of metabolism are regulated by:

hormones, including glucagon, epinephrine, and the glucocorticoids.

Insulin is released in response:

to increased blood glucose

Insulin triggers:

glycogen synthesis when blood glucose rises

Between meals, blood glucose is:

70-90 mg/dL

Glucose rises to ____ m/dL after a meal and then returns to normal within ____ hours.

150; 2-3

Insulin is secreted from:

the pancreas (to liver) in response to an increase in blood glucose

Insulin acts to lower blood glucose rapidly in several ways by:

stimulating glycogen synthesis and inhibiting glycogen breakdown

What stimulates glycogen breakdown?

Glucagon and epinephrine.

Binding of insulin to plasma membrane receptors in the _____ and ______ triggers:

the (liver) and (muscles) triggers protein kinase cascades that stimulate glycogen synthesis.

Effect of Insulin

Stimulation of lipid synthesis, glycogen synthesis, protein synthesis, glycolysis, and active transport, and inhibition of gluconeogenesis and lipid breakdown.

Glucose uptake provides:

substrate for glycogen synthesis and glucose-6-P, which allosterically activates the otherwise inactive form of glycogen synthase.

_________ and __________ use covalent control via cAMP dependent protein kinase.

Glucagon; Epinephrine

__________, __________, __________, and _________ are used in allosteric regulation.

Glc-6-Phosphate, ATP, Pi, AMP.

Phosphocreatine

A phosphorylated creatine molecule that serves as a rapidly mobilizable reserve of high-energy phosphates. It provides a short-term source of ATP in muscle (and brain), because it can phosphorylate ADP to make ATP.

Dietary supplementation with creatine increases:

muscle store of phosphocreatine and improves performance during brief intense exercise (but FDA recommends doctor's approval)

During intense exercise:

free ATP is depleted within seconds, phosphocreatine prolongs ATP availability for a few more seconds.

During brief intense exercise (100m sprint) energy sources are:

free ATP, phosphocreatine and anaerobic glycolysis.

Anaerobic glycolysis cannot persist for long:

because ATP and phosphocreatine are quickly used up, and anaerobic glycolysis would cause acidosis.

In longer term exercise (1000m):

glycogen breakdown and aerobic metabolism become important