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Biochemistry I Fall Term, 2004 November 19, 2004

Lecture 30: Glycogen Metabolism/Gluconeogenesis

Assigned reading in Campbell: Chapter 15.1-15.3 (& 21.4)

Key Terms:

Glycogen phosphorylase a & b

Glycogen synthase

UDP-glucose

Phosphorylation cascade

Gluconeogenesis

Fructose-2,6-bisphosphate (F2,6P)

Substrate cycling

Hormonal control:

Insulin, Glucagon, & Epinephrine

Adenyl cyclase & cAMP

Protein kinase A

Phosphorylase kinase

Phosphoprotein phosphatase

Links:

(I) Review Quiz on Lecture 30 concepts

(S) Muscle Glycogen Phosphorylase: Chime page showing the R and T state conformations.

Regulation of anabolic (synthesis) and catabolic (degradation) pathways

1. Synthetic and degradative pathways usually use the same enzyme for steps in whichG = 0.

2. Synthetic and degradative pathways always utilize different enzymes for steps in which G

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Glycogen Synthesis and Degradation

Glycogen synthesis is catalyzed by glycogen synthase.

Glycogen degradation is catalyzed by glycogen phosphorylase.

Phosphorolysis is cleavage of an ester by a phosphate group (instead of water).

The major regulation of these two pathways is by phosphorylation of the enzymes involved. The

phosphorylation changes the structure of the enzyme. Depending on the enzyme, either

activation or inhibition can occur. Consider phosphorylation as functionally equivalent to an

allosteric inhibitor/activator that has a very high affinity.

In the case of glycogen synthesis/degradation the regulatory signals are amplified by a

"phosphorylation cascade" produced by the activation of additional kinases/phosphatases that act

on the enzymes of glycogen synthesis/degradation.

Enzyme Active Form Inactive Form

Glycogen phosphorylase Ser/Thr-phosphate "a" Ser/Thr (no phosphate) "b"

Glycogen synthase Ser/Thr (no phosphate) Ser/Thr-phosphate

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Thus, phosphoryation leads to glycogen degradation, and dephosphorylation leads to glycogen

synthesis.

Gluconeogenesis

Gluconeogenesis, the formation of glucose from 3- and 4-carbon carboxylic acids, is essential for

the maintenance of constant blood glucose levels. The liver, and to a lesser extent the kidneys,

are the only organs that carry out this process. All of our other tissues and organs (especially the

brain) require this newly-synthesized glucose during periods of fasting, i.e. between meals and

during sleep.

All of the reversible reactions of glycolysis are used in gluconeogenesis, however, the three

irreversible steps require alternative reactions. These reactions are catalyzed and regulated by the

enzymes described here.

1. Pyruvate carboxylase

Carboxylation is coupled to ATP hydrolysis.Allosterically activated by acetyl coenzyme A.

Biotin is the covalently attached cofactor.

Found in the mitochondria.(where it also functions to provide oxaloacetate for the TCA cycle.)

The oxaloacetate formed by pyruvate carboxylase is converted to PEP in the cytosol (byPEP carboxykinase).

GTP is the phosphoryl donor.

CO2 is a product.

(The steps from PEP --> F-1,6-DP are a reversal of glycolysis.)

2. Fructose-1,6-bisphosphatase

Allosterically activated by ATP; inhibited by AMP. Inhibited by fructose-2,6-bisphosphate (F2,6P).

F2,6P is synthesized by [PFK-2]; phosphorylation inhibits.

F2,6P is hydrolyzed by [FBPase-2]; phosphorylation activates.Both of these enzyme activities reside on the same polypeptide and are sensitive to

phosphorylation.

3. Glucose-6-phosphatase

Hydrolyzes G-6-P to glucose + Pi.

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Overall, the "cost" of making glucose from pyruvate is two ATP equivalents. Although this

corresponds to the net yield from anaerobic glycolysis, the price is cheap when ATP is plentiful.

Regulation of PFK (glycolysis) by fructose-2,6-bisphosphate (F2,6P)

High glucose levels: increase in glycolysis Low glucose levels: decrease in glycolysis, increase in the synthesis of glucose. Fructose-2,6-bisphosphate activates PFK, thus increasing glycolysis.

When blood sugar is low, glucose must be mobilized from the liver stores; therefore glycolysis

must be inhibited and glucose must be produced from glycogen. Thus, protein kinases will be

active.

Phosphorylation of PFK-2/FBPase in the liver: inhibits PFK-2 activity and activates FBPase

activity. Thus, the levels of F2,6P decrease and glycolysis is inhibited.

F2,6P also activates the synthesis of glucose from pyruvate.

When blood sugar is high, glucose can be used to generate energy and NADH for other

metabolic pathways. In addition, the excess glucose is stored as glycogen. During this time

protein phosphatases are active:

Dephosphorylation of PFK-2/FBPase in the liver activates PFK-2 and inhibits FBPase. Thus, the

levels of F2,6P increase and glycolysis is activated.

Regulation of Biochemical Pathways

1. Change in levels of enzymes by regulation of their synthesis/degradation (slow)2. Change in the activity of enzymes by covalent modification (moderately fast)3. Change in the activity of enzymes by feedback inhibition (fast)4. Product inhibition (very fast)

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