KAPLAN_USMLE_STEP_1_LECTURE_NOTES_2018_BIOCHEMISTRY_and_GENETICS

Glycogen, Gluconeogenesis, and the	14
Hexose Monophosphate Shunt

Learning Objectives

Interpret scenarios about glycogenesis and glycogenolysis

Know how glycogen synthesis is regulated

Know how glycogenolysis is regulated

Know the glycogen storage diseases

Solve problems concerning gluconeogenesis

Use knowledge of hexose monophosphate shunt

GLYCOGENESIS AND GLYCOGENOLYSIS

Glycogen, a branched polymer of glucose, represents a storage form of glucose. Glycogen synthesis and degradation occur primarily in liver and skeletal muscle, although other tissues such as cardiac muscle and the kidney store smaller quantities.

Glycogen is stored in the cytoplasm as single granules (skeletal muscle) or as clusters of granules (liver). The granule has a central protein core with polyglucose chains radiating outward to form a sphere.

•	Glycogen granules composed entirely of linear chains have the highest	Figure I-14-1. Glycogen Granule
	density of glucose near the core.
•	If the chains are branched, glucose density is highest at the periphery
	of the granule, allowing more rapid release of glucose on demand.

Glycogen stored in the liver is a source of glucose mobilized during hypoglycemia. Muscle glycogen is stored as an energy reserve for muscle contraction. In white (fast-twitch) muscle fibers, the glucose is converted primarily to lactate, whereas in red (slow-twitch) muscle fibers, the glucose is completely oxidized.

GLYCOGEN SYNTHESIS

Synthesis of glycogen granules begins with a core protein glycogenin. Glucose addition to a granule begins with glucose 6-phosphate, which is converted to glucose 1-phosphate and activated to UDP-glucose for addition to the glycogen chain by glycogen synthase. Glycogen synthase is the rate-limiting enzyme of glycogen synthesis.

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Part I ● Biochemistry

Epinephrine

Insulin

(liver and muscle)

(liver

Glycogen

Glucagon

AMP

muscle)

Pi

(liver)

muscle

+

UDP

+

+ +

Glycogen

Glycogen

phosphorylase

synthase

(and debranching

(and branching

enzyme)

enzyme)

UDP-Glucose

PPi

UTP

Glucose 1-P

Glucose

Glucose 6-P

Glycolysis (ATP)

(muscle)

CO2+H2O

6-phosphatase

(liver)

Glucose

Pyruvate

Lactate

Figure I-14-2. Glycogen Metabolism

Glycogen Synthase

High-Yield

Glycogen synthase forms the α1,4 glycosidic bond found inMEDIUMthe linearYIELDglucose chains of the granule. Note the differences in the control of glycogen synthase in the liver and skeletal muscle.

Table I-14-1. Comparison of Glycogen Synthase in Liver and Muscle

	Glycogen Synthase			Liver			Skeletal Muscle
	Activated by		Insulin			Insulin
Inhibited by			Glucagon,			Epinephrine
			epinephrine

Branching Enzyme (Glycosyl α1,4: α1,6 Transferase)

Branching enzyme is responsible for introducing α1,6-linked branches into the granule as it grows. Branching enzyme:

•Hydrolyzes one of the α1,4 bonds to release a block of oligoglucose, which is then moved and added in a slightly different location

•Forms an α1,6 bond to create a branch

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Chapter 14 ● Glycogen, Gluconeogenesis, and the Hexose Monophosphate Shunt

α1,4 bond

Core

1.Glycogen synthase makes a linear α1,4-linked polyglucose chain ().

2.Branching enzyme hydrolyzes an α1,4 bond.

α1,6 bond

Core

3.Transfers the oligoglucose unit and attaches it with an α1,6 bond to create a branch.

4.Glycogen synthase extends both branches.

Figure I-14-3. Branching Enzyme

GLYCOGENOLYSIS

The rate-limiting enzyme of glycogenolysis is glycogen phosphorylase (in contrast to a hydrolase, a phosphorylase breaks bonds using Pi rather than H2O). The glucose 1-phosphate formed is converted to glucose 6-phosphate by the same mutase used in glycogen synthesis.

Glycogen Phosphorylase

High-Yield

Glycogen phosphorylase breaks α1,4 glycosidic bonds,MEDIUMreleasingYIELDglucose 1-phosphate from the periphery of the granule. Control of the enzyme in liver and muscle is compared below.

Table I-14-2. Comparison of Glycogen Phosphorylase in Liver and Muscle

	Glycogen Phosphorylase			Liver			Skeletal Muscle
	Activated by			Epinephrine		Epinephrine
				Glucagon		AMP
						Ca2+ (through calmodulin)
	Inhibited by			Insulin		Insulin
						ATP

Glycogen phosphorylase cannot break α1,6 bonds and thus stops when it nears the outermost branch points.

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Part I ● Biochemistry

α1,4 bond nearest the branch point

to core

1.Glycogen phosphorylase releases glucose 1-P from the periphery of the granule until it encounters the first branch points.

2.Debranching enzyme hydrolyzes the α1,4 bond nearest the branch point, as shown.

α1,6 bond

to core

3. Transfers the oligoglucose unit to the end of another chain, then

4. Hydrolyzes the α1,6 bond releasing the single glucose from the former branch.

Figure I-14-4. Debranching Enzyme

Debranching Enzyme (Glucosyl α1,4: α1,4 Transferase and α1,6 Glucosidase)

Debranching enzyme deconstructs the branches in glycogen that have been exposed by glycogen phosphorylase. This is a 2-step process. Debranching enzyme:

•Breaks an α1,4 bond adjacent to the branch point and moves the small oligoglucose chain released to the exposed end of the other chain

•Forms a new α1,4 bond

•Hydrolyzes the α1,6 bond, releasing the single residue at the branch point as free glucose; this represents the only free glucose produced directly in glycogenolysis

GENETIC DEFICIENCIES OF ENZYMES IN GLYCOGEN METABOLISM

Important genetic deficiencies are classed as glycogen storage diseases since all are characterized by an accumulation of glycogen.

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Chapter 14 ● Glycogen, Gluconeogenesis, and the Hexose Monophosphate Shunt

Table I-14-3. Glycogen Storage Diseases

	Type			Deficient Enzyme			Cardinal Clinical Features			Glycogen Structure
	I: von Gierke			Glucose-6-phosphatase			Severe hypoglycemia, lactic acidosis,			Normal
							hepatomegaly, hyperlipidemia,
							hyperuricemia, short stature, doll-like
							facies, protruding abdomen emaciated
							extremities
	II: Pompe			Lysosomal			Cardiomegaly, muscle weakness,			Glycogen-like material in
				α1, 4-glucosidase			death by 2 years			inclusion bodies
	III: Cori			Glycogen debranching			Mild hypoglycemia, liver enlargement			Short outer branches; single
				enzyme						glucose residue at outer branch
	IV: Andersen			Branching enzyme			Infantile hypotonia, cirrhosis, death			Very few branches, especially
							by 2 years			toward periphery
	V: McArdle			Muscle glycogen			Muscle cramps and weakness on			Normal
				phosphorylase			exercise, myoglobinuria
	VI: Hers			Hepatic glycogen			Mild fasting hypoglycemia,			Normal
				phosphorylase			hepatomegaly, cirrhosis

Glucose-6-Phosphatase Deficiency	High-Yield
(von Gierke Disease)	MEDIUM YIELD

Deficiency of hepatic glucose-6-phosphatase produces a profound fasting hypoglycemia, lactic acidosis, and hepatomegaly. Additional symptoms include:

•Glycogen deposits in the liver (glucose 6-P stimulates glycogen synthesis, and glycogenolysis is inhibited)

•Hyperuricemia predisposing to gout. Decreased Pi causes increased AMP, which is degraded to uric acid. Lactate slows uric acid excretion in the kidney.

•Hyperlipidemia with skin xanthomas

•Fatty liver

In a person with glucose-6-phosphatase deficiency, ingestion of galactose or fructose causes no increase in blood glucose, nor does administration of glucagon or epinephrine.

Lysosomal α1,4 Glucosidase Deficiency (Pompe Disease)

Pompe disease is different from the other diseases described here because the enzyme missing is not one in the normal process of glycogenolysis. The deficient enzyme normally resides in the lysosome and is responsible for digesting glycogen-like material accumulating in endosomes. In this respect, it is more similar to diseases like Tay-Sachs or even I-cell disease in which indigestible substrates accumulate in inclusion bodies.

In Pompe disease, the tissues most severely affected are those that normally have glycogen stores. With infantile onset, massive cardiomegaly is usually the cause of death, typically age <2.

Note

Glycogen storage diseases are the favorite biochemistry topic of the exam. Be sure to know:

•Clinical features

•Deficient enzyme

•Accumulating by-products

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Part I ● Biochemistry

A 12-month-old girl had slowly progressing muscle weakness involving her arms and legs and developed difficulty breathing. Her liver was enlarged, and a CT revealed cardiomegaly. A muscle biopsy showed muscle degeneration with many enlarged, prominent lysosomes filled with clusters of electron-dense granules. Her parents were told that without treatment, the child’s symptoms would continue to worsen and likely result in death in 1–2 years. Enzyme replacement therapy was initiated.

This child has a defect of the enzyme lysosomal α1,4 glucosidase (also called acid maltase). Coordinated glycogen breakdown with phosphorylase and debranching enzyme occurs in the cytoplasm. Although the α1,4 glucosidase participates in glycogen breakdown, the purpose of this enzyme and the reason for its location in the lysosome are unknown. Nevertheless, tissues that contain most of the body glycogen (liver and muscle) are severely affected in Pompe disease.

Recall Question

Which of the following activates the enzyme responsible for breaking alpha 1,4 glycosidic bonds?

A.ATP

B.AMP

C.Epinephrine

D.Glucagon

Answer: B

Myophosphorylase Deficiency

(McArdle Disease)

High-Yield

A 25-year-old woman had a lifelong history of exercise intolerance that was often accompanied by episodes of cramping. The episodes were somewhat ameliorated by drinking sucrose-rich soft drinks immediately before exercise. The latest episode occurred during her first spin class (stationary bicycling with a resistance load) at her local bicycle shop. She initially had extreme weakness in both legs and muscle cramps and later excreted red-brown urine. In subsequent sessions, in addition to the high-sucrose drink, she reduced the load on the bicycle and was better able to tolerate the initial phase of exercise. After 10–15 minutes, she experienced a “second wind” and was able to continue her exercise successfully.

Myophosphorylase is another name for the muscle glycogen phosphorylase.

Symptoms of myophosphorylase deficiency include exercise intolerance during the initial phase of intense exercise, muscle cramping, possible myoglobinuria, and recovery (or “second wind”) after 10–15 minutes of exercise.

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Chapter 14 ● Glycogen, Gluconeogenesis, and the Hexose Monophosphate Shunt

The woman in our vignette has myophosphorylase deficiency and is unable to properly break down glycogen to glucose 6-phosphate in her muscles. Without an adequate supply of glucose, sufficient energy via glycolysis for carrying out muscle contraction cannot be obtained, explaining why the muscles are not functioning well (weakness and cramps). The situation is improved by drinking the sucrosecontaining drink, which provides dietary glucose for the muscles to use.

Hepatic Glycogen Phosphorylase Deficiency (Hers Disease)

Hepatic glycogen phosphorylase deficiency is usually a mild disease because gluconeogenesis compensates for the lack of glycogenolysis. If present, hypoglycemia, hyperlipidemia, and hyperketosis are mild. Hepatomegaly and growth retardation may be present in early childhood, although hepatomegaly may improve with age.

GLUCONEOGENESIS

During fasting, the liver maintains glucose levels in blood through glycogenolysis or gluconeogenesis. These pathways are promoted by glucagon and epinephrine and inhibited by insulin. In fasting, glycogen reserves drop dramatically in the first 12 hours, during which time gluconeogenesis increases. After 24 hours, it represents the sole source of glucose.

Important substrates for gluconeogenesis are:

•Glycerol 3-phosphate (from triacylglycerol in adipose)

•Lactate (from anaerobic glycolysis)

•Gluconeogenic amino acids (protein from muscle)

Table I-14-4. Glucogenic and Ketogenic Amino Acids

	Ketogenic			Ketogenic and Glucogenic			Glucogenic
	Leucine		Phenylalanine			All others
	Lysine		Tyrosine
			Tryptophan
			Isoleucine
			Threonine

Dietary fructose and galactose can also be converted to glucose in the liver.

In humans, it is not possible to convert acetyl-CoA to glucose. Inasmuch as most fatty acids are metabolized solely to acetyl-CoA, they are not a major source of glucose either. One minor exception is odd-number carbon fatty acids (e.g., C17), which yield a small amount of propionyl-CoA that is gluconeogenic.

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Part I ● Biochemistry

Pi	Glucose

Glucose-6-phosphatase

Glucokinase

Glucose 6-P

Pi

Fructose 6-P

Fructose-1,6-bisphosphatase

PFK-1

Fructose 1,6-bis P

Most steps in this figure

NADH

NAD

represent a reversal of

Glyceraldehyde 3-P

DHAP

Glycerol 3-P

glycolysis, and several of

these have been omitted

3 reversible

here.

reactions

PEP carboxykinase

PEP

Pyruvate kinase

Alanine

GDP

CO2

GTP

NADH NAD

OAA

Cytoplasm

Pyruvate

Lactate

Malate shuttle

Mitochondria

ATP

Pyruvate

PDH

CO2

–

ADP

OAA

Pyruvate carboxylase

Acetyl-CoA

(biotin)

+

(from ß-oxidation

of fatty acids)

Figure I-14-5. Gluconeogenesis

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Chapter 14 ● Glycogen, Gluconeogenesis, and the Hexose Monophosphate Shunt

In the pathway of gluconeogenesis, lactate is oxidized to pyruvate by lactate dehydrogenase. The important gluconeogenic amino acid alanine is converted to pyruvate by alanine aminotransferase (ALT or GPT). Glycerol 3-phosphate is oxidized to dihydroxyacetone phosphate (DHAP) by glycerol 3-phosphate dehydrogenase.

The 4 important enzymes are those required to catalyze reactions that circumvent the irreversible steps:

•Pyruvate carboxylase is a mitochondrial enzyme requiring biotin. It is activated by acetyl-CoA (from β-oxidation). The product oxaloacetate (OAA), a citric acid cycle intermediate, cannot leave the mitochondria but is reduced to malate that can leave via the malate shuttle. In the cytoplasm, malate is reoxidized to OAA.

•Phosphoenolpyruvate carboxykinase (PEPCK) in the cytoplasm is induced by glucagon and cortisol. It converts OAA to phosphoenolpyruvate (PEP) in a reaction that requires GTP. PEP continues in the pathway to fructose 1,6-bisphosphate.

•Fructose-1,6-bisphosphatase in the cytoplasm is a key control point of gluconeogenesis. It hydrolyzes phosphate from fructose 1,6-bisphos- phate rather than using it to generate ATP from ADP. A common pattern to note is that phosphatases oppose kinases. Fructose- 1,6-bisphosphatase is activated by ATP and inhibited by AMP and fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate, produced by PFK-2, controls both gluconeogenesis and glycolysis (in the liver). Recall that PFK-2 is activated by insulin and inhibited by glucagon. Thus, glucagon will lower F 2,6-BP and stimulate gluconeogenesis, whereas insulin will increase F 2,6-BP and inhibit gluconeogenesis.

•Glucose-6-phosphatase is in the lumen of the endoplasmic reticulum. Glucose 6-phosphate is transported into the ER, and free glucose is transported back into the cytoplasm from which it leaves the cell. Glucose-6-phosphatase is only in the liver. The absence of glucose- 6-phosphatase in skeletal muscle accounts for the fact that muscle glycogen cannot serve as a source of blood glucose (see Chapter 17).

Although alanine is the major gluconeogenic amino acid, 18 of the 20 (all but leucine and lysine) are also gluconeogenic. Most of these are converted by individual pathways to citric acid cycle intermediates, then to malate, following the same path from there to glucose.

It is important to note that glucose produced by hepatic gluconeogenesis does not represent an energy source for the liver. Gluconeogenesis requires expenditure of ATP that is provided by β-oxidation of fatty acids.

Therefore, hepatic gluconeogenesis is always dependent on β-oxidation of fatty acids in the liver. During hypoglycemia, adipose tissue releases these fatty acids by breaking down triglyceride.

Although the acetyl-CoA from fatty acids cannot be converted to glucose, it can be converted to ketone bodies as an alternative fuel for cells, including the brain. Chronic hypoglycemia is thus often accompanied physiologically by an increase in ketone bodies.

Note

Biotin deficiency is caused by raw egg whites (avidin) or long-term home TPN. Symptoms include:

•Alopecia

•Scaly dermatitis

•Waxy pallor

•Acidosis (mild)

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