KAPLAN_USMLE_STEP_1_LECTURE_NOTES_2018_BIOCHEMISTRY_and_GENETICS

Part I ● Biochemistry

Coordinate Regulation of Pyruvate Carboxylase and Pyruvate Dehydrogenase by Acetyl-CoA

The 2 major mitochondrial enzymes which use pyruvate, pyruvate carboxylase and pyruvate dehydrogenase are both regulated by acetyl-CoA. This control is important in these contexts:

•Between meals, when fatty acids are oxidized in the liver for energy, accumulating acetyl-CoA activates pyruvate carboxylase and gluconeogenesis and inhibits PDH, thus preventing conversion of lactate and alanine to acetyl-CoA.

•In the well-fed, absorptive state (insulin), accumulating acetyl-CoA is shuttled into the cytoplasm for fatty acid synthesis. OAA is necessary for this transport, and acetyl-CoA can stimulate its formation from pyruvate (see Chapter 15, Figure I-15-1).

Cori Cycle and Alanine Cycle

During fasting, lactate from red blood cells (and possibly exercising skeletal muscle) is converted in the liver to glucose that can be returned to the red blood cell or muscle. This is called the Cori cycle. The alanine cycle is a slightly different version of the Cori cycle, in which muscle releases alanine, delivering both a gluconeogenic substrate (pyruvate) and an amino group for urea synthesis.

Alcoholism and Hypoglycemia

High-Yield

Alcoholics are very susceptible to hypoglycemia. In addition to poor nutrition and the fact that alcohol is metabolized to acetate (acetyl-CoA), the high amounts of cytoplasmic NADH formed by alcohol dehydrogenase and acetaldehyde dehydrogenase interfere with gluconeogenesis. High NADH favors the formation of:

•Lactate from pyruvate

•Malate from OAA in the cytoplasm

•Glycerol 3-phosphate from DHAP

The effect is to divert important gluconeogenic substrates from entering the pathway.

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

Alcohol

NAD

Alcohol dehydrogenase

NADH

Acetaldehyde

NAD

Acetaldehyde dehydrogenase

NADH

Acetate

Figure I-14-6. Alcohol Metabolism

Accumulation of cytoplasmic NADH and glycerol 3-P may also contribute to lipid accumulation in alcoholic liver disease. Free fatty acids released from adipose in part enter the liver where β-oxidation is very slow (high NADH). In the presence of high glycerol 3-P, fatty acids are inappropriately stored in the liver as triglyceride.

Extreme Exercise and Alcohol Consumption

Immediately after completing a 26-mile marathon race, a healthy 24-year-old man was extremely dehydrated and thirsty. He quickly consumed a 6-pack of ice-cold beer and shortly thereafter became very weak and light-headed and nearly fainted. He complained of muscle cramping and pain.

Although the effect of alcohol is unrelated to the hormonal control of gluconeogenesis, excessive consumption of alcohol can cause severe hypoglycemia after running a marathon. In exercising muscle, lactic acid builds up in muscle due to anaerobic glycolysis, causing muscle cramping and pain. The lactate spills into blood and is converted to glucose in the liver, as part of the Cori cycle. But to carry out gluconeogenesis, NAD is required by lactate dehydrogenase to oxidize lactate to pyruvate. However, much of the available NAD is being used for ethanol metabolism and is unavailable for lactate oxidation. The result is metabolic acidosis and hypoglycemia.

Clinical Correlate

Alcohol abuse may lead to hepatic steatosis, which is fatty degeneration of liver tissue.

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Part I ● Biochemistry

Abbreviated diagram of the pathway

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HEXOSE MONOPHOSPHATE SHUNT

The hexose monophosphate (HMP) shunt (pentose phosphate pathway) occurs in the cytoplasm of all cells, where it serves 2 major functions:

•NADPH production

•Source of ribose 5-phosphate for nucleotide synthesis

The first part of the HMP shunt begins with glucose 6-phosphate and ends with ribulose 5-phosphate and is irreversible. This part produces NADPH and involves the important rate-limiting enzyme glucose 6-phosphate dehydrogenase (G6PDH). G6PDH is induced by insulin, inhibited by NADPH, and activated by NADP.

Glucose

NADP NADPH NADP NADPH

Glucose 6-P

6-Phosphogluconate

Ribulose 5-P

Glucose

6-P dehydrogenase

CO2

Fructose 6-P

Erythrose 4-P

Ribose 5-P

Xylulose 5-P

Sedoheptulose 7-P

Glyceraldehyde 3-P

Transketolase (TPP)

Nucleotide

Pyruvate

synthesis

Figure I-14-7. Hexose Monophosphate Shunt

The second part of the pathway, beginning with ribulose 5-phosphate, represents a series of reversible reactions that produce an equilibrated pool of sugars for biosynthesis, including ribose 5-phosphate for nucleotide synthesis. Because fructose 6-phosphate and glyceraldehyde 3-phosphate are among the sugars produced, intermediates can feed back into glycolysis; conversely, pentoses can be made from glycolytic intermediates without going through the G6PDH reaction. Transketolase, a thiamine-requiring enzyme, is important for these interconversions; it is the only thiamine enzyme in red blood cells.

Functions of NADPH	High-Yield

Cells require NADPH for a variety of functions, including:

•Biosynthesis

•Maintenance of a supply of reduced glutathione to protect against reactive oxygen species (ROS)

•Bactericidal activity in polymorphonuclear leukocytes (PMN)

These important roles are cell-specific.

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

Liver

Glucose 6-Phosphate

HMP
		G6PDH			NADPH
shunt

Biosynthesis

• fatty acids

• cholesterol

Pentose phosphates

• nucleotides

Neutrophil

Glucose 6-Phosphate

NADP+

HMP

NADPH

G6PDH

shunt

NADPH oxidase

O2

O2–

H2O2

Pentose

Kill bacteria

phosphates

Erythrocyte

Glucose 6-Phosphate

+

HMP

NADPH Glutathione NADP

G6PDH

shunt

reductase

Oxidant stress

• infection

Oxidized

Reduced

• drugs

Pentose

glutathione

glutathione

• fava

beans

phosphates

H2O2

O2

Glutathione

Spontaneous

peroxidase

(Se)

If accumulates

H2O

• Hemoglobin denaturation (Heinz bodies)

• Membrane damage (hemolytic anemia)

Figure I-14-8. Role of HMP Shunt in Hepatocytes, Phagocytes, and Erythrocytes

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Part I ● Biochemistry

Clinical Correlate

Broad beans (or fava beans) are common to diets in Mediterranean countries (Greece, Italy, Spain, Portugal, Turkey). Their ingestion may cause severe hemolysis in G6PDH individuals (favism). Symptoms include pallor, hemoglobinuria, jaundice, and severe anemia 24–48 hours after ingestion of the beans.

Clinical Correlate

CGD is typically caused by genetic deficiency of NADPH oxidase in the PMN. Patients are susceptible to infection by catalase-positive organisms such as S. aureus, Klebsiella, E. coli, Candida, and

Aspergillus. A negative nitroblue tetrazolium test helps to confirm the diagnosis.

Bridge to Microbiology

Many parasites such as Plasmodium are deficient in

antioxidant mechanisms, making them particularly susceptible to oxygen radicals. In G6PDH deficiency, the ability of erythrocytes to detoxify oxygen radicals is impaired. Ironically, the accumulation of the radicals in erythrocytes in G6PDH deficiency gives protection against malaria.

Glucose 6-Phosphate Dehydrogenase	High-Yield
Deficiency

Deficiency of G6PDH may result in hemolytic anemia and, in rare cases, symptoms resembling chronic granulomatous disease (CGD). The disease shows significant allelic heterogeneity (>400 different mutations in the G6PDH gene are known). The disease is X-linked recessive.

•Major symptom is an acute episodic or chronic hemolysis (rare)

•Female heterozygous for G6PDH deficiency have increased resistance to malaria; consequently, the deficiency is often seen in families where malaria is endemic

Because red blood cells contain a large amount of oxygen, they are prone to spontaneously generate ROS that damage protein and lipid in the cell. In the presence of ROS, hemoglobin may precipitate (Heinz bodies) and membrane lipids may undergo peroxidation, weakening the membrane and causing hemolysis. As peroxides form, they are rapidly destroyed by the glutathione peroxidase/glutathione reductase system in the red blood cell, thus avoiding these complications. NADPH required by glutathione reductase is supplied by the HMP shunt in the erythrocyte.

Persons with mutations that partially destroy G6PDH activity may develop an acute, episodic hemolysis. Certain mutations affect the stability of G6PDH, and, because erythrocytes cannot synthesize proteins, the enzyme is gradually lost over time and older red blood cells lyse. This process is accelerated by certain drugs and, in a subset of patients, ingestion of fava beans. In the United States, the most likely cause of a hemolytic episode in these patients is overwhelming infection, often pneumonia (viral and bacterial) or infectious hepatitis.

In rare instances, a mutation may decrease the activity of G6PDH sufficiently to cause chronic nonspherocytic hemolytic anemia. Symptoms of CGD may also develop if there is insufficient activity of G6PDH (<5% of normal) in the PMN to generate NADPH for the NADPH oxidase bactericidal system.

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

Review Questions

Select the ONE best answer.

1.A liver biopsy is done on a child with hepatomegaly and mild fasting hypoglycemia. Hepatocytes show accumulation of glycogen granules with single glucose residues remaining at the branch points near the periphery of the granule. The most likely genetic defect is in the gene encoding a(n):

A.α-1,4 phosphorylase

B.α-1,4:α-1,4 transferase

C.phosphoglucomutase

D.α-1,6 glucosidase

E.lysosomal α-1,4 glucosidase

2.When fatty acid β-oxidation predominates in the liver, mitochondrial pyruvate is most likely to be

A.carboxylated to phosphoenolpyruvate for entry into gluconeogenesis

B.oxidatively decarboxylated to acetyl CoA for entry into ketogenesis

C.reduced to lactate for entry into gluconeogenesis

D.oxidatively decarboxylated to acetyl CoA for oxidation in Krebs cycle

E.carboxylated to oxaloacetate for entry into gluconeogenesis

Items 3 and 4

A 44-year-old man from Limpopo Province in South Africa, living in the United States and receiving antibiotic therapy for a urinary tract infection, has a self-limiting episode of hemolysis, back pain, and jaundice. The peripheral blood smear reveals a nonspherocytic, normocytic anemia, and Heinz bodies are seen in some of his erythrocytes.

3.Which of the following genetic deficiencies is most likely related to his hemolytic episode?

A.Homocysteine methyltransferase

B.Pyruvate kinase

C.Dihydrofolate reductase

D.Ferrochelatase

E.Glucose 6-phosphate dehydrogenase

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Part I ● Biochemistry

4.Which of the following sets of lab results would most likely have been obtained for this patient?

	Direct Bilirubin	Indirect Bilirubin	Urinary Bilirubin
A.	Increased	Increased	Absent
B.	Increased	Increased	Present
C.	Normal	Increased	Absent
D.	Normal	Decreased	Present
E.	Increased	Decreased	Present

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

Answers

1.Answer: D. This activity of the debranching enzyme removes 1,6-linked glucose residues from the branch points during glycogenolysis.

2.Answer: E. Hepatic fatty acid oxidation generates energy in the postabsorptive period when pyruvate is being converted to OAA for glucose biosynthesis.

3.Answer: E. Only option E is consistent with the constellation of clinical findings presented. Major clue is the positive Heinz body preparation.

4.Answer: C. Only choice C is characteristic of hemolytic jaundice; indirect hyperbilirubinemia with no spillover of the water-insoluble unconjugated form into the urine.

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Lipid Synthesis and Storage 15

Learning Objectives

Answer questions about fatty acid nomenclature

Understand lipid digestion

Answer questions about fatty acid biosynthesis

Demonstrate understanding of lipoprotein metabolism

Explain information related to hyperlipidemias

Use knowledge of cholesterol metabolism

FATTY ACID NOMENCLATURE

Fatty acids are long-chain carboxylic acids. The carboxyl carbon is number 1, and carbon number 2 is referred to as the α carbon. When designating a fatty acid, the number of carbons is given along with the number of double bonds (carbons:double bonds).

Palmitic acid (palmitate) is the primary end-product of fatty acid synthesis.

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COO-

Palmitate

C16:0 or 16:0

Saturated fatty acids have no double bonds.

Unsaturated fatty acids have ≥1 double bonds. Humans can synthesize only a few of the unsaturated fatty acids; the rest come from essential fatty acids in the diet that are transported as triglycerides from the intestine in chylomicrons.

Linolenic acid and linoleic acid are 2 important essential fatty acids. These polyunsaturated fatty acids—as well as other acids formed from them—are important in membrane phospholipids to maintain normal fluidity of cell membranes essential for many functions.

The omega (ω) numbering system is used for unsaturated fatty acids. The ω-family describes the position of the last double bond relative to the end of the chain. The omega designation identifies the major precursor fatty acid, e.g., arachidonic acid is formed from linoleic acid (ω-6 family). Arachidonic acid is itself an important precursor for prostaglandins, thromboxanes, and leukotrienes.

Linoleic	C18:2 (9,12) or 18		9,12	ω-6 family (18 - 12 = 6)
Linolenic	C18:3	(9,12,15) or	18 9,12,15	ω-3 family
Arachidonic	C20:4	(5,8,11,14) or 20 5,8,11,14		ω-6 family

Clinical Correlate

Omega-3 fatty acids in the diet are correlated with a decreased risk of cardiovascular disease. These appear to replace some of the arachidonic acid (an omega-6 fatty acid) in platelet membranes and may lower the production of thromboxane and the tendency of the platelets to aggregate.

High omega-3 fatty acids have also been associated with a decrease in serum triglycerides; they are high in some cold-water fish (salmon, tuna, herring), nuts (walnuts), and seeds (flaxseed).

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