KAPLAN_USMLE_STEP_1_LECTURE_NOTES_2018_BIOCHEMISTRY_and_GENETICS

Part I ● Biochemistry

Protein (such as Gαs)

O

O:

NH2

O

O

;

ADP-ribosylated

P

O

N

protein

Toxin

O

O

O

O

P

OH OH

NH2

:

O

O

N

HO

OH N

NH2

N

O

P

O

O

:O

O O

P

O

N

N

N

N

O:

N

HO

OH

OH OH

Nicotinamide adenine dinucleotide (NAD)

+nicotinamide

Figure I-9-7. ADP-Ribosylation of a Protein

LIPID-SOLUBLE HORMONES

Lipid-soluble hormones diffuse through the cell membrane, where they bind to their respective receptors inside the cell. The receptors have a DNA-binding domain (usually Zn-fingers) and interact with specific response elements in enhancer (or possibly silencer) regions associated with certain genes.

For example, the cortisol receptor binds to its response element in the enhancer region of the phosphoenolpyruvate carboxykinase (PEPCK) gene. By increasing the amount of PEPCK in the hepatocyte, cortisol can increase the capacity for gluconeogenesis, one of its mechanisms for responding to chronic stress often associated with injury.

The enhancer mechanism was reviewed in Chapter 5.

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Chapter 9 ● Hormones

Review Questions

1.A patient with manic depressive disorder is treated with lithium, which slows the turnover of inositol phosphates and the phosphatidyl inositol derivatives in cells. Which of the following protein kinases is most directly affected by this drug?

A.Protein kinase C

B.Receptor tyrosine kinase

C.Protein kinase G

D.Protein kinase A

E.Protein kinase M

Items 2 and 3

Tumor cells from a person with leukemia have been analyzed to determine which oncogene is involved in the transformation. After partial sequencing of the gene, the predicted gene product is identified as a tyrosine kinase.

2.Which of the following proteins would most likely be encoded by an oncogene and exhibit tyrosine kinase activity?

A.Nuclear transcriptional activator

B.Epidermal growth factor

C.Membrane-associated G protein

D.Platelet-derived growth factor

E.Growth factor receptor

3.A kinetic analysis of the tyrosine kinase activities in normal and transformed cells is shown below.

Normal

cells

1/V

Tumor

cells

1/[ATP]

Which of the following conclusions is best supported by these results?

A.The tumor cell kinase has a higher-than-normal affinity for ATP

B.A kinase gene has been deleted from the tumor cell genome

C.A noncompetitive inhibitor has been synthesized in the tumor cells

D.A kinase gene has been amplified in the tumor cell genome

E.The tumor cell kinase has a lower-than-normal affinity for ATP

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Part I ● Biochemistry

4.In a DNA sequencing project, an open reading frame (ORF) has been identified. The nucleotide sequence includes a coding region for an SH2 domain in the protein product. This potential protein is most likely to

A.bind to an enhancer region in DNA

B.be a transmembrane hormone receptor

C.transmit signals from a tyrosine kinase receptor

D.bind to an upstream promoter element

E.activate a soluble guanyl cyclase enzyme in vascular smooth muscle

γ	β
	α	+	Enzyme	Second
				messenger
			Substrate

5.The diagram above represents a signal transduction pathway associated with hormone X. The receptor for hormone X is most likely to be characterized as a(n)

A.seven-helix transmembrane domain receptor

B.intracellular receptor with a zinc-finger domain

C.helix-turn-helix transmembrane domain receptor

D.transmembrane receptor with a guanyl cyclase domain

E.tyrosine kinase domain receptor

6.A 58-year-old man with a history of angina for which he occasionally takes isosorbide dinitrate is having erectile dysfunction. He confides in a colleague, who suggests that sildenafil might help and gives him 3 tablets from his own prescription. The potentially lethal combination of these drugs relates to

		Isosorbide Dinitrate			Sildenafil
A.		Activates nitric oxide			Inhibits guanyl cyclase in vascular
		synthase in vascular			smooth muscle
		endothelium
B.		Activates nitric oxide			Inhibits guanyl cyclase in corpora
		synthase in vascular			cavernosa smooth muscle
		endothelium
C.		Releases cyanide as a			Inhibits cGMP phosphodiesterase in
		byproduct			corpora cavernosa smooth muscle
D.		Activates guanyl cyclase in			Inhibits cGMP phosphodiesterase
		vascular smooth muscle			in vascular smooth muscle
E.		Activates the ANF receptor in			Inhibits protein kinase G in vascular
		vascular smooth muscle			smooth muscle

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Chapter 9 ● Hormones

Answers

1.Answer: A. The description best fits the PIP2 system in which protein kinase C is activated.

2.Answer: E. Although any of the listed options might be encoded by an oncogene, the “tyrosine kinase” description suggests it is likely to be a growth factor receptor.

3.Answer: D. Because the y-axis is 1/V, a smaller value for the 1/V means

an increase in Vmax. An increase in Vmax (with no change in Km) means an increase in the number of enzymes (a kinase in this problem). Gene amplification (insertion of additional copies of the gene in the chromosome) is a well-known mechanism by which oncogenes are overexpressed and by which resistance to certain drugs is developed. For instance, amplification of the dihydrofolate reductase gene can confer resistance to methotrexate.

4.Answer: C. Proteins with SH2 domains might bind to the insulin receptor substrate-1 (IRS-1) to transmit signals from the insulin receptor, a tyrosine kinase type of receptor. PI-3 kinase is an example of an SH2 domain protein. SH2 domains are not involved in DNA binding (choices A and D). Examples of protein domains that bind DNA include zinc fingers (steroid receptors), leucine zippers (CREB protein), and helix- turn-helix proteins (homeodomain proteins).

5.Answer: A. The diagram indicates that the receptor activates a trimeric G-protein associated with the inner face of the membrane and that the G-protein subsequently signals an enzyme catalyzing a reaction producing a second messenger. Receptors that activate trimeric G-proteins have a characteristic seven-helix transmembrane domain. The other categories of receptors do not transmit signals through trimeric G-proteins.

6.Answer: D. Nitrates may be metabolized to nitric oxide (NO) that activates a soluble guanyl cyclase in vascular smooth muscle. The increase in cGMP activates protein kinase G and subsequently leads to vasodilation. Sildenafil inhibits cGMP phosphodiesterase (PDE), potentiating vasodilation that can lead to shock and sudden death. Although sildenafil has much higher potency for the cGMP PDE isozyme in the corpora cavernosa, it can also inhibit the cGMP PDE in vascular smooth muscle. Nitric oxide synthase (choices A and B) is the physiologic source of nitric oxide in response to vasodilators such as acetylcholine, bradykinin, histamine, and serotonin.

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Vitamins 10

Learning Objectives

Understand differences between vitamins and coenzymes of watersoluble hormones

Know pathologies associated with water-soluble vitamins

Know metabolism and functions of the 4 fat-soluble vitamins

VITAMINS

Vitamins have historically been classified as water-soluble or lipid-soluble. Water-soluble vitamins are precursors for coenzymes and are reviewed in the context of the reactions for which they are important.

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Part I ● Biochemistry

Table I-10-1. Water-Soluble Vitamins

	Vitamin or
				Enzyme			Pathway			Deficiency
	Coenzyme
	Biotin			Pyruvate carboxylase Acetyl CoA			Gluconeogenesis			MCC* (rare): excessive consumption
				carboxylase			Fatty acid synthesis			of raw eggs (contain avidin, a
										biotin-binding protein); also caused
										by biotinidase deficiency
				Propionyl CoA carboxylase			Odd-carbon fatty acids, Val,			Alopecia (hair loss), bowel
							Met, Ile, Thr			inflammation, muscle pain
	Thiamine (B1)			Pyruvate dehydrogenase			PDH			MCC: alcoholism (alcohol interferes
										with absorption)
				α-Ketoglutarate dehydrogenase			TCA cycle			Wernicke (ataxia, nystagmus,
										ophthal-moplegia)
				Transketolase			HMP shunt			Korsakoff (confabulation, psychosis)
										Wet beri-beri (high-output cardiac
										failure, fluid retention, vascular leak)
										and dry beri-beri (peripheral
				Branched chain ketoacid dehy-			Metabolism of valine			neuropathy)
				drogenase			isoleucine and leucine
	Niacin (B3)			Dehydrogenases			Many			Pellagra: diarrhea, dementia,
										dermatitis, and, if not treated, death
	NAD(H)									Pellagra may also be related to
	NADP(H)									deficiency of tryptophan (corn is low
										in tryptophan), which supplies a
										portion of the niacin requirement
	Folic acid			Thymidylate synthase			Thymidine (pyrimidine)			MCC: alcoholism and pregnancy
							synthesis			(body stores depleted in 3 months),
										hemodialysis
	THF			Enzymes in purine synthesis need			Purine synthesis			Homocystinemia with risk of deep
				not be memorized						vein thrombosis and atherosclerosis
										Megaloblastic (macrocytic) anemia
										Deficiency in early pregnancy causes
										neural tube defects in fetus
	Cyanocobala-			Homocysteine methyltransferase			Methionine, SAM			MCC: pernicious anemia. Also in aging,
	min (B12)			Methylmalonyl CoA mutase			Odd-carbon fatty acids, Val,			especially with poor nutrition, bacterial
							Met, Ile, Thr			overgrowth of terminal ileum, resection
										of the terminal ileum secondary to
										Crohn disease, chronic pancreatitis,
										and, rarely, vegans, or infection with
										D. latum
										Megaloblastic (macrocytic) anemia
										Progressive peripheral neuropathy
*MCC, most common cause										(Continued)

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Chapter 10 ● Vitamins

Table I-10-1. Water-Soluble Vitamins (continued )

Vitamin or

Enzyme

Pathway

Deficiency

Coenzyme

Pyridoxine (B6)

Aminotransferases (transaminase):

Protein catabolism

MCC: isoniazid therapy

Pyridoxal-P (PLP)

AST (GOT), ALT (GPT)

Sideroblastic anemia

δ-Aminolevulinate synthase

Heme synthesis

Cheilosis or stomatitis (cracking or

scaling of lip borders and corners of

the mouth)

Convulsions

Riboflavin (B2)

Dehydrogenases

Many

Corneal neovascularization

FAD(H2)

Cheilosis or stomatitis (cracking or

scaling of lip borders and corners of

the mouth)

Magenta-colored tongue

Ascorbate (C)

Prolyl and lysyl hydroxylases

Collagen synthesis

MCC: diet deficient in citrus fruits

Dopamine hydroxylase

and green vegetables

Dopamine hydroxylase

Catecholamine synthesis

Scurvy: poor wound healing, easy

bruising (perifollicular hemorrhage),

Absorption of iron in

bleeding gums, increased bleeding

GI tract

time, painful glossitis, anemia

Pantothenic acid

Fatty acid synthase

Fatty acid metabolism

Rare

CoA

Fatty acyl CoA synthetase

Pyruvate dehydrogenase

PDH

α-Ketoglutarate dehydrogenase

TCA cycle

Scurvy

A 7-month-old infant presented in a “pithed frog” position, in which he lay on his back and made little attempt to lift the legs and arms because of pain. The infant cried when touched or moved, and there appeared to be numerous areas of swelling and bruising throughout the body. The mother informed the pediatrician that the infant was bottle-fed. However, the mother stated that she always boiled the formula extensively, much longer than the recommended time, to ensure that it was sterile.

The patient has infantile scurvy, which often occurs in infants 2–10 months of age who are bottle-fed with formula that is overheated for pasteurization and not supplemented with vitamin C. Vitamin C is destroyed by excessive heat.

Although bleeding in an infant with scurvy might occur similarly as in an adult, gum bleeding does not unless there are erupted teeth. Biochemically, vitamin C is necessary as a cofactor by proline and lysine hydroxylases in collagen synthesis. In scurvy, because proline and lysine residues are not hydroxylated, hydrogen bonding within the triple helices does not take place. Consequently, collagen fibers are significantly less stable than normal. Vitamin C also has roles as 1) an antioxidant, 2) in reducing iron in the intestine to enable the absorption of iron, and 3) in hepatic synthesis of bile acids.

Bridge to Pharmacology

High-dose niacin can be used to treat hyperlipidemia.

Note

What to Know for the Exam

Vitamins:

•Clinical manifestations of deficiencies

•Enzymes that accumulate

•Pathways

•Preventions of deficiencies

•Treatments of deficiencies

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Part I ● Biochemistry

Note

Vitamin D deficiency is caused

by insufficient sunlight, inadequate fortified foods (milk), or end-stage renal disease (renal osteodystrophy).

Symptoms:

•Bone demineralization

•Rickets (children)

•Osteomalacia (adults)

Vitamin A deficiency is caused by fat malabsorption or a fat-free diet.

Symptoms:

•Night blindness

•Keratinized squamous epithelia

•Xerophthalmia, Bitot spots

•Keratomalacia, blindness

•Follicular hyperkeratosis

•Alopecia

Vitamin E deficiency is caused by fat malabsorption or premature birth.

Symptoms:

•Hemolytic anemia

•Acanthocytosis

•Peripheral neuropathy

•Ataxia

•Retinitis pigmentosum

There are 4 important lipid-soluble vitamins: D, A, K, and E.

•Two of them (A and D) work through enhancer mechanisms similar to those for lipid-soluble hormones.

•In addition, all 4 have more specialized mechanisms through which they act.

Table I-10-2. Lipid-Soluble Vitamins

	Vitamin			Important Functions
	D (cholecalciferol)			In response to hypocalcemia, helps normalize
				serum calcium levels
	A (carotene)			Retinoic acid and retinol act as growth regulators,
				especially in epithelium
				Retinal is important in rod and cone cells for vision
	K (menaquinone,			Carboxylation of glutamic acid residues in many
	bacteria; phytoquinone,			Ca2+-binding proteins, importantly coagulation
	plants)			factors II, VII, IX, and X, as well as protein C and
				protein S
	E (α-tocopherol)			Antioxidant in the lipid phase. Protects membrane
				lipids from peroxidation

VITAMIN D AND CALCIUM HOMEOSTASIS

Hypocalcemia (below-normal blood calcium) stimulates release of parathyroid hormone (PTH), which in turn binds to receptors on cells of the renal proximal tubules. The receptors are coupled through cAMP to activation of a 1α-hydroxylase important for the final, rate-limiting step in the conversion of vitamin D to 1,25-DHCC (dihydroxycholecalciferol or calcitriol).

Once formed, 1,25-DHCC acts on duodenal epithelial cells as a lipid-soluble hormone. Its intracellular receptor (a Zn-finger protein) binds to response elements in enhancer regions of DNA to induce the synthesis of calcium-binding proteins thought to play a role in stimulating calcium uptake from the GI tract.

1,25-DHCC also facilitates calcium reabsorption in the kidney and mobilizes calcium from bone when PTH is also present. All these actions help bring blood calcium levels back within the normal range.

The relation of vitamin D to calcium homeostasis and its in vivo activation are shown below.

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Chapter 10 ● Vitamins

7-Dehydrocholesterol

Skin

UV light

Cholecalciferol

(Vitamin D3)

Liver

25-Hydroxylase

25-Hydroxycholecalciferol

Dietary source required if insufficient exposure to UV light. Vitamin D3 is found in saltwater fish (salmon) and egg yolks.

Vitamin D3, prepared from animal products and then irradiated with UV light, is added to milk and some fortified cereals.

Cirrhosis and liver failure may produce bone demineralization.

	Kidney	Hypocalcemia
				+ PARATHYROID
	1α-Hydroxylase (induce)
			Parathyroid hormone (PTH)
		Patients with end-stage renal disease develop renal
1,25-Dihydroxycholecalciferol
(Calcitriol, 1,25-DHCC)		osteodystrophy; IV or oral 1,25-DHCC may be given.

	BONE: Osteoclasts; Ca2+		INTESTINE (Duodenum): increase
	mineralization or		calcium uptake from intestine
	demineralization (with PTH)
	Figure I-10-1. Synthesis and Activation of Vitamin D
Synthesis of 1, 25-Dihydroxycholecalciferol				High-Yield
(Calcitriol)

Humans can synthesize calcitriol from 7-dehydrocholesterol derived from cholesterol in the liver. Three steps are involved, each occurring in a different tissue:

Step 1. Activation of 7-dehydrocholesterol by UV light in the skin produces cholecalciferol (vitamin D3); this step is insufficient for many people in cold, cloudy climates, and vitamin D3 supplementation is necessary.

Step 2. 25-hydroxylation in the liver (patients with severe liver disease may need to be given 25-DHCC or 1,25-DHCC).

Step 3. 1α-hydroxylation in the proximal renal tubule cells in response to PTH; genetic deficiencies or patients with end-stage renal disease develop renal osteodystrophy because of insufficiency of 1,25-DHCC and must be given 1,25-DHCC or a drug analog that does not require metabolism in the kidney. Such patients include those with:

•End-stage renal disease secondary to diabetes mellitus

•Fanconi renal syndrome (renal proximal tubule defect)

•Genetic deficiency of the 1α-hydroxylase (vitamin D-resistant rickets)

Bridge to Pharmacology

Bisphosphonates are a class of drugs used in the treatment of osteoporosis.

•Function by inhibiting osteoclast action and resorption of bone; results in a modest increase in bone mineral density (BMD)

•Will lead to strengthening of bone and decrease in fractures

•Commonly used bisphosphates are ibandronate, risedronate, and alendronate

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