Usmle Biochemistry Qbank 3

Front 1

Clinical features of sickle cell anemia

Back 1

Exertional dyspnea, pneumonia resulting in life-threatening acute chest syndrome, and recurrent abdominal and bone pain

Front 2

Most common cause of CF

Back 2

Phenylalanine deletion (delta F508)

Front 3

Explain the meaning of polycistronic

Back 3

Bacterial mRNA is said to be polycistronic, meaning that one mRNA may code for several proteins.

- one example is the lac operon, which codes for proteins necessary for lactose metabolism by E. coli.

Front 4

Explain pathogenesis of HbC

What would you see on electrophoresis relative to HbA and HbS?

Back 4

missense mutation that causes a substitution of glutamate with lysine int eh beta globin chain (6th position), resulting in creased positive charge of the molecule.

- HbC moves more slowly than both HbA and HbS towards the positive anode

Front 5

If the partial pressure at which hemoglobin is 50% saturated (P50) is decreased, what does this mean?

Back 5

Hb has an increased O2 affinity --> causes less O2 to be released into the tissues --> resultant hypoxia produces reflex polycythemia

Front 6

Name sources of oxidative stress for G6PD deficiency

Back 6

Antimalarials, sulfonamides, fava beans

Front 7

Hereditary spherocytosis results from what molecular defect?

Back 7

defects in genes that code for spectrin and ankyrin

Front 8

An infant that is exclusively breast fed may still need what supplementation?

Back 8

Vit D and Vit K

Front 9

Which sugar has the highest rate of metabolism in the glycolytic pathway?

Back 9

Fructose

Front 10

Describe the process of collagen synthesis

Back 10

see picture

Front 11

Which prokaryotic DNA pol have proof reading activity and remove mismatched nucleotides via 3' to 5' exonuclease activity?

Which have 5' to 3' exonuclease activity? What is the purpose of this?

Back 11

All 3 prokaryotic DNA pol have proofreading activity 3' to 5'

DNA pol I has 5' to 3' exonuclease activity to excise and replace RNA primers and damaged DNA sequences.

Front 12

What is the difference between exonucleases and endonucleases?

Back 12

Exonucleases remove nucleotides from the end of a DNA molecule, whereas endonucleases cut DNA at very specific DNA sequences within the molecule.

Front 13

The nucleolus is the site of synthesis of what?

Back 13

The nucleolus is the site of rRNA synthesis from rDNA, the site of ribosomal protein synthesis, nad the site of ribosome formation.

Q: which enzyme produces rRNA?

Hint 13

RNA pol I

Front 14

What does RNA pol III transcribe?

Back 14

small RNA molecules such as:
tRNA, the 5S ribosomal RNA, and small nuclear RNA (snRNP)

Front 15

Pompe dz results from deficiency in what enzyme?

Back 15

Acid maltase (alpha glucosidase)

Front 16

Which step in the TCA cycle requires thiamine as a cofactor?

- Talk about the results of administration of glucose to thiamine deficient patients

Back 16

alpha-KG DH

-Results in Wernicke encephalopathy due to increased thiamine demand --> acute confusion, ophthalmoplegia, and ataxia

Front 17

Which enzymes require thiamine in the various metabolic pathways?

Back 17

Glycolysis: Pyruvate dehydrogenase
TCA: alpha-KG DH
HMP shunt: Transketolase

Front 18

List hormone receptors that utilize the G-protein/adenylate cyclase second messenger system.

Back 18

TSH, glucagon, PTH, and beta adrenergic receptors

Front 19

Telomerase: discuss everything about it

Back 19

Telomerase is an RT enzyme (RNA dependent DNA polymerase) that adds TTAGGG repeats to the 3' end of DNA strands at the terminal end of chromosomes, the telomere region.

Normally expressed in stem cells (present in most tissues where they are responsible for replacement of dead cells - epidermal stem cells present in the basal cell layers, bone marrow stem cells) as well as cancer cells.

Front 20

Describe four patient populations that are most likely to develop Vit K deficiency:

Back 20

malabsorption syndromes, broad-spectrum antibiotics that destroy intestinal flora, neonates (secondary to their limited hepatic reserves, unestablished intestinal flora, and limited bioavailability of Vit K in breast milk), and those afflicted with generalize liver dz

Front 21

Inhibition of lactate DH in exercising muscles would eventually lead to an inhibition of glycolysis due to intracellular depletion of _______.

Back 21

NAD+

Under anaerobic conditions, NADH transfers protons to pyruvate to form lactate and to regenerate NAD+. NAD+ is required to convert glyceraldehyde-3-phosphate to 1,2-bisphosphoglycerate in glycolysis.

Front 22

Which amino acid becomes essential in patients with homocystinuria? What is the defective enzyme in this case?

Back 22

Cysteine

Defective enzyme is cystathione synthetase

Hint 22

Front 23

Impaired formation of N-acetylglutamate is the cause of a child's increased blood ammonium level.

Which reaction is most likely impaired in this patient?

Back 23

CPS I

N-acetylglutamate is an essential activator of CPS I

(low yield?)

Front 24

Explain the characteristics of tRNA

Back 24

1) small, noncoding form of RNA that contains unusual nucleosides such as pseudouridine and thymidine.
2) has a CCA sequence at its 3' end that is used as a recognition sequence by proteins, and that the 3' terminal hydroxyl group of the CCA tail is used as the binding site for the AA.

Front 25

What happens to a protein that undergoes a splice site mutation?

Back 25

Frequently result in production of larger proteins with altered function but preserved immune reactivity.

Front 26

2 major functions of HMP shunt

Back 26

1) production of NADPH as a reducing equivalent
2) synthesis of ribose 5-phosphate for nucleotide synthesis

Front 27

Afr. American pt with malaise and dark urine. Recent UTI. Anemia is evident on the CBC and erythrocyte fragments are seen on peripheral smear.

What does pt most likely have?

Back 27

G6PD deficiency

NADPH is necessary to keep glutathione reduced, which in turn detoxes free radicals and peroxides.

Decrased NADPH in RBCs leads to hemolytic anemia due to poor RBC defense against oxidizing agents.

Infection can also precipitate hemolysis.

X-linked recessive; more prevalent among blacks; increase malarial resistance

Front 28

Where would you find glucokinase?

Back 28

Liver and beta cells of pancreas

Front 29

What might you find on autopsy of someone who died with chronic thiamine deficiency (ie homeless alcoholic)?

How would you be able to diagnose this?

Back 29

Foci of hemorrhage and necrosis in the mamillary bodies and periaqueductal gray matter

- look at enzyme activity of the enzymes that require thiamine: pyruvate DH, transketolase, alpha-KG DH

Front 30

Marfan vs Ehlers-Danlos differences in clinical presentation

Back 30

Marfan: fibrillin gene mutation; connective tissue disorder affecting skeleton, heart, and eyes; Sx: tall with long extremities, pectus excavatum, hyperextensive joints, arachnodactyly, cystic medial necrosis of aorts (dissecting aneurysms), floppy mitral

Ehlers-Danlos: Type 3 collagen disorder; Hyperextensible skin, tendency to bleed, hypermobile joints, may have berry aneurysms and organ rupture

bonus: name some of the mutations leading to EDS phenotypes

Hint 30

- deficiencies in lysyl-hydrolase and pro-collagen peptidase enzymes responsible for collagen synthesis

Front 31

For which metabolic reactions is riboflavin (vit b2) used?

Back 31

Dehydrogenase reactions involving FMN and FAD

Front 32

Where is biotin used?

Back 32

Cofactor for all four carboxylase enzymes: pyruvate carboxylase, acetyl CoA carboxylase, propionyl CoA carboxylase, and 3-methylcrotonyl-CoA-carboxylase

Hint 32

Front 33

OAA forms aspartate while reacting with glutamate. Which vitamin most likely participates in this reaction?

Back 33

Transamination reactions typically occur between an amino acid and an alpha keto acid. The amino group from the AA is transferred to the alpha keto acid, and the alpha keto acid in turn becomes an AA.

Pyridoxal phosphate (vit B6) serves as a cofactor in AA transamination and in decarboxylation reactions.

Front 34

Describe the process of conjugation of bilirubin starting from the RBC to the feces/urine

Back 34

Front 35

Signaling pathways of endocrine hormones:

Back 35

Front 36

Explain the clinical manifestations of porphyria cutanea tarda and describe the pathogenesis:

Back 36

see image