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USMLE Step 1 Biochemistry

Subjects : health-sciences, usmle-step-1

Instructions:

Answer 50 questions in 15 minutes.
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Match each statement with the correct term.
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This is a study tool. The 3 wrong answers for each question are randomly chosen from answers to other questions. So, you might find at times the answers obvious, but you will see it re-enforces your understanding as you take the test each time.

1. Which amino acids are acidic? are they negatively or positively charged at body pH?
Asp and glu; negatively
SAM; Vit C
Fe2+; Fe3+
Vit C

2. What are three ketone bodies? Which is not detected in urine? Which do you see most in alcoholic ketoacidosis?
Steroid - cholesterol FA synthesis; HMP shunt
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate
They increase the permeability of the inner mit membrane resulting in a loss of gradient and loss of ATP production BUT electron transport continues! heat is generated instead
Prolonged deficiency of Vitamin C results in decreased tensile strength of collagen because lack of hydroxylated lysines to crosslink (hydroxylation required vit C); bleeding gums - bone pain - poor wound healing - hemarthroses - glossitis - hemorrha

3. What is the fate of glucose from glycogenolysis?
Glucosekinase to glucose 6 P
Collagen; fibroblasts; 4; ECM
AD (trinucleotide expansion is not a mode of inheritance)
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA

4. What is this reaction dependent on?
Inbit Na+K+ ATPase; Cardiac glycosides
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional
Oxygen!
Pellagra

5. Which end carries the triphosphate? Which end of DNA makes the hydoxyl attack?

6. What type exonuclease activity does DNA polymerase III have? in What organisms is it found?

7. _______ can not get inside cells
Incoming Aminoacyl tRNA; site Where peptidyltransferase bond is made between growing peptide aa's; the holds the empty tRNA as it exits; the initiator mRNA (met or f - met) starts off in E site rather than A site
fructose
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Sphingomyelinase; sphingomyelin

8. On Which side of the cytoplasm is the ATP site in the Na+K+ ATPase pump? is a phosphorylated pump active or inactive?
VHL (Von Hippa Lindau)
Cytoplasmic side; active
Tryptophan; melatonin
Limit dextrans

9. What type of aa is phenylalanine? What implications does this have on the presentation of phenylketonuria?
Aromatic; musty body odor
No protein!
Aminoglycosides and tetracyclines
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis

10. What happens in an aminotransferase reaction
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
An aminotransferase enzyme transfers an amino group from an amino acid on to alpha ketoglutarate making it a glutamate (to join the urea cycle or excreted in the kidney) and the amino acid that lost the amino group is then turned into a glycolytic in

11. What disease is associated with chromosome 4?
CPSII
osteosarcomas (Rb mutation is in all cells)
Hemolytic anemia with a blood smear with heinz bodies and bite cells (phagocytic removal of heinz bodies)
Hungtington

12. What form of Vitamin D is in plants? milk fortified with vit D? formed in sun exposed skin?
Acidic
Asp and glu; negatively
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Arylsulfatase A; cerebroside sulfate

13. Heterozygous females in X linked recessive disease...
Phenylketones that build up in phenylketonuria
Acetyl coA activated pyruvate carboxylase in gluconeogenesis
can be d/t lyonization
No protein!

14. How would one use a PCR to id an RNA virus?
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Acetazolamide to alkalinize the urine
Do an RT PCR (reverse transcriptase)- RT the RNA first - then PCR the DNA
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting

15. What else becomes cystic often in APCKD? other than cystic presentations What else do APCKD patients present with?
Intestine - renal - bone
Folinic acid; does not require DHF
Liver; berry aneurysms (d/t hypertension!) - and mitral valve prolapse
Incoming Aminoacyl tRNA; site Where peptidyltransferase bond is made between growing peptide aa's; the holds the empty tRNA as it exits; the initiator mRNA (met or f - met) starts off in E site rather than A site

16. What is the difference between methotrexate and trimethroprim?
Arylsulfatase A; cerebroside sulfate
Trimethroprim inhibits bacterial dihyrofolate reductase
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Catalase positive because they reduce their H2O2 and WBCs cant use that to make bleach; because the way they protect is by building a wall around the organisms with granulomas

17. in What organ is D3 converted from 25 OH D3? with What enzymes?
III
Neimann pick and tay sachs
Liver; P450 hydroxylation
Glucose 6 phosphate dehydrogenase

18. What does 6 mercaptopurine do?
Helps make Vit K and antioxidant
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
6MP blocks de novo purine synthesis
Nacetyl cysteine; breaks disulfide bridges of mucus plugs

19. Where are ketone bodies made? What is the rate limiting enzyme? What are they made from?
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Abnormal glycogen metabolism results in accumulation of glycogen in cells

20. Which antifungal acts on MT?
Griseofulvin
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
TPP (thiamine pyrophosphate)
SAM; Vit C

21. What vesicular trafficking protein takes proteins from the Golgi to lysosomes? in between the golgi? from the plasma membrane to endosomes (receptor mediated endocytosis)?
Neurons and RBCs
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
All clathrin
linked recessive

22. Which amino acids are required during periods of growth?
Vit C
Heme synthesis - urea cycle - gluconeogenesis
Arg and his
Intestine - renal - bone

23. How can hyperammonia be hereditary? acquired?

24. What is the metabolism of fructose?
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
to ions
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face

25. What is the inheritance of myoclonic epilepsy?
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
Mitochondrial inheritance
Carnitine shuttle
BM; laminins; most flexible; different aa composition

26. How is the nitrogen of amino acids transferred to the liver for the urea cycle? explain this process.
As alanine; amino acids donate their amino group to alpha ketoglutarate making it glutamate and the aa then becomes an alpha ketoacid (joins TCA cycle); glutamate then converts pyruvate to alanine by donating its amino group to pyruvate (with ALT); a
Increased nuchal translucency
Arginne
Codons may encode same amino acid since there are only ~20 aa

27. What two compounds provide the two N's of urea? What else is in urea other than two NH2's?
Glutamate and aspartate; CO2 provides an ketone
Formation of a proton gradient that is then used to create ATP through ATP synthase
Creates nicks in DNA to prevent supercoiling; both!
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's

28. What is needed after the glycosylase in base excision repair?
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
An AP (apyrimidinic) endonuclease to cut the DNA - exonuclease then removes and then filled and sealed by enzymes
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA

29. What is the pathogenesis of fragile x syndrome?
Collagen; fibroblasts; 4; ECM
Uracil; thymine
Calcium and calmodulin activate glycogen phosphorylase kinase
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)

30. How does glycerol from triacylglycerols in adipose tissue used for gluconeogenesis?
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
kidney

31. What disease is associated with chromosome 13?
Rb
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine
Ouabain

32. The ________ is 50% cholesterol - 50% phoshpolipids
plasma membrane
Creates nicks in DNA to prevent supercoiling; both!
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
TPP (thiamine pyrophosphate)

33. What are the two main function of Vit E?
Helps make Vit K and antioxidant
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
Cheilosis and corneal vascularization
Ortinithine transcarbamoylase - orotic acid phosphoribosyltransferase - and orotidine 5 phosphate decarboxylase; urea cycle; last two are involved in conversion of orotic acid to UMP (adding PRPP to orotic acid)

34. What does oligomycin do?
Block ATP synthase resulting in no ATP production and an increased proton gradient
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
Adenosine deaminase deficiency; Severe combined immunodeficiency disease
Hemolytic anemia; jaundice in newborn if mom gets too much vit K

35. What is the function of Vitamin A?
Differentiation of cells - vision in reduced light (retinol makes up rhodopsin cells) - antioxidant
Tyrosine; phenylaline
both liver and muscle (unlike glucagon)
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )

36. Which amino acids makes up histones?
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Fe2+; Fe3+
Smoking (FR takes up all the antioxidants); diets lacking fruit
Lys and arg

37. What is fructose 2 -6 phosphate? Where does it come from? by What enzyme?
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
An allosteric regulator; fructose 1 phosphate; phosphofructokinase 2
30S ribosomal RNA (small one)
Heterochromatin; euchromatin

38. What carries CO2 as energy?
Biotin
Enriched whole grain cereals and other things; thiamine; malnutrition (nonenriched white processed bread and rice) and alcohol second to malnutrition
Phenylalanine hydroxylase; tetrahydrobiopterin factor
It is apart of thiamine pyrophosphate Which is a cofactor enzymes involved in ATP production: pyruvate dehydrogenase (glycolysis) - alpha ketoglutarate dehydrogenase (TCA cycle) - transketolase (HMP shunt) - branched chain AA dehydrogenase

39. NAD+ is used in catabolic processes and NADPH is used in...
Double strand breaks; nonhomologous end joining
Less; more
anabolic processes
S - adenosyl methionine

40. What are the findings in Metachromatic leukodystrophy?
degradation
Central and peripheral demyelination with ataxia and dementia
TPP (thiamine pyrophosphate)
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)

41. When are there low levels of cAMP? What does this result in?
Branched chain alpha keto acid dehydrogenase; inability to break down branched chain aa
Where there is insulin; low protein kinase A means dePhosphorylation of the complex Which results in increased PFK2 Which results in more fructose 2 -6 bp and thus more PFK1!!
Histidine; B6
only L form

42. why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased production of orotic acid - the pyrimidine precursor?
Dopamine
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation
Cu; hydroxylation

43. Which lysosomal storage diseases have enzymes that are def in cells that are neuronal mostly?
COP II
dimers
Dopa decarboxylase; B6
Neimann pick and tay sachs

44. What are the clinical symptoms of Huntingtons disease? When does it present?
6MP blocks de novo purine synthesis
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate

45. What does thymine have on it?
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count
A methyl
Becomes methionine
Homogentistic acid oxidase

46. Fructose is the _______ metabolized - because fructokinase in the liver allows it to bypass the rate limiting step in glycolysis (PFK1) - and it enters through a different pathway (either through DHAP to glycogenolysis or to glyeraldehyde for glycoly
fastest sugar
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Colchine
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

47. From Where is alk phos release in the bone? What enzyme does it act on? What is the result?
2-3 days after birth because maternal enzyme during fetal like might still be there
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization
Riboflavin makes up FAD and FMN both used in redox reactions; vegans

48. What is the swollen belly from in a child with kwashiokor?
Pataus
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Ascites from liver damage
Mebendazole and thiabendazole

49. What are the findings in Krabbes disease?
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells
Histidine; B6
= q
Glycogen and FFA oxidation - glucose conserved for final sprinting!

50. What converts fructose 2 -6 phosphate back to fructose 2 phosphate?
reabsorbs
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy
Carbomyl phosphate synthetase II
Fructose bisphosphatase 2