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

2. What is the deficient enzyme in Krabbes disease? accumulated Substrate?
Adrenal medulla
Galactocerebrosidase; Galactocerebroside
Endonucleases - exonucleases - DNA polymerase - DNA ligase
Double strand breaks; nonhomologous end joining

3. Is DNA acidic or basic?
Acidic
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly
Folinic acid; does not require DHF
Collagen; fibroblasts; 4; ECM

4. What causes the pathology in I- cell disease? What does I cell sidease
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
Nucleus
I; nucleolus
Acidic

5. In What syndrome are their immotile cilia? why?

6. What two substrates (other than pyruvate) does pyruvate carboxylase require?
Dihydrofolate reductase; vitamin B12; homocysteine
the RER
ATP and biotin
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)

7. What is the result of arginase deficiency?
DHF
RER; free ribosomes
Chorea and spastic paresis
To amplify a specific desired fragment of DNA

8. What enzyme is deficient in maple syrup urine disease?
Telomerase; an RNA polymerase that uses a RNA template to add DNA to the end of shortening 3' ends
Chromosomal imbalance (downs for ex)
Branched alpha ketoacid dehydrogenase
This depends on if phosphofructokinase 1 is active and this depends on whether or not fructose 2.6 bisphophate is activating it or not - this can only occur if PFK 2 is on and not FBPase 2. then fructose 6 phosphate will participate in glycolysis. Ot

9. What is the difference between wet and dry beriberi?
Dry is just the symmetrical neuropathy with both motor and sensory symptoms; wet includes both the neuropathy and cardiac involvement of high output failure with peripheral edema - tachycardia - cardiomegaly
Tyrosinase
Pellagra (dermatitis - diarrhea - dementia); INH use - B6 deficiency - Hartnup disease - malignant carcinoid syndrome - and a corn based diet (lacks trp and niacin in corn cant be absorbed)
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM

10. Do RNA polymerases have proofreading function? What does this mean for HIV?
Hepatic fatty change
No; increased mutation rate!
Vit C
show disease

11. deamination of What pyrimidine makes what?
Formation of the alpha chains out of glycine - lysine - proline in the RER to make preprocollagen; hydroxylation of the proline and lysine residues in the RER; glycosylation of the lysine residues to make procollagen Which is triple helix of alpha ch
Cytosine to uracil
Oxaloacetate depleted for gluconeogenesis; excess NADH shunts oxaloactetate towards malate
Von gierkes

12. What 4 diseases specifically results with B1 deficiency?
NF2 - DiGeorge/Velocardiofacial syndromes
1) Dry beriberi: peripheral neuropathy demyelination (symmetricul muscle wasting) 2) Wernicke: ataxia - confusion - nystagmus - mammilary body and medial dorsal nuc of thalamus hemorrhage 3) Korsakoff: anterograde and retrograde amnesia - demyelinati
ATP is an allosteric inhibitor on both phosphofructokinase and pyruvate kinase and pyruvate hydrogenase; and fructose 2 -6 BP Which is an allosteric enhancer on phosphofructokinase is an allosteric downregulator on fructose 1 -6 bisphophatase
Beta glucocerebrosidase; glucocerebroside

13. What happens in RBCs after their membranes get oxidized?
Less; more
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
low Vmax
HnRNA; mRNA

14. What disease is associated with chromosome 18?
Glutamine - aspartate - glycine; aspartate
6MP blocks de novo purine synthesis
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
Edwards

15. How is urea measured
SER and mitochondria; NADPH
As BUN (blood urea nitrogen)
Succinate hydrogenase; matrix
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+

16. Amino acids are found in...
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
calorie deficient
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
only L form

17. What is the most common cause of Osteogenesis Imperfecta? What is the inheritance pattern?
BM; laminins; most flexible; different aa composition
Type I collagen; AD
Inbit Na+K+ ATPase; Cardiac glycosides
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells

18. What is cystinuria due to?
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes
Ile - Phe - Thr - Trp
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination
Starts before age 5 with with pelvic girdle weakness with use of Gowers maneuver to stand up - it then progresses superiorly with muscle weakness (type 1 and 2!); waddling duck gait; hyporeflexia; pseudohypertrophy of calf muscles d/t fibrofatty repl

19. How does an enzyme know to go to lysosome?
Cortisol; inhibits insulin action
3 hydrogen bonds; 2 hydrogen bonds
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
By addition of mannose 6 phosphate to proteins in the Golgi

20. From What aa does dopa come from?
Tyrosine
Vit C
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
Lens and neural tissue; converts galactose to osmotically active galactitol

21. With What tool are Prader Willi and Angelman diagnosed?
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
Nitric Oxide - creatinine - Urea
FISH
Dermatitis - glossitis - diarrhea

22. What is the pyruvate dehydrogenase complex used for? What is produced in that reaction?
Glutamate and aspartate; CO2 provides an ketone
S. aureus - Aspergillus
FA coA synthetase combines coA with FA to make acyl coA
Conversion of pyruvate to Acetyl CoA; acetyl coA - CO2 - and NADH (one of each per pyruvate)

23. What is imprinting disease? Give 2 examples
Dopa decarboxylase; B6
MT!
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann
Methionine; f Met (formyl methyl methionine)

24. Glycogenolysis ________ maintains blood sugar
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
in hepatocytes
Robertsonian translocations
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count

25. What are the symptoms of infantile beriberi and When do they present?
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)
Cardiomegaly - cyanosis - tachycardia - dyspnea - vomitting; 2-3 months
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Each codon specifies only one amino acid

26. What 3 genetic disorders can cause Marfanoid habitus?
Marfans - homocystinuria - MEN 2B
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine
Phenylaline

27. Can a disorder be congential but not hereditary?

28. What does a def in folic acid cause? is it more or less common than cobalamin def? How do you differentiate it with cobalamin def?
Fabrys Disease and Hunters Syndrome
Phosphofructokinase 1
7 dehydrocholesterol
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine

29. What type of diseases exhibit anticipation?
When different combination of exons get combined together; in Beta thallasemia
Galactokinase deficiency and classic galactosemia; classic galactosemia
Trinucleotide expansion diseases
Uses HMG coA reductase

30. What two compounds provide the two N's of urea? What else is in urea other than two NH2's?
Glucokinase
Bacterial synthesis in the intestine
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
Glutamate and aspartate; CO2 provides an ketone

31. What does alcohol do to the P450 system?
Limit protein in diet; give benzoate or phenylbutyrate Which bind amino acids and lead to excretion - lactulose (hydrogen ion binds to ammonia to make excretable ammonium) and neomycin (to kill bacteria that release ammonia from aa)
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism

32. What does a deficiency cause?
Night blindness - dry skin
IMP
Alpha 1 -4 linkages; alpha 1 -6 linkages
All and Alzheimers

33. What enzyme converts phenylalanine to tyrosine? What cofactor is used?
Arginine
high Vmax
The active form of Vit D (1 -25 (OH)2 D3)
Phenylalanine hydroxylase; tetrahydrobiopterin factor

34. Which amino acids makes up histones?
Lys and arg
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
AD

35. What happens outside the fibroblast to procollagen?
Proteolytic processing (of water soluble parts to less soluble) to turn into troprocollagen and then crosslinking between hydroxylated lysine residues by lysyl oxidase to make collagen fibrils
Phenylketones that build up in phenylketonuria
Urea cycle enzyme deficiency (mc: ornithine transcarbamoylase); liver disease - alcoholic cirrhosis and reye's
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly

36. What can cause an excees of vit A? What are the symptoms?
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Glutamine PRPP amidotransferase
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Fomepizole; antidote for methanol or ethylene glycol poisoning

37. What enzyme is deficient in Fructose intolerance? What is the result of the enzyme def? What are the symptoms?
Riboflavin; dairy and meat
anabolic processes
G6PD dehydrogenase; transketolases
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting

38. Which cells have the most FA and thus are the most susceptible to FR injury?
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
Neurons and RBCs
Tyrosine
Cytoplasmic side; active

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

40. What carries electrons (as energy)?
NADH - NADPH - FADH2
Familial Adenomatous Polyposis (APC gene)
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
only yield acetyl coA

41. When is there high levels of cAMP? What does this result in?
To amplify a specific desired fragment of DNA
When there is glucagon; protein kinase A activation and thus phosphorylation of the complex resulting in the activation of fructose 2 -6 BPase and no PFK2
By addition of mannose 6 phosphate to proteins in the Golgi
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis

42. What is the findings in Fabrys disease?
Glycogen phosphorylase
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
In hepatocytes; B6
cell cycle events

43. What disease is associated with chromosome 3?
VHL (Von Hippa Lindau)
Oxaloacetate depleted for gluconeogenesis; excess NADH shunts oxaloactetate towards malate
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA
AD

44. What is the net production of glycolysis from one glucose?
Fructokinase; benign - fructose in the blood and urine
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20

45. Where does splicing occur?
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)
SAM
Nucleus
Enriched whole grain cereals and other things; thiamine; malnutrition (nonenriched white processed bread and rice) and alcohol second to malnutrition

46. What three enzyme def. can cause homocysteinura? What is their mode of inheritance?
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic
golgi apparatus
Pyruvate (through OXA); lactate - alanine - malate (and all other TCA intermediates through OXA) - glycerol - odd chain fatty acids yield propionyl coA Which can be converted to succinyl coA and enter as TCA cycle

47. What is the main source of FA? Where else can they be released from?
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens

48. is CPSII or CPSI involved in pyrimidine synthesis?
Isocitrate dehydrogenase
Decreased phenylalanine - increase tyrosine
AR
CPSII

49. When are there low levels of cAMP? What does this result in?
paternal
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!!
Williams
Met - arg - val - his

50. What is the most common disorder of the urea cycle? What is the result? What is its mode of inheritance? What is the mode of inheritance of the other urea cycle enzyme def?
Ornithine transcarbomyalse; orotic aciduria; X linked recessive; autosomal recessive
Complex I - Complex II (lower energy)
Starts before age 5 with with pelvic girdle weakness with use of Gowers maneuver to stand up - it then progresses superiorly with muscle weakness (type 1 and 2!); waddling duck gait; hyporeflexia; pseudohypertrophy of calf muscles d/t fibrofatty repl
SnRNPs; RNA polymerase III