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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 vesicular trafficking protein takes vesicles from the Golgi to the ER (retrograde)?
Ret
III
Increased CPK and muscle biopsy
COP I

2. What is the rate determining enzyme in steroid synthesis? cholesterol synthesis?
The active form of Vit D (1 -25 (OH)2 D3)
HMG CoA synthetase; HMG CoA reductase
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Increased blood levels of ammonia - decreased BUN

3. What disease is associated with chromosome 13?
Carbomyl phoshpate synthetase I
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
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA
Rb

4. in What organ is the storage form of D3 converted to its active form? What enzyme? What hormone stimulates this action?
Blood group types
Kidney; 1 alpha hydroxylase; Parathyroid hormone
MT!
tyrosine hydroxylase

5. What enzyme is deficient in Essential fructosuria? What are the symptoms?
Arg - lys - his; positive except His has no charge at body pH
Glucokinase
Kidney; 1 alpha hydroxylase; Parathyroid hormone
Fructokinase; benign - fructose in the blood and urine

6. Which RNA polymerase synthesizes rRNA? where?
Aldolase B
Type IV collagen defect; X linked recessive; hereditary glomerulonephritis - deafness - ocular disturbances
Liver; P450 hydroxylation
I; nucleolus

7. Vitamin c is needed in...
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine
fructose
Oral uridine administration; provides nucleosides and provides feedback inhibition
catecholamine synthesis (dopamine to norepinephrine)

8. What are the clinical findings in edwards syndrome?
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
high Vmax
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP

9. How do hepatocytes release glucose 6 P from glycogenolysis into the blood ? in What disease is this enzyme deficient?
Fructose 1 -6 bisphosphatase
Glutamate
With glucose 6 phosphatase; Von Gierkes disease
Metanephrine

10. What three steps are included in RNA processing? Where does it occur?

11. What causes hypoglycemia in alcoholism?
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid
With glucose 6 phosphatase; Von Gierkes disease
Type I collagen; AD
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis

12. What aa does tyrosine come from?
Phenylaline
30S ribosomal RNA (small one)
Gain of function gene after expansion results in overproduction of huntingtin protein that in overproduction results in toxicity to neurons in the caudate --> caudate atrophy and decreased levels of GABA and Ach
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B

13. What is the difference between a kinase and a phophorylase?
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
Neimann pick and tay sachs
Acetone
Actin/myosin - MT - intermediate filaments

14. Which amino acids are basic? are they negatively or positively charged at body pH?
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
Cycloheximide
Chromosomal imbalance (downs for ex)
Arg - lys - his; positive except His has no charge at body pH

15. What is there on increased risk of with Downs?
All and Alzheimers
Mitochondrial inheritance
Each codon specifies only one amino acid
Dermatitis - glossitis - diarrhea

16. What is the mc chromosomal disorder?
4 -4 -9
Downs
VMA
Phosphorylation of glucose

17. What is unique about the diarrhea in lactase deficiency?
Tyrosine
Via fructose 1 P by fructokinase in the liver; as glucose 1 by being GALT- ed ;)
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating

18. What type of diseases exhibit anticipation?
No protein!
Make it stiffer/more tensile; continues as we get older so gets less and less elastic
Lesch Nyhan syndrome; X linked recessive
Trinucleotide expansion diseases

19. What does cross linking to collagen do? What does this have to do with age?
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP
Make it stiffer/more tensile; continues as we get older so gets less and less elastic
3 hydrogen bonds; 2 hydrogen bonds
AR; trinucleotide deletion results in loss of a Phe aa on CFTR gene on chrom 7- the result is a mutated CFTR channel that gets degraded in the ER before reaching the surface; the purpose of CFTR is to increase Cl secretion in luminal secretions and r

20. What are rotenon - CN - antimycin A - CO?
Block Electron transport
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )

21. What are causes of zinc deficiencies?
Alcoholism - RA - inflammatory disease - chronic diarrhea
only L form
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
Increased concentration of Cl ions in sweat test

22. What is the end of anaerobic glycolysis?
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
NADH - NADPH - FADH2
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
To provide NADPH since there is an abundance of glucose 6 phosphate; also yields ribose for nucleotide synthesis and glycolysis intermediates

23. What are the findings of homocystinuria?
ATP and biotin
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase
Increased homocysteine in the urine - mental retardation - osteoporosis - marfanoid habitus - and atherosclerosis

24. What are the causes of folate def?
Cancers (rapidly synthesizing); MTX (blocks dihyrdofolate reductase); phenytoin (prevent conjugase from breaking down polyglutamate form to monoglutamate form; reabsorption in jejunum is blocked by alcohol and OCPs; also blocked by sulfonamides
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
Uric acidemia; HGPRT; purine salvage
Delayed wound healing - hypogonadism - decreased adult hair - dysgeusia - anosmia - and alcoholic cirrhosis - perioral rash

25. What is the other name of Vitamin B6? What is its function?
A topoisomerase II
Southern - DNA - Northern - RNA - Western - Protein; blotting procedures are used to detect a specific nucleic acid or aa sequence; the sample is electrophoresed and then transferred to a filter Which is then soaked in a denaturant and then a radiola
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)

26. Findings for McArdle's disease? deficient enzyme?
Inbit Na+K+ ATPase; Cardiac glycosides
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
High glycogen in muscle - but cannot break it down - leading to painful muscle cramps - myoglobinuria with strenuous exercise - no increase in lactic acid after exercise; skeletal muscle glycogen phophorylase
Heterochromatin; euchromatin

27. Which antigout drugs act on MT?
2-3 days after birth because maternal enzyme during fetal like might still be there
Colchine
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA

28. For What three enzymes is arginine a precursor for?
tyrosine hydroxylase
are AR
Type IV collagen defect; X linked recessive; hereditary glomerulonephritis - deafness - ocular disturbances
Nitric Oxide - creatinine - Urea

29. Where are secreted proteins synthesized? Where are cellular/organellar proteins synthesized?
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
RER; free ribosomes
Increased G3P for TG synthesis - increased acetyl CoA (Fatty acid synthesis for TG synthesis) - and decreased oxidation of FA in the mitochondria due to alcohols toxicity on the mit
It contains the peptidyltransferase activity (for elongation of protein

30. What Abs block the binding between aminoacyl tRNA and rRNA by binding the 30S ribosome?
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
Aminoglycosides and tetracyclines
Oral uridine administration; provides nucleosides and provides feedback inhibition
Endonucleases - exonucleases - DNA polymerase - DNA ligase

31. What three enzymes lead to orotic acid accumulation? What reactions are they involved in?
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)
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound

32. What are the two main function of Vit E?
Renal calculi of uric acid (replaces uric acid in excretion)
AR
Steroid - cholesterol FA synthesis; HMP shunt
Helps make Vit K and antioxidant

33. What systems are affected by CF?
Carbamoyl phosphate to orotic acid
Riboflavin; dairy and meat
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
3 hydrogen bonds; 2 hydrogen bonds

34. What does the excess NH4+ in hyperammonia eat up? What is the result?
Methionine; f Met (formyl methyl methionine)
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
Alpha ketoglutarate; no TCA cycle intermediates
NAD+; through ETC you produce NAD+; through production of Lactate from pyruvate you create lactate

35. What ensures that during gluconeogenesis all the ATP wont be used in glycolysis as youre producing it?
Carbomyl phoshpate synthetase I
Vit D levels
GALT
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase

36. What is scurvy caused by? rickets?
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
Def in Vit C; def in Vit D
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann

37. What are the results of the pregnancy screen for Downs?
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Rb
Decrease AFP - increased BCHG - decreased estriol - increased inhibin A
Abnormal glycogen metabolism results in accumulation of glycogen in cells

38. What is alternative splicing? in What disease state does alt splicing occur unintentionally d/t early stop codons?
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
It contains the peptidyltransferase activity (for elongation of protein
TCA cycle - FA oxidation - acetyl coA production; ox phos
When different combination of exons get combined together; in Beta thallasemia

39. Kwashiokor patients are not...
calorie deficient
Also have fair skin - blond hair - blue eyes - eczema
Glucose 6 phosphate dehydrogenase
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA

40. What enzyme is involved in the degradation of tyrosine to fumarate?
enzymatic
pleiotropic
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue
Homogentistic acid oxidase

41. What are the first three enzymes in the first three steps in respiratory burst? Where do they occur? What do they result in? What does it do?
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
UV nonionizing radiation causing thymidine dimers
MT!

42. deamination of What pyrimidine makes what?
Methionine; f Met (formyl methyl methionine)
Cytosine to uracil
ATP synthase - ATP- ADP translocase - ETC
COP II

43. What is the function of Vit K?
Catalyze gamma carboxylation of glutamic acid residues on coag factors so they can bind to Ca complex
1) if it is stable/labile or permanent 2) intact BM and 3) intact ECM
FISH
Glycogen phosphorylase; phosphorylation and dephosphorylation

44. What is the mode of inheritance of Marfans?
AD
Limit dextrans
Alpha ketoglutarate; no TCA cycle intermediates
A folic acid analogue

45. Where does gluconeogenesis occur?
Liver mit and liver cytosol
Fomepizole; antidote for methanol or ethylene glycol poisoning
MT!
Lupus

46. What enzyme converts phenylalanine to tyrosine? What cofactor is used?
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
Phenylalanine hydroxylase; tetrahydrobiopterin factor
golgi apparatus

47. What is methotrexate?
A folic acid analogue
3'--> 5'; prokaryote only
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Griseofulvin

48. What goes on with G0 cell cycle phase?
Cells that are stable go in and out of the G1 to G0 Which is the stable/quiescent (can go into replication but dont) type of cells; cells that stay in G0 are permanent and can not enter G1
A methyl
Williams
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself

49. What are the findings in Tay Sachs disease?
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
Dopamine beta hydroxylase; vit C
NADH - NADPH - FADH2
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption

50. What are the glucogenic amino acids?
Dactinomycin - doxorubicin
Met - arg - val - his
Following gastroenteritis and kwashiokor (protein deficiency)
Glucose 6 phosphatase