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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 is the deficient enzyme in Tay Sachs disease? accumulated Substrate?
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
Hexosaminidase A; GM2 ganglioside
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count

2. What is given to prevent side effects When methotrexate is given? why does this work?
Folinic acid; does not require DHF
Marfans - homocystinuria - MEN 2B
NADPH- reductive biosynthesis!
10 - 9 - 7 - 2; protein C and S

3. What are two examples of X linked Autosomal Dominant disease?
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)
Hypophosphatemia rickets and Alports
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA
Carnitine shuttle

4. What is an example of a nonsense mutation?
Biotin; cofactor for carboxylation; pyruvate carboxylase (turning pyruvate to oxaloacetate) - acetyl coA carboxylase (acetyl coA to malonyl Co A) - and proprionyl coA caroboxylase for (propionyl CoA to methylmalonyl coA)
Defective neutral amino acid transporter on renal and intestinal cells; AR
= q
Beta thalassemia

5. No offspring of affected males with a mit disease can...
Fabrys Disease and Hunters Syndrome
Trinucleotide expansion diseases
Hypophosphatemia rickets and Alports
show disease

6. Where does a majority of galactose in our diet come from?
Mental retardation - obesity - short stature - hypogonadism - hypotonia - hyperphagia; father
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
Lactose (glucose and galactose)
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary

7. What is the deficient enzyme in Niemann Pick disease? accumulated Substrate?
Sphingomyelinase; sphingomyelin
Damaged 'ubiquinated' cytokeratin intermediate filaments build up in hepatocytes - damaged neurofilaments in idiopathic parkinsons disease build up to form Lewy bodies - Neurofibrillary tangles are build up of neurofilaments in alzheimers
RBCs - renal medulla - leukocytes - lens - testes - cornea
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50

8. What converts dihydrobioterin back to tetrahydrobioterin factor? What energy carrier is used? in What reactions is this cofactor used?
Tyrosine; tryptophan
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP
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)
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)

9. What is the difference between the promoter and enhancer? in What process are they used for?
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
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
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!!
Promoter binds RNA Pol II and is very close (25 bases) to initiation site and is AT rich; enhancer/silencers bind TFs only - can be located close or far and regulate gene expression but not specifically initiation; transcription

10. What enzyme is deficient in Essential fructosuria? What are the symptoms?
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Fructokinase; benign - fructose in the blood and urine
'9+2' 9 pairs of microtubules connected by an axonemal dynein ATPase in a cylinder with 20 MTs in the center; the connecting ATPase allows bending and differential sliding
Cysteine; tyrosine

11. What is hartnup disease? What is the mode of inheritance?
Neimann pick and tay sachs
Alcoholism - RA - inflammatory disease - chronic diarrhea
Defective neutral amino acid transporter on renal and intestinal cells; AR
Severe fasting hypoglycemia - very high levels glycogen in liver - high blood lactate - hepatomegaly

12. What is the mode of inheritance for the multiple endocrine neoplasias?
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
Ehler Danlos; type III collagen def; joint dislocation - berry aneurysm - ecchymoses - organ rupture
Hepatic fatty change
AD

13. What drug blocks acetaldehyde dehyrogenase? What are its side effects?
COP II
Carbomyl phoshpate synthetase I
Endonucleases - exonucleases - DNA polymerase - DNA ligase
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover

14. What is the net production of glycolysis from one glucose?
Bacteria produce lactate acid and gases from lactose and produce acidic diarrhea and bloating
glucagon - insulin and epinephrine
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -

15. What can cause deficiency in Vit K?
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
Gets converted to pyruvate via the Cori cycle in the liver
Aromatic; musty body odor
Prolonged Ab use

16. What is the first step of collagen synthesis? 2nd step? 3rd step? 4th step? Where do all these steps occur?
LDL; LDL oxidize atherosclerosis
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
Tyrosine
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate

17. Where does synthesis of longer chain fatty acids occur? What does it require?
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
aldose reductase
SER and mitochondria; NADPH

18. fatty acid synthase requires ...
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
Central and peripheral demyelination with ataxia and dementia
NADPH- reductive biosynthesis!
Vomiting - rice water stools - garlic breath

19. All glutamate goes to _______ d ammonia is used as a buffer in urine
Vit C; Vit E
S - adenosyl methionine
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly
kidney

20. What are the findings in alkaptonuria? why do they occur?
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
RBCs - renal medulla - leukocytes - lens - testes - cornea
Dihydropterin reductase; NADP+; phenylalanine to tyrosine and tyrosine to DOP
Coenzyme A and lipoamides

21. Phosphorylated glycogen phosphorylase...
RBCs - renal medulla - leukocytes - lens - testes - cornea
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
FA coA synthetase combines coA with FA to make acyl coA
is active

22. What is the RNA called before processing? and after?
HnRNA; mRNA
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
RER in nucleus (synthesize NTs)
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+

23. ____________ disorders are worse than fructose disorders.
ATP and dATP
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
Galactose
alk phos

24. What is the rate determining enzyme in TCA cycle?
AD (trinucleotide expansion is not a mode of inheritance)
Isocitrate dehydrogenase
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
Fomepizole; antidote for methanol or ethylene glycol poisoning

25. What is imprinting?
Excreted from the kidney in the form of urea in hepatocytes
Calcium and calmodulin activate glycogen phosphorylase kinase
Von gierkes
When one allele on a gene is only expressed and the other is methylated/inactivated; during gametogenesis one of the alleles is methylated because the gamete from the other sex will provide those characteristics for the child

26. What is 'charging' of tRNA? What enzyme regulates this? What does this reaction require? in What process is this?
Complex I - Complex II (lower energy)
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation
TCA cycle - FA oxidation - acetyl coA production; ox phos
Phospholipids - sphingolipids - glycolipids

27. What is def of Vit E caused by?
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional
Tryptophan; niacin
linked recessive
Fat malabsorption (CF) and abetalipoproteinemia (damage takes up all antioxidants)

28. How can hyperammonia be hereditary? acquired?

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29. Where does catecholamine synthesis take place?
Biotin; cofactor for carboxylation; pyruvate carboxylase (turning pyruvate to oxaloacetate) - acetyl coA carboxylase (acetyl coA to malonyl Co A) - and proprionyl coA caroboxylase for (propionyl CoA to methylmalonyl coA)
VHL (Von Hippa Lindau)
Neural tissue and adrenal medulla
Phosphorylation of glucose to yield glucose 6 phosphate; ATP; first step of glycolysis

30. In What genetic code is uracil found in? in replace of what?
Uracil; thymine
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility
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
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly

31. What metabolic reactions occur in the cytoplasm?
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
Arginne
32 or 30; malate aspartate shuttle or glycerol 3 phosphate shuttle; heart/liver and muscle
Succinyl coA

32. What causes hypoglycemia in alcoholism?
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Thinned cornea from lack of connective tissue so see underlying choroidal lens
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination

33. How many RNA polymerases do prokaryotes have? What drug blocks prokaryotic RNA polymerase only? What is it used for?
1 for all three classes of RNA; Rifampin; TB
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Collections of abnormal mit collecting under sarcolemmal membranes and results in distortion of myofibrils
Cysteine; tyrosine

34. What are the last three steps of the respiratory burst? why do they need to occur? Where do they occur?
Phenylalanine build up
Glutathione peroxidase (catalase) reduces H202 to H20 with glutathione; oxidized glutathione gets reduced by glutathione reductase with NADPH; NAP+ is regenerated to NADPH with Glucose 6 phosphate dehydrogenase; because H2O2 from the resp burts in th
show disease
Also have fair skin - blond hair - blue eyes - eczema

35. What disease is associated with chromosome 17?
Arg and his
NF1
TRNA wobble
O2 consumption (last electron acceptor) gets consumed also

36. From Where is alk phos release in the bone? What enzyme does it act on? What is the result?
Release alk phos
Whether or not the patient is also hyperammonemia (if yes - then OTC def)
Inability to taste
Osteoblasts; de phosphorylates pyrophosphate Which inhibits bone mineralization; bone mineralization

37. What kind of DNA damage does ionizing radiation cause? What type of repair helps with this?
Double strand breaks; nonhomologous end joining
Alpha ketoglutarate; no TCA cycle intermediates
Ret
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12

38. With hypervitaminosis D you will see increased ______ - it causes bone turnover - and stimulates osteoblasts also
Glucagon - insulin and epinephrine
alk phos
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
Vit D and K

39. Ca2+ is an ___________ on pyruvate deyhdrogenase - this links glycolysis with muscle contraction
activator
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
Inhibits lipoic acid; lipoic acid is a cofactor for pyruvate dehydrogenase
Isocitrate dehydrogenase

40. What does fructose 6 phosphate do - depending On what?
Delayed wound healing - hypogonadism - decreased adult hair - dysgeusia - anosmia - and alcoholic cirrhosis - perioral rash
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
Phenylaline
be anorexic

41. How do you treat fructose intolerance?
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)
Decrease intake of fructose and sucrose (fructose and glucose)
Lactic acidosis and ketoacidosis
Each codon specifies only one amino acid

42. What is the findings in Fabrys disease?
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Lack of proper dietary therapy during pregnancy resulting in infant's microcephaly - mental retardation - growth retardation and congenital heart defects
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
only L form

43. What type of proteins have AD mode of inheritance?
4 -4 -9
Structural (constitutively active)
Pyruvate to lactate (results in lactic acidosis and decreased pyruvate for oxaloacetate for gluconeogenesis) - increased production of glycerol 3 phosphate from DHAP for TG synthesis; and increased betahydroxybutyrate ketoacid production from increas
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells

44. What are the findings for Pompe's disease? What is the deficient enzyme?
Biotin (vit B7) and Vit K
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Lime and CaCO3 to displace the niacin

45. Because electron transport continues...
RER in nucleus (synthesize NTs)
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio
O2 consumption (last electron acceptor) gets consumed also

46. Ocular albinism is...
1 gene has greater than 1 phenotypic effect; locus heterogeneity (many different mutations can cause the same phenotype); PKU; albinism - deafness - Marfanoid habitus (Marfans - MEN 2B - homocystinuria)
Collections of abnormal mit collecting under sarcolemmal membranes and results in distortion of myofibrils
linked recessive
Mebendazole and thiabendazole

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

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48. What is the most common cause of Osteogenesis Imperfecta? What is the inheritance pattern?
Acetyl CoA carboxylase
Type I collagen; AD
Adenosine deaminase deficiency; Severe combined immunodeficiency disease
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

49. What is a common result of unbalanced robertsonian translocation?
Chromosomal imbalance (downs for ex)
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes
Arg
RER; free ribosomes

50. other than in the kidney - Where else are gluconeogenesis enzymes found?
Increased nuchal translucency
Def in Vit C; def in Vit D
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
Kidney and intestinal epithelium