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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 antigout drugs act on MT?
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine
Smoking (FR takes up all the antioxidants); diets lacking fruit
Alcoholism - RA - inflammatory disease - chronic diarrhea
Colchine

2. What carries 1 carbon units as energy?
can be d/t lyonization
G6PD dehydrogenase; transketolases
Glucosekinase to glucose 6 P
Tetrahydrofolates

3. What are permanent cells? Which cells are permanent?
5' end; 3' end
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
Metabolizes them with transferase enzymes to acetyl coA
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs

4. What is significant about 50S/60S ribosomal unit?
Hemolytic anemia (fragility of RBCs) - muscle weakness and posterior column and spinocerebellar tract demyelination (neuron damage) resulting in decreased proprioception and vibration sensation - and peripheral neuropathy - and retinal degeneration
It contains the peptidyltransferase activity (for elongation of protein
Block ATP synthase resulting in no ATP production and an increased proton gradient
fastest sugar

5. name three glucogenic AA and how they enter gluconeogenesis. through What cycle do they enter gluconeogenesis?
Increased nuchal translucency
NADPH- reductive biosynthesis!
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA

6. What carries activated Phosphate groups?
Carbomyl phosphate synthetase I turns N acetyl glutamate into carbomyl phosphate and then ornithine transcarbomyalse turns carbomyl phosphate into citrulline (using ornithine)
ATP
NF2 - DiGeorge/Velocardiofacial syndromes
Aminoglycosides and tetracyclines

7. What aa is most accumulated in hartnup disease? What vitamin becomes deficient?
Tryptophan; niacin
Isoleucine - valine - leucine
glycogenolysis
Galactose 1 phosphate uridyltransferase (GALT) results in an accumulation of toxic substances; failure to thrive - jaundice - hepatomegaly - infantile cataracts - mental retardation; exclude galactose and lactose from diet

8. What is the primary source of nitrogen in the urea cycle
Arg - lys - his; positive except His has no charge at body pH
Glutamate
Citrulline is combined with aspartate to make arginosuccinate Which is then split into fumarate (which leaves to enter TCA) and arginine. Arginine is then converted to Ornithine (via arginase) giving off Urea Which then goes to the kidney and ornithi
NF2 - DiGeorge/Velocardiofacial syndromes

9. why is there decreased immunity in kwashiokor and marasmus?
No protein!
Neimann pick and tay sachs
Oxaloacetate gets shifted to malate d/t excess NADH
Silent - misssense - nonsense

10. What ensures that during gluconeogenesis all the ATP wont be used in glycolysis as youre producing it?
Inosine/IMP; adenosine deaminase
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase
Double stranded RNA is created that is complementary to the mRNA of interest and When inserted into cells the strands separate and attach to the mRNA preventing it from being translated
CPSII

11. To What type of DNA damage are xeroderma pigmentation susceptible to?
Mitochondria - archaebacteria - mycoplasma - and some yeasts
AD
UV nonionizing radiation causing thymidine dimers
25 OH D3; 1 -25 (OH)2 D3 (Calcitriol)

12. What is a robertsonian translocation? What does it mean if its balanced?
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Fabrys Disease and Hunters Syndrome
Beta glucocerebrosidase; glucocerebroside
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional

13. Where is anaerobic glycolysis used often?
5' end; 3' end
Carbamoyl phosphate to orotic acid
RBCs - renal medulla - leukocytes - lens - testes - cornea
6MP blocks de novo purine synthesis

14. What are ragged red fibers?
Block ATP synthase resulting in no ATP production and an increased proton gradient
4 -4 -9
ATP and dATP
Collections of abnormal mit collecting under sarcolemmal membranes and results in distortion of myofibrils

15. Which amino acids are required during periods of growth?
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
Arg and his
AD
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)

16. What is the rate determining enzyme in steroid synthesis? cholesterol synthesis?
activator
Decrease intake of fructose and sucrose (fructose and glucose)
HMG CoA synthetase; HMG CoA reductase
Sickle cell disease - conversion of hydroophillic glutamic acid to hyrdophobic valine

17. What is the rate determining enzyme in galactose metabolism?
GALT
Cortisol; inhibits insulin action
7 dehydrocholesterol
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy

18. What aa does AUG mRNA code for in eukaryotes? prokaryotes?
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Methionine; f Met (formyl methyl methionine)
Chorea and spastic paresis
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. What is the degradation product of Norepinephrine?
VMA
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Alcoholism - RA - inflammatory disease - chronic diarrhea
Phenylaline

20. Where are secreted proteins synthesized? Where are cellular/organellar proteins synthesized?
RER; free ribosomes
osteosarcomas (Rb mutation is in all cells)
Uses HMG coA reductase
is active

21. How does insulin both deactivate glycogenolysis and activates glycogenesis at same time?
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
Glucokinase
Carbamoyl phosphate
Collagen; fibroblasts; 4; ECM

22. How does lactate contribute to gluconeogenesis?
Calcium and calmodulin activate glycogen phosphorylase kinase
HnRNA; mRNA
Gets converted to pyruvate via the Cori cycle in the liver
6MP blocks de novo purine synthesis

23. From What aa does thyroxine come from?
Lysine and leucine
all carriers
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
Tyrosine

24. What is the RNA called before processing? and after?
No protein!
Mental retardation - excessive laughter - seizures - ataxia - 'happy puppet'
Pernicious anemia; older - mentally slow european woman who is anemic and icteric with a smooth shiny tongue and has broad based shuffling gait
HnRNA; mRNA

25. What is the rate determining enzyme in fructose metabolism?
Cartilage (including hyaline) - vitreous body - nucleus pulposus
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid
Aldolase B
Block ATP synthase resulting in no ATP production and an increased proton gradient

26. What is the difference between a nucleotide and a nucleoside?
I; nucleolus
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond
Phenylalanine build up
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD

27. What enzyme is involved in the degradation of tyrosine to fumarate?
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs
Homogentistic acid oxidase
Aldolase B
Aminoglycosides and tetracyclines

28. What form of Vitamin D is in plants? milk fortified with vit D? formed in sun exposed skin?
allosteric regulation
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
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
AD

29. What is an ELISA? What are the two ways to perform it? What is the sensitive and specificity? give a relative clinical example.
Tyrosine; phenylaline
Testing for antigen - antibody reactivity; can either add an antibody to a persons blood sample to see if there is binding or add an antigen to a persons blood sample to see if there immune system recognizes it; close to 100% for both; HIV eliza - an
Chloramphenicol
Some viruses

30. How is entry into cell cycle phases regulated?
Phosphorylation of glucose to yield glucose 6 phosphate; ATP; first step of glycolysis
By cyclin CDK complexes
AR
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count

31. What are the symptoms involved in lactase deficiency?
Osmotic diarrhea - bloating - cramps
liver
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine
Mitochondria - archaebacteria - mycoplasma - and some yeasts

32. What enzyme is used in the oxidative phase of HMP shunt? nonoxidative?
FA coA synthetase combines coA with FA to make acyl coA
G6PD dehydrogenase; transketolases
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan

33. How does an enzyme know to go to lysosome?
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
By addition of mannose 6 phosphate to proteins in the Golgi
Glutamate
Branched alpha ketoacid dehydrogenase

34. Where is type IV collagen found? to What protein is bound? What is special about this type? How do the different types of collagen differ?
Biotin (vit B7) and Vit K
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis
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)
BM; laminins; most flexible; different aa composition

35. What is imprinting disease? Give 2 examples
When the disease manifestation depends on from who the gene deletion was inherited from d/t methylation that occurs; Prader Willi and AngelMann
Peripheral neuropathy - developmental delay - optic atrophy - globoid cells
Uses HMG coA reductase
Liver; P450 hydroxylation

36. What explains the excess phenylketones in the urine in phenylketonuria?
Phenylalanine build up
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)
G6PD; provides malarial resistance; X linked recessive
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles

37. other than being returned to methionine - What else is the fate of homocysteine? through What enzyme? with What cofactor?
Cystic Fibrosis (CFTR gene)
Cystathionine (which then goes to cysteine) via cystathionine reductase; B6
Lys and arg
Collagen; fibroblasts; 4; ECM

38. Why do mitochondrial diseases exhibit variable expression?
Dopamine beta hydroxylase; vit C
Acetazolamide to alkalinize the urine
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
Heteroplasmy

39. What two enzymes are in involved in catecholamine degradation?
N linked oligosaccharides
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
Mitosis then Interphase (G1 - S - G2); in G1 is presynthesis phase - S is synthesis phase (DNA rep. etc) - G2 is premitotic (making of MT spindle etc)
COMT and MAO

40. By What products are gluconeogenesis and glycolysis linked so that they are not both occurirng at the same time?
Dopamine
Orotic aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
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
Steroid synthesis and detoxification

41. What are the symptoms of B2 deficiency?
Cheilosis and corneal vascularization
3 hydrogen bonds; 2 hydrogen bonds
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Dermatitis - glossitis - diarrhea

42. What liver enzymes are increased in alcoholism? why?
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Cu; hydroxylation
Defective neutral amino acid transporter on renal and intestinal cells; AR
Lime and CaCO3 to displace the niacin

43. What are the findings in ornthinie transcarbamoylase?
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Bacterial synthesis in the intestine
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
When child gets two chromosomes from one parent; imprinting dx

44. How does a kwashiokor patient present?
Dihydrofolate reductase; vitamin B12; homocysteine
Night blindness - dry skin
Malnutrition - edema (hypoalbuminemia) - anemia liver (fatty change) - diarrhea d/t no brush border enzymes (no protein!)
homocysteine transferase

45. What is the precursor for pyrimidines?
Assist in spontaneous refolding of proteins
Leucine and lysine
Increased blood levels of ammonia - decreased BUN
Orotate

46. What causes the lysosomal storage diseases?
Block ATP synthase resulting in no ATP production and an increased proton gradient
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products
tubulin dimer
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)

47. What converts dUMP to dTMP? What drug blocks this?
Citrulline is combined with aspartate to make arginosuccinate Which is then split into fumarate (which leaves to enter TCA) and arginine. Arginine is then converted to Ornithine (via arginase) giving off Urea Which then goes to the kidney and ornithi
Thymidylate synthase; 5- Flourouracil
Niacin treatment for hyperlipidemia; facial flushing
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond

48. What does methotrexate do? What step is this enzyme involved in?
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
Osmotic diarrhea - bloating - cramps
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness

49. What does severe deficiency of B3 lead to? What are the causes?
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
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)
Tyrosine; tryptophan
Glucagon - insulin and epinephrine

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?
NF2 - DiGeorge/Velocardiofacial syndromes
Ornithine transcarbomyalse; orotic aciduria; X linked recessive; autosomal recessive
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
catecholamine synthesis (dopamine to norepinephrine)