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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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1. What disease is associated with chromosome 16?
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
Edwards
Presence of normal and mutated mtDNA (like mosaicism except in mit)
APCKD

2. After citrulline is produced and in the cytoplasm What happens next in the urea cycle?
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
glucagon - insulin and epinephrine
Metabolizes them with transferase enzymes to acetyl coA
Heterochromatin; euchromatin

3. What is the difference between exonucleases and endonucleases?
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside
Dopamine
Neurologic defects - myopathy - lactic acidosis; giving ONLY ketogenic nutrient high fat content and aa (lysine and leucine)
Phosphorylation of glucose

4. What is lebers hereditary optic neuropathy? What is the main clinical symptom? and What is its mode of inheritance?
ATP depletion and highly aerobic tissues (heart and brain) are affected first; false - glucose infusion worsens
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Tryptophan; B6
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance

5. What are the four fates of pyruvate metabolism?
Phenylalanine build up
Actin/myosin - MT - intermediate filaments
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD
Pyruvate to alanine (to carry amino groups to liver) - pyruvate to OAA for gluconeogenesis (or replenish for TCA cycle) - acetyl coA (for TCA cycle transition or FA or chol. synthesis) - or the end of anaerobic glycolysis

6. What is another way to trap glucose in cells but not by phosphorylating it? What enzymes are used?
Make it stiffer/more tensile; continues as we get older so gets less and less elastic
Glycogen phosphorylase; phosphorylation and dephosphorylation
It induces its action!
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase

7. What is cystinuria due to?
Phosphorylation of glucose
Tyrosine; tryptophan
Acetyl coA and NADH inhibit pyruvate dehyrogenase
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes

8. What does the brain do with ketone bodies?
Hydroxurea; deoxygenates ribose
Metabolizes them with transferase enzymes to acetyl coA
Ret
A 16C FA! the product in FA synthesis; saturated; no double bonds

9. What carries electrons (as energy)?
NADH - NADPH - FADH2
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
Hydroxurea; deoxygenates ribose
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy

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

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11. What enzyme is deficient in galactokinase deficiency? What are the symptoms?
Hemorrhage with increased PT and PTT time but normal bleeding time
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Dopamine
Calcium and calmodulin activate glycogen phosphorylase kinase

12. What is the presentation for cystinuria?
Lesch Nyhan syndrome; X linked recessive
It induces its action!
Cystine kidney stones (cystine staghorn calculi)
Highly condensed forms of DNA; mitosis; neoplasia - highly dividing and thus hyperchromatic

13. How do you differentiate the causes of orotic aciduria?
Whether or not the patient is also hyperammonemia (if yes - then OTC def)
Helps make Vit K and antioxidant
Vit C
BM; laminins; most flexible; different aa composition

14. What is the other name of Vitamin B6? What is its function?
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
Cystic Fibrosis (CFTR gene)
Marfans - homocystinuria - MEN 2B
reabsorbs

15. What are the functions of Microtubule proteins?
Isocitrate dehydrogenase
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Type I collagen; AD
all are diseased!

16. What are three ways to poison Ox phos?
Prolonged Ab use
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis
Lactating mammary glands - adrenal cortex (sites of fatty acid or steroid synthesis) - RBCS (for glutathione reduction for FR injury)

17. What are the resp. clinical symptoms of CF?
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract
Tay Sachs - Niemann Picks - Gauchers
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase

18. What toxins prevent elongation in translation in humans (by binding 40s)?
Homocystinuria (homocysteine damages endothelial cells); alkaptonuria (tyrosine degradation); homocystinuria
DHF
Shiga toxin and ricin (protein in castor beans)
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)

19. What are the two molecular proteins that are used on MT for transporting in neurons? Which direction do they go?
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Dynein - retrograde - kinesin - anterograde
Heme synthesis - urea cycle - gluconeogenesis
Thick mucus plugs lungs recurrent pulmonary infections (pseudomonas and aureus) - chronic bronchitis - bronchiectasis (which can lead to cor pulmonale!); biofilms form on resp tract

20. What explains the fatty change in kwashiokor?
Renal calculi of uric acid (replaces uric acid in excretion)
Abetalipoproteinemia and increased G3P production from all the carbs!
In ability to reabsorb phosphate from renal tubular cells results in osteomalacia due to defective bone mineralization; no
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons

21. in Fabrys - What accumulates in tissue because fibroblasts express the def enzyme highly?
Ceramide trihexoside
Leucine and lysine
Hexosaminidase A; GM2 ganglioside
Downs (21) - Edwards (18) - Pataus (13); also in increasing severity

22. How is muscular dystrophies diagnosed?
Methionine is only encoded by AUG
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
Lys and arg
Increased CPK and muscle biopsy

23. What disease is associated with chromosome 18?
Edwards
TRNA wobble
Glycogen phosphorylase; phosphorylation and dephosphorylation
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia

24. What are the functions of zinc?
Gets converted to pyruvate via the Cori cycle in the liver
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
Thymidylate synthase; 5- Flourouracil
Nucleus

25. What are two examples of X linked Autosomal Dominant disease?
Hypophosphatemia rickets and Alports
Prolonged Ab use
Lactic acidosis and ketoacidosis
Dopamine beta hydroxylase; vit C

26. What does ultrasound show in Downs?
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
NF1
Glycogen synthase
Increased nuchal translucency

27. Where is type III collagen found?
NADPH oxidase; CGD
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)
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)

28. What is the treatment for cystinuria?
Acetazolamide to alkalinize the urine
Biotin
UV nonionizing radiation causing thymidine dimers
Increased NADH causes lactate production and pyruvate dehydrogenase def causes increased lactate (d/t thiamine def that often occurs with alcoholics)

29. What is rickets? What is it caused by?
Galactocerebrosidase; Galactocerebroside
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol
Structural (constitutively active)
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary

30. What are the glucogenic amino acids?
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
In hepatocytes; B6
Met - arg - val - his

31. why is there hepatosplenomegaly in many of the lysosomal storage diseases?
Liver; P450 hydroxylation
Because cells of mononuclear phagocytic are esp rich in lysosomes
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
Lens and neural tissue; converts galactose to osmotically active galactitol

32. What converts dopa to dopamine? using What cofactor?
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
only L form
Dopa decarboxylase; B6
NADH - NADPH - FADH2

33. No offspring of affected males with a mit disease can...
show disease
AD
BM; laminins; most flexible; different aa composition
Tyrosine

34. What is bleomycin?
An anti cancer agent that is a topoisomerase II and creates nicks in DNA
SAM
ATP
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

35. What is the storage form of vit D? active form?
Synthesis in liver and then hydrolysis of chylomicrons and VLDLs by capillary lipoprotein lipase in peripheral tissue; adipose tissue by lipases
Galactokinase deficiency and classic galactosemia; classic galactosemia
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones
25 OH D3; 1 -25 (OH)2 D3 (Calcitriol)

36. Which cells have the most FA and thus are the most susceptible to FR injury?
Familial Adenomatous Polyposis (APC gene)
Neurons and RBCs
No
degradation

37. How is Vit K activated? What drug blocks this? What is the source of Vit K?
Pyruvate carboxylase (pyruvate to OXA) (mit) - PEP carboxykinase (OXA to PEP) (cyt) - Fructose 1 -6 bisphophatase (fructose 1 -6 bisphosphate to fructose 6 P) (cyt) and glucose 6 phosphatase (in ER); glucose 6 phosphatase
allosteric regulation
Increased acid production competes with uric acid for excretion
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables

38. What is the link between adenosine deaminase deficiency and decreased lymphocyte count?
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)

39. Other than ragged red fibers and lactic acidosis - What else is common in mit myopathies?
H1
Tyrosine; phenylaline
Neurologic lesions and cardiomyopathies (ox phos!)
Pyruvate carboxylase (pyruvate to OXA) (mit) - PEP carboxykinase (OXA to PEP) (cyt) - Fructose 1 -6 bisphophatase (fructose 1 -6 bisphosphate to fructose 6 P) (cyt) and glucose 6 phosphatase (in ER); glucose 6 phosphatase

40. What is the rate limiting step in fatty acid synthesis? What four things inhibit this rate limiting step? What stimulates it?
plasma membrane
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
Increased acid production competes with uric acid for excretion
Shiga toxin and ricin (protein in castor beans)

41. What is the rate determining enzyme in gluconeogenesis?
Lysine and arginine; basic
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
Fructose 1 -6 bisphosphatase
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )

42. What are the types of lipids found in the plasma membrane?
Phospholipids - sphingolipids - glycolipids
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)

43. What two processes maintain the plasma glucose levels?
Glycolysis and aerobic respiration
Tay Sachs - Niemann Picks - Gauchers
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
When child gets two chromosomes from one parent; imprinting dx

44. Daughters of Xlinked recessive diseased fathers are...
Osmotic diarrhea - bloating - cramps
all carriers
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine
A methyl

45. From What aa does thyroxine come from?
Carnitine shuttle
Chloramphenicol
Fe2+; Fe3+
Tyrosine

46. Only the tyrosinase one is...
Arg
By addition of mannose 6 phosphate to proteins in the Golgi
Down's Syndrome
AR

47. All the glycogen storage diseases...
An anti cancer agent that is a topoisomerase II and creates nicks in DNA
GTP: gluconeogenesis - OXA to phosphoenolpyruvate
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol
are AR

48. The mode of inheritance for both NF disease is...
Both AD
Cytosine to uracil
Williams
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine

49. Walk through the process that adipose tissue makes triacylglycerol.
Williams
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
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts
Lipoprotein lipases take FA from VLDL and chylomicrons Which are then combined with glycerol 3 P (a carb intermediate) to make triacylglycerol

50. What is the result of lactase deficiency? in What nationalities is it seen the most?
1. denature by heating 2. during cooling anneal with DNA primers 3. elongation - add heat stable DNA polymerase 4. repeat
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
Lack of lactase (brush border enzyme) so can not break down lactose (a dissaccharide of galactose and glucose); African Americans and Asians