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USMLE Step 1 Biochemistry

Subjects : health-sciences, usmle-step-1

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Answer 50 questions in 15 minutes.
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1. What toxin causes liver failure by inhibiting RNA polymerase II if ingested? Where does it come from?
Alpha amanitin; mushroom caps
Cytosine to uracil
the RER
Mit encephalopathy - lactic acidosis - stroke like episodes

2. What is achondroplasia? What type of inheritance?
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD
Acetyl CoA carboxylase
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
Carnitine shuttle

3. What is most often mutated in xeroderma pigmentosum? What is the inheritance pattern? What are the clinical symptoms?
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs
only L form
Trimethroprim inhibits bacterial dihyrofolate reductase
UV specific endonucleases for excision repair; AR; dry skin - photosensitivity - poikiloderma - susceptibility to skin cancers

4. After the PCR reaction - How do you actually visualize the DNA?
An anti cancer agent that is a topoisomerase II and creates nicks in DNA
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)

5. What is the rate determining enzyme in gluconeogenesis?
Retinol; Beta carotenes (dark and yellow vegetables)
Increased concentration of Cl ions in sweat test
Phosphoenolpyruvate; pyruvate kinase; fructose 1 - 6 BP; ATP - alanine
Fructose 1 -6 bisphosphatase

6. in What organ is the storage form of D3 converted to its active form? What enzyme? What hormone stimulates this action?
Mental retardation - excessive laughter - seizures - ataxia - 'happy puppet'
Kidney; 1 alpha hydroxylase; Parathyroid hormone
Neurons and RBCs
glycogen

7. What carries 1 carbon units as energy?
Cobalamin; cofactor for homocysteine methyltransferase (transfers SAM to THF to give to dUMP) and for methylmalonyl coA mutaseto make succinyl coA
Pyruvate dehydrogenase; ATP - NADH - acetyl CoA
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
Tetrahydrofolates

8. Phosphorylated glycogen phosphorylase...
Dihydrofolate reductase; vitamin B12; homocysteine
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD
is active
NADH - NADPH - FADH2

9. The ________ is 50% cholesterol - 50% phoshpolipids
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
Hemorrhage with increased PT and PTT time but normal bleeding time
I; nucleolus
plasma membrane

10. What does the excess NH4+ in hyperammonia eat up? What is the result?
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid
Alpha ketoglutarate; no TCA cycle intermediates
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
NADPH- reductive biosynthesis!

11. What is PRPP? How is it made?
Hungtington
Inhibits lipoic acid; lipoic acid is a cofactor for pyruvate dehydrogenase
Tryptophan; niacin
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt

12. Which TCA cycle enzyme is found in the inner mit membrane? Where are the rest found?
Central and peripheral demyelination with ataxia and dementia
Acetyl coA carboxylase converting Acetyl coA into malonyl coA; increased AMP - glucagon - epinephrine - palmitate; citrate
Succinate hydrogenase; matrix
Cheilosis and corneal vascularization

13. What is the first enzyme step in glycogenolysis? What activates it ? inactivates?
Asp and glu; negatively
Glycogen phosphorylase; phosphorylation and dephosphorylation
COP I
XR - iduronate sulfatase

14. What is the mc chromosomal disorder?
Riboflavin makes up FAD and FMN both used in redox reactions; vegans
Downs
Make it stiffer/more tensile; continues as we get older so gets less and less elastic
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

15. At What point is the amino acid correspondence to the mRNA code finalized?
Type I collagen; AD
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
Formation of a proton gradient that is then used to create ATP through ATP synthase

16. What is the deficient enzyme in Gauchers disease? accumulated Substrate?
Beta glucocerebrosidase; glucocerebroside
Mucus secreting goblet cells - ab secreting plasma cells - nt secreting neurons
Glucose 6 phosphate dehydrogenase
Trimethroprim inhibits bacterial dihyrofolate reductase

17. Which oxidant is most attributable to damage of RBCs?
paternal
LDL; LDL oxidize atherosclerosis
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables
Increased TG synthesis from increased Glycerol 3 P and abetalipoproteinemia

18. How do WBCs of CGD patients do some damage?
They use the H2O2 produced by some bacteria to produce bleach with myeloperoxidase Which they still have
S adenosyl homocysteine; homocysteine transferase; B12 and THF; M- THF
To amplify a specific desired fragment of DNA
RER; free ribosomes

19. AD disease...
Familial Adenomatous Polyposis (APC gene)
Tyrosine
enzymatic
usually present late

20. No offspring of affected males with a mit disease can...
show disease
Cycloheximide
Thymidylate synthase; 5- Flourouracil
kidney

21. NAD+ is used in catabolic processes and NADPH is used in...
Kidney and intestinal epithelium
Cystic Fibrosis (CFTR gene)
anabolic processes
succinyl coA (and then to OXA)

22. Which cells are rich in SER?
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
2 -4 dinitrophenol - aspirin - and thermogenin (in brown fat)
Alcoholism - RA - inflammatory disease - chronic diarrhea

23. Do RNA polymerases have proofreading function? What does this mean for HIV?
Mental retardation - rocker bottom feet - micropthalmia - microcephaly - cleft lip/palate - holoprosoncephaly - polydactyly
When different combination of exons get combined together; in Beta thallasemia
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis
No; increased mutation rate!

24. What are three ketone bodies? Which is not detected in urine? Which do you see most in alcoholic ketoacidosis?
S. aureus - Aspergillus
Carnitine shuttle
No
B hydroxybutyrate - acetone - acetoacetate; beta hydroxybutyrate; beta hydroxybutyrate

25. To What type of DNA damage are xeroderma pigmentation susceptible to?
Type I collagen; AD
VHL (Von Hippa Lindau)
Calcium and calmodulin activate glycogen phosphorylase kinase
UV nonionizing radiation causing thymidine dimers

26. Other than ragged red fibers and lactic acidosis - What else is common in mit myopathies?
Mitochondrial inheritance
normal lactate levels
Neurologic lesions and cardiomyopathies (ox phos!)
golgi apparatus

27. What is the mode of inheritance of Huntingtons?
Glutamate and aspartate; CO2 provides an ketone
AD (trinucleotide expansion is not a mode of inheritance)
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?

28. What is the deficient enzyme in Niemann Pick disease? accumulated Substrate?
Sphingomyelinase; sphingomyelin
calorie deficient
Leucine and lysine
Decreased phenylalanine - increase tyrosine

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

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30. 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?
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
All and Alzheimers
FA coA synthetase combines coA with FA to make acyl coA
N5N10 methylene THF

31. What is the difference between exonucleases and endonucleases?
Colchine
Endonucleases are specific and break specific linking phosphodiester bonds - exonucleases are not sequence specific and just remove the nucleoside
Make it stiffer/more tensile; continues as we get older so gets less and less elastic
ATP

32. _____________ prevents fluidity of plasma membrane
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine
Inability to transport LCFA into mitochondria results in their build up and causes weakness - hypotonia And HYPOKETOTIC HYPOGLYCEMIA
high cholesterol
VHL (Von Hippa Lindau)

33. What are the findings in Metachromatic leukodystrophy?
COP II
A 16C FA! the product in FA synthesis; saturated; no double bonds
Central and peripheral demyelination with ataxia and dementia
Hemolytic anemia; jaundice in newborn if mom gets too much vit K

34. What are the two main function of Vit E?
Helps make Vit K and antioxidant
ATP and biotin
Branche aa alpha ketoacid dehydrogenase; thiamine; Isoleucine - leucine - valine
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding

35. Name two proapoptotic/tumor suppressor genes. How do they each act?
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
Prevents neural tube defects in 1st trimester
Glycolysis - fatty acid synthesis - HMP shunt - protein synthesis (RER) - steroid synthesis (SER) - glycogenolysis - glycogenesis

36. Heterozygous females in X linked recessive disease...
Very Poor Carb Metabolism (Von Gierke - Type 1 - Pompes - Type II - Cori's - Type III - McArdles disease - Type IV)
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate
Heme synthesis - urea cycle - gluconeogenesis
can be d/t lyonization

37. What is scurvy caused by? rickets?
Cycloheximide
Leucine and lysine
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
Def in Vit C; def in Vit D

38. How does glucagon activate glycogenolysis? What regulator does similar?
Downs
DNA dependent RNA polymerase
A folic acid analogue
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine

39. Breast milk has a little of...
be anorexic
Vit D and K
in hepatocytes
All clathrin

40. What are the mucopolysaccharidoses? Which one is XR? AR? What accumulates in them?
Marfans - homocystinuria - MEN 2B
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Glucose 6 phosphatase
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate

41. Glucokinase has a LOW affinity (high Km) and...
high Vmax
Homogentistic acid oxidase
Phenylalanine
Glutamate

42. What tissues contain aldose reductase? What does this enzyme do to what?
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Lens and neural tissue; converts galactose to osmotically active galactitol
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
XR - iduronate sulfatase

43. Name two diseases that have to do with elastin; discuss their pathogenesis.
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
Marfans and emphysema; marfans is a fibrillin protein defect - fibrillin is a supporting protein in tropoelastin scaffolding and results in aorta - skeletal and eye defects; emphysema is due to a deficiency in the function of alpha 1 antitrypsin
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
Catalyze gamma carboxylation of glutamic acid residues on coag factors so they can bind to Ca complex

44. What is the main use of glycogen?
Increased CPK and muscle biopsy
3'--> 5'; prokaryote only
Phenylalanine build up
Reserve supply of glucose in the fasting state - liver gives to the blood and skeletal muscle uses for itself

45. Which is the most common lysosomal storage disease? What are its findings?
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
glutton
Glucose 6 phosphate dehydrogenase
Galactose gets converted to galactose 1 Phosphate by galactokinase Which then gets turned into glucose 1 phosphate by Galactose 1 Uridyl Transferase (GALT) Which also produces UDP Gal Which is used for lactose production in the breast; the fate of gl

46. What does degenerate/redundant mean in terms of the genetic code?
Pellagra
Codons may encode same amino acid since there are only ~20 aa
Self mutilation - aggression - mental retardation - gout - hyperuricemia - choreathetosis
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -

47. What is the difference between rickets and osteomalacia?
Beckers is less severe (instead of deletion is either defect or deficiency) with later presentation
Cystic Fibrosis (CFTR gene)
Osteomalacia is vit D def seen in adults and causes bowed legs and results in increased fractures
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

48. What control over pyruvate kinase does insulin have?
It induces its action!
enzymatic
Type IV collagen defect; X linked recessive; hereditary glomerulonephritis - deafness - ocular disturbances
Phenylalanine hydroxylase or tetrahydrobiopterin factor

49. Phenylethanolamine N- methyl is only located in the...
Schilling test
AR
adrenal medulla
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50

50. What does a pyridoxine deficiency look like? What causes it?
Asp and glu; negatively
Aldolase B; Fructose 1 phosphate accumulates Which uses up Phosphate and the result is inhibition of gluconeogenesis without phosphate; hypoglycemia - jaundice - cirrhosis - vomitting
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption
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

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USMLE Step 1 Biochemistry

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