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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. Fructose 2 -6 bisphophate is involved in...
Smoking (FR takes up all the antioxidants); diets lacking fruit
Ouabain
allosteric regulation
can be d/t lyonization

2. What is a common result of unbalanced robertsonian translocation?
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
homocysteine transferase
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)
Chromosomal imbalance (downs for ex)

3. What is used in a marathon?
Coenzyme A and lipoamides
Dark connective tissue - brown pigmented sclera - debilitating athralgias; homogentistic acid builds up and can be toxic to cartilage
UV nonionizing radiation causing thymidine dimers
Glycogen and FFA oxidation - glucose conserved for final sprinting!

4. What control over pyruvate kinase does insulin have?
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
Retinol; Beta carotenes (dark and yellow vegetables)
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
It induces its action!

5. What is the inheritance pattern of hereditary spherocytosis?
linked recessive
Block Electron transport
AD
Hydroxurea; deoxygenates ribose

6. What causes scurvy? What are the symptoms of scurvy?
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
Hungtington
Hexosaminidase A; GM2 ganglioside
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)

7. What is acyl coA dehydrogenase used in? What does a deficiency in this enzyme produce?
Prolonged Ab use
Fatty acid (acyl coA) oxidation to acetyl coA; increase in dicarboxylic acids - and decreased glucose and ketones
Reticulin - skin - blood vessels - uterus - fetal tissue - early phase of granulation tissue
Later

8. What four enzymes are required for nucleotide excision repair?
Succinate hydrogenase; matrix
4 -4 -9
Central and peripheral demyelination with ataxia and dementia
Endonucleases - exonucleases - DNA polymerase - DNA ligase

9. What is the mode of inheritance for the multiple endocrine neoplasias?
III
Cytoplasmic side; active
AD
HMG CoA reductase

10. What are the general symptoms of 22q11 chromosomal deletion? why is it that these occur?
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
Carbidopa
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
anabolic processes

11. What does the Cre - lox system allow in Gene expression modifications?
Can inducibly manipulate genes at specific developmental points
Niacin/Nicotinic acid; NAD+ and NADH used in redox reactions - and catabolic and anabolic reactions; B6; tryptophan
Beta glucocerebrosidase; glucocerebroside
AD: yes! most common presentation; very rare (700 mg/dl cholesterol level!)

12. What is the treatment for phenylketonuria?
Fe2+; Fe3+
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary
Increased CPK and muscle biopsy
Decreased phenylalanine - increase tyrosine

13. Which syndrome results from microtubule polymerization defect? What are the clinical features?
Amniocentesis karyotyping
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
They use the H2O2 produced by some bacteria to produce bleach with myeloperoxidase Which they still have
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine

14. What aa does melanin come from? melatonin?
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Prevents neural tube defects in 1st trimester
Tyrosine; tryptophan
normal lactate levels

15. What is RNAi? and How is it used in gene expression modification?
Essential fructosuria and fructose intolerance; fructose intolerance
AR
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
III

16. What are the symptoms of cri du chat syndrome? What is the pathogenesis?
Microcephaly - mental retardation - high pitched mewing/crying - epicanthal folds - cardiac abnormalities; microdeletion on short arm of chrom 5
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease
Alkaptonuria; ochronosis

17. The ________ is 50% cholesterol - 50% phoshpolipids
Tetrahydrofolates
HMG CoA reductase; converts HMG CoA to mevalonate
plasma membrane
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

18. Which is the most common lysosomal storage disease? What are its findings?
Asp and glu; negatively
alk phos
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
3 hydrogen bonds; 2 hydrogen bonds

19. 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
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products
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

20. What is heteroplasmy?
Presence of normal and mutated mtDNA (like mosaicism except in mit)
Orotic acid in blood and urine (from pyrimidine synthesis) - decreased BUN - and symptoms of hyperammonemia
Arginine
Convulsions - hyperirritiability - peripheral neuropathy - sideroblastic anemias; alcoholics and INH and fortified goats milk consumption

21. What regulates Fructose 1 -6 bisphosphatase? What reaction is it found in?
Becomes methionine
Exons
Citrate is positive on it - AMP and fructose 2 -6 bisphosphate is negative on it; gluconeogenesis
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis

22. What does adipose tissue contribute for hepatic gluconeogenesis during starvation? muscle?
ATP synthase - ATP- ADP translocase - ETC
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Glycerol and propionyl coA; lactate and alanine
Increased blood levels of ammonia - decreased BUN

23. What is the first step in fatty acid synthesis? Where does it occur? Where does triacylglycerol synthesis occur?
Citrate shuttle in the inner mitochondrial membrane transports acetyl coA into the cytoplasm; liver; liver and adipose tissue
Lupus
Tyrosinase deficiency; defective tyrosine transporters; lack of migration of neural crest cells; skin cancer
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism

24. What converts dopamine to norepinephrine? using What cofactor?
Dopamine beta hydroxylase; vit C
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
Microvilli - muscle contraction - cytokinesis - adherens junctions
Yes! congenital simply means 'born with'- hereditary is familial

25. What is the order of collagen synthesis - just the terms of the intermediate forms?
Glycerol goes to liver for gluconeogenesis; free fatty acids bind to albumin in blood and travel to cells to be used as for energy
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
Preprocollagen - procollagen - tropocollagen - collagen
Actin/myosin - MT - intermediate filaments

26. How many stages are there in HMP shunt? Where do they occur? how much energy does HMP shunt use up?
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
Conversion to UDP glucose via UDP glucose pyrophosphorylase; glycogen synthase (rate limiting step )
Phenylalanine hydroxylase or tetrahydrobiopterin factor
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid

27. Which amino acids are required during periods of growth?
Because cells of mononuclear phagocytic are esp rich in lysosomes
No protein!
VMA
Arg and his

28. What is the deficient enzyme in Tay Sachs disease? accumulated Substrate?
Hexosaminidase A; GM2 ganglioside
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
Chloramphenicol
G6PD dehydrogenase; transketolases

29. What are lisch nodules?
Tay Sachs - Niemann Picks - Gauchers
Pigmented iris hamartomas
Endonucleases - exonucleases - DNA polymerase - DNA ligase
show disease

30. What are the lab findings see in hyperammonemia?
APCKD
30S + 50S= 70S
Increased blood levels of ammonia - decreased BUN
Decreased would healing; microcytic anemia (d/t decreased iron [not absorbable]; macrocytic anemia (d/t Fh4 not reduced)

31. For What three enzymes is arginine a precursor for?
is active
Pataus
Nitric Oxide - creatinine - Urea
RER in nucleus (synthesize NTs)

32. _______ can not get inside cells
fructose
False; GTP hydrolysis
glutton
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -

33. What are two reasons why pyruvate would be pushed to lactate in an alcoholic?
Increased NADH causes lactate production and pyruvate dehydrogenase def causes increased lactate (d/t thiamine def that often occurs with alcoholics)
HMG CoA synthetase; HMG CoA reductase
Neurons and RBCs
Decreased substrate for gluconeogenesis (pyruvate to lactate and not oxaloacetate); and pyruvate doing only lactic acidosis and not glycolysis

34. What is the net production of glycolysis from one glucose?
Glutamine - aspartate - glycine; aspartate
2 NADH - 2 ATP - 2 pyruvate - 2H+ - 2H20
SnRNPs; RNA polymerase III
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth

35. What is the pathogenesis of fragile x syndrome?
RBCs - renal medulla - leukocytes - lens - testes - cornea
A trinucleotide repeat disorder results in faulty methylation and thus expression of the FMR1 gene on the X chromosome (x linked recessive mode of inheritance)
SnRNPs; RNA polymerase III
Vit K; neonates have sterile intestine and can not synthesize vit K

36. If a cell has a hyperchromatic or condensed nucleus is it undergoing transcription etc?
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)
fatty acid degradation
Pataus
No

37. Other than ragged red fibers and lactic acidosis - What else is common in mit myopathies?
Iatrogenic - granulomatous disease (sarcoidosis d/t epitheloid macrophage release of calcitriol); hypercalcemia - hypercalciuria (renal calculi!) - loss of appetite - stupor
3'--> 5'; prokaryote only
Neurologic lesions and cardiomyopathies (ox phos!)
I; nucleolus

38. What is the difference between a nucleotide and a nucleoside?
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond
Edwards
glycogen
tubulin dimer

39. Is hexokinase or glucokinase activated by insulin?
Hexosaminidase A; GM2 ganglioside
Glucokinase
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
Both AD

40. What cofactor is required for lysyl oxidase cross linking? What reaction is required beforehand for crosslinking to occur?
Uric acidemia; HGPRT; purine salvage
A lysosomal storage disease; mannose 6 phosphate can not be added to lysosomal proteins
= q
Cu; hydroxylation

41. How do you treat fructose intolerance?
Decrease intake of fructose and sucrose (fructose and glucose)
Histidine; B6
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Inability to taste

42. What causes the lysosomal storage diseases?
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
tubulin dimer
Adrenal cells (steroid synthesis) - hepatocytes (p450 detox)
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products

43. What functions are completed in the SER?
Its the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt
Phenylalanine hydroxylase or tetrahydrobiopterin factor
Heart - SKELETON - and EYES: arachnodactyly - long limbs - cystic medial necrosis of aorta resulting in aneurysms (and eventual dissection) - floppy mitral valve (prolapse and regurg!) - subluxation of lens
Steroid synthesis and detoxification

44. What is SAM?
Alcohol dehydrogenase; cytosol; mitochondria; NAD+
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
S - adenosyl methionine

45. What aa does AUG mRNA code for in eukaryotes? prokaryotes?
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
Cycloheximide
Methionine; f Met (formyl methyl methionine)
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)

46. What toxin causes liver failure by inhibiting RNA polymerase II if ingested? Where does it come from?
Tremor (asterixis) - slurring of speech - decrease temperature - somnolence - vomitting - cerebral edema - blurring of vision - neural focal deficits
Acetone
is asymmetric
Alpha amanitin; mushroom caps

47. What are the findings with phenylketonuria?
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
Retinol; Beta carotenes (dark and yellow vegetables)
It inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
Karyotyping

48. What are the key intermediates in the TCA?
To supply sufficient glucose to the brain and the RBCs and to preserve protein
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination
3 hydrogen bonds; 2 hydrogen bonds
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate

49. What is the inheritance of CF? What is the pathogenesis of the disease?
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
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
to ions
COMT and MAO

50. What tissues contain aldose reductase? What does this enzyme do to what?
Lesch Nyhan syndrome; X linked recessive
Glycine; heme
Lens and neural tissue; converts galactose to osmotically active galactitol
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