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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.
Don't refresh. All questions and answers are randomly picked and ordered every time you load a test.

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. Where are receptors for Vit D? (think How does it increase serum Ca and Phosphate)
Cylindrical made up of a helical array of polymerized alpha and beta tubulin dimers - each dimer has 2 GTP bound
Macrocytic megaloblastic anemia; it is the most common vit def in the US; no neuro symptoms and no methylmalonyl coA in the urine
Tremor (asterixis) - slurring of speech - decrease temperature - somnolence - vomitting - cerebral edema - blurring of vision - neural focal deficits
Intestine - renal - bone

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

3. Phosphorylated glycogen phosphorylase...
Tyrosine
In hepatocytes; B6
is active
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

4. In What stage of the cell cycle must cells be in order to be analyzed for karyotyping? and From What fluids can you draw the cells? and for What is karyotyping is used for?
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Vit C
Metaphase (all lined up); blood - bone marrow - amniotic fluid - placental tissue; used to diagnose chromosomal abnormalities (deletions - trisomies - sex chromosomal disorders)
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

5. All the glycogen storage diseases...
Zero order; increased production of NADH no matter the amount of end products produced
are AR
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
NADH - NADPH - FADH2

6. What two processes maintain the plasma glucose levels?
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
glucagon - insulin and epinephrine
cell cycle events
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos

7. By What products are gluconeogenesis and glycolysis linked so that they are not both occurirng at the same time?
SER and mitochondria; NADPH
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
succinyl coA (and then to OXA)
When the amino acid is covalently binded to its corresponding tRNA; Aminoacyl tRNA synthetase; ATP; translation

8. What is the major cause of SCID? What does SCID stand for?
DNA dependent RNA polymerase
DiGeorge and Velocardiofacial; deletion on chromosome 22q11 results in aberrant dev of 3rd and 4th branchial pouches; digeorge has thymic and parathyroid prob - velocardiofacial has palate and facial instead
Mitochondrial inheritance
Adenosine deaminase deficiency; Severe combined immunodeficiency disease

9. What carries electrons (as energy)?
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
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells
NADH - NADPH - FADH2
Telomerase; an RNA polymerase that uses a RNA template to add DNA to the end of shortening 3' ends

10. Why do mitochondrial diseases exhibit variable expression?
Heteroplasmy
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Defective neutral amino acid transporter on renal and intestinal cells; AR
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)

11. What is the inheritance of Tuberous Sclerosis?
AD (trinucleotide expansion is not a mode of inheritance)
Block ATP synthase resulting in no ATP production and an increased proton gradient
Mitochondria in liver; HmG CoA synthetase; amino acids and beta oxidation of FA
AD

12. What cofactor is necessary for conversion of dopamine to NE? NE to epinephrine?
SAM; Vit C
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
Adipose tissue for brain and heart (ketone bodies) and then go to vital protein degradation (organ failure)
Metanephrine

13. What carries aldehydes as energry?
Cartilage (including hyaline) - vitreous body - nucleus pulposus
HMG CoA synthetase; HMG CoA reductase
Phenylalanine hydroxylase or tetrahydrobiopterin factor
TPP (thiamine pyrophosphate)

14. What ensures that during gluconeogenesis all the ATP wont be used in glycolysis as youre producing it?
Macrocytic megaloblastic anemia - hypersegmented PMNs - subacute combined degeneration myelopathy Which results in degeneration of the dorsal and lateral columns (loss of position and vibration sense - spastic paresis and ataxia) due to abnormal myel
An anti cancer agent that is a topoisomerase II and creates nicks in DNA
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
Protein Kinase A inhibits pyruvate kinase also ATP and alanine have negative effects on pyruvate kinase

15. What does adipose tissue contribute for hepatic gluconeogenesis during starvation? muscle?
Smoking (FR takes up all the antioxidants); diets lacking fruit
Glycerol and propionyl coA; lactate and alanine
Back up of pyruvate and alanine and thus lactic acidosis occurs; can be d/t alcoholism (vit B1 def)
An aminotransferase enzyme transfers an amino group from an amino acid on to alpha ketoglutarate making it a glutamate (to join the urea cycle or excreted in the kidney) and the amino acid that lost the amino group is then turned into a glycolytic in

16. What two enzymes are the apart of the same complex? What dictates What form they are in? and What dictates that?
Pigmented iris hamartomas
2/3; LCAT (lecithin cholesterol acyltransferase)
Fructose bisphosphatase 2 and Phosphofructokinase 2; phosphorylation by protein kinase A; inuslin/glucagon ratio
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate

17. What is heteroplasmy?
Presence of normal and mutated mtDNA (like mosaicism except in mit)
Galactokinase deficiency and classic galactosemia; classic galactosemia
Carnitine shuttle
To increase serum Ca and phosphate for bone deposition

18. What type of aa are histones made of? are they acidic or basic?
Meconium ileus and failure to thrive; negative serum immunoreactive trypsin
Succinyl coA
Lysine and arginine; basic
Chorea and spastic paresis

19. What drug blocks acetaldehyde dehyrogenase? What are its side effects?
calorie deficient
Both add phosphate; phosphorylase uses no ATP and inorganic phosphate
Disulfiram (antabuse); acetaldehyde accumulate contributing to hangover
all carriers

20. Which amino acids are acidic? are they negatively or positively charged at body pH?
Asp and glu; negatively
Glutamate
Schwann cells - lens - retina - kidneys because they only have aldose reductase and no sorbital dehydrogenase
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells

21. What is the first step in the respiratory burst/oxidative burst? in What disease is this deficient?
An aminotransferase enzyme transfers an amino group from an amino acid on to alpha ketoglutarate making it a glutamate (to join the urea cycle or excreted in the kidney) and the amino acid that lost the amino group is then turned into a glycolytic in
NADPH oxidase; CGD
Tyrosine hydroxylase; tetrahydrobiopterin factor; NADP+
Yes! congenital simply means 'born with'- hereditary is familial

22. What does SAM turn into after it loses a methyl? How does it get turned back to SAM?
Myoclonus epilepsy
Tyrosine; tryptophan
Coarse facial features - psychomotor retardation - clouded corneas - restricted joint movement - high plasma levels of lysosomal enzymes
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate

23. Which aa are only ketogenic?
Phenylalanine hydroxylase; tetrahydrobiopterin factor
Lysine and leucine
Yes! congenital simply means 'born with'- hereditary is familial
Imprinting Dx's: prader Willi and Angelmann

24. What are the four trinucleotide expansion disease and What are the expansions?
NADPH- reductive biosynthesis!
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
Either salvage by HGPRT and PRPP or conversion to xanthine and then uric acid by xanthine oxidase
glycogenolysis

25. Is heterochromatic more accessible or less accessible to TF? euchromatin?
Less; more
Glycogen phosphorylase
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
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis

26. What is the rate determining enzyme in TCA cycle?
DOPA
Isocitrate dehydrogenase
32 or 30; malate aspartate shuttle or glycerol 3 phosphate shuttle; heart/liver and muscle
Lysine and leucine

27. What is the confirmation of Downs?
Beta thalassemia
Isocitrate dehydrogenase
Incoming Aminoacyl tRNA; site Where peptidyltransferase bond is made between growing peptide aa's; the holds the empty tRNA as it exits; the initiator mRNA (met or f - met) starts off in E site rather than A site
Amniocentesis karyotyping

28. What is the deficient enzyme in Krabbes disease? accumulated Substrate?
By addition of mannose 6 phosphate to proteins in the Golgi
Abnormal glycogen metabolism results in accumulation of glycogen in cells
They can enter G1 from G0 and replicate if they like; hepatocytes - lymphocytes - fibroblasts
Galactocerebrosidase; Galactocerebroside

29. What is alternative splicing? in What disease state does alt splicing occur unintentionally d/t early stop codons?
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!!
Vit D and K
When different combination of exons get combined together; in Beta thallasemia
ATP and dATP

30. What explains the excess phenylketones in the urine in phenylketonuria?
Phenylalanine build up
TFs
Glycogen synthase
Acetyl coA; gluconeogenesis

31. What is the end of anaerobic glycolysis?
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Homogentistic acid oxidase
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
Succinate hydrogenase; matrix

32. Heterozygous females in X linked recessive disease...
Hemorrhage with increased PT and PTT time but normal bleeding time
Acetyl coA and NADH inhibit pyruvate dehyrogenase
can be d/t lyonization
By activating adenylyl cylase Which increases cAMP Which increases levels of protein kinase A Which activates glycogen phosphorylase kinase; epinephrine

33. AR disease is usually...
severe
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
Heteroplasmy
Von gierkes

34. What is the rate determining enzyme in de novo pyrimidine synthesis?
usually present late
Macroorchitism - big ears - big jaw - mental retardation (2nd mcc!) - mitral valve prolapse - long face
Carbomyl phosphate synthetase II
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables

35. What is the main hormonal regulation of glycogen?
Glucagon - insulin and epinephrine
Biotin
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
A folic acid analogue

36. What are some catalase positive bugs?
Phenylethanolamine N- methyl transferase; SAM
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
liver
S. aureus - Aspergillus

37. Hexokinase has a HIGH affinity (low Km) and...
Vitamin B1 (thiamine) - Vitamin B2 (riboflacin) - Vitamin B3 (niacin) - Vitamin B5 (pantothenate) - Vitamin B6 (pyridoxine) - Vitamin B7 (Biotin) - Vitamin B12 (cobalamin) - folate - Vit. C
False; GTP hydrolysis
low Vmax
kidney

38. Which intermediate is involved in both pyrimidine synthesis and the urea cycle?
Carbamoyl phosphate
Metanephrine
glycogen
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis

39. What is ascorbic acid? What is its 5 main function?
fructose
Adenosine deaminase deficiency; Severe combined immunodeficiency disease
Vitamin C; facilitates iron absorption by making sure its in its reduced state (Fe2+) - hydroxylation for lysine and proline Which is necessary for crosslinking; necessary for dopamine Beta hydroxylase for conversion of dopamine to NE; keeps FH4 in i
Tetrahydrofolates

40. What liver enzymes are increased in alcoholism? why?
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)
Intestine - renal - bone
Methionine; f Met (formyl methyl methionine)
Karyotyping

41. What converts dopa to dopamine? using What cofactor?
Dopa decarboxylase; B6
Acidic
is asymmetric
TFs

42. What is the rate determining enzyme in HMP shunt?
Flouroquinolones
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
Glucose 6 phosphate dehydrogenase
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active

43. How does a kwashiokor patient present?
Malnutrition - edema (hypoalbuminemia) - anemia liver (fatty change) - diarrhea d/t no brush border enzymes (no protein!)
Neimann pick and tay sachs
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
3 NADH - 1 FADH2 - 2 CO2 - 1 GTP = 12 ATP/acetyl CoA

44. What creates a frameshift mutation? What are some clinical examples?
Acetazolamide to alkalinize the urine
Insertion or deletion of a number of nucleotides indivisible by 3; tay sachs - duchennes MD
Glycogen and FFA oxidation - glucose conserved for final sprinting!
G6PD dehydrogenase; transketolases

45. Prokaryotes have What ribosomal units?
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Bilateral acoustic schwannomas - juvenile cataracts
low Vmax
30S + 50S= 70S

46. After N5N10 methylene THF donates a methyl for conversion of dUMP to dTMP What is it converted to?
DHF
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
Central and peripheral demyelination with ataxia and dementia
5' end; 3' end

47. What type of drug are alkylating agents? How do they work? give a couple examples
Antineoplastic agents; alkylate DNA!; cyclophosphamide and nitrosurea (aka lomustrine - a nitrosource)
Pyruvate! (not lactate - that is generate just in order to produce more NAD+)
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
Vincristine - vinblastine - and paclitaxel; the vinca alkaloids block formation of the spindle and paclitaxel prevents breakdown of the mT spindle

48. In protein translation - What is the 'A' site for? 'P' site? 'E' site? What is the exception to this?

49. How many enzymes does the pyruvate dehydrogenase complex have? and How many cofactors? Where is it located? to What enzyme is it similar to?
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex
Debranching enzyme; alpha 1 - 4 glucosidase; no!; Pompes disease

50. Which antifungal acts on MT?
Uses HMG coA reductase
Pulmonary - GI - reproductive (infertility in male due to absence of vas deferens and seminal fluid thickness)
CNS defects - mental retardation - death - feeding difficulties - vomiting - hypoglycemia; thiamine
Griseofulvin