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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. What is the job of S adenosyl methionine? What is it made of ? give an example of a reaction it participates in.
To transfer methyls; a methionine and an ATP; norepinephrine plus a methyl makes epinephrine
Stored ATP - creatine phosphate - anaerobic glycolysis; as distances increase - ATP is obtained from additional resources; above plus ox phos
= q
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs

2. What is the mc chromosomal disorder?
Increased G3P for TG synthesis - increased acetyl CoA (Fatty acid synthesis for TG synthesis) - and decreased oxidation of FA in the mitochondria due to alcohols toxicity on the mit
liver and muscle
Downs
30S + 50S= 70S

3. From What aa does porphyrin come from? What is porphyrin the precursor of?
Glycine; heme
Some viruses
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Arginne

4. What are the lab findings see in hyperammonemia?
Increased blood levels of ammonia - decreased BUN
Leucine and lysine
Branching increases the rate of synthesis and breakdown of glycogen by having more sites to add on to and break off
With glucose 6 phosphatase; Von Gierkes disease

5. Sporadic Retinoblastoma results is unilateral - but only familial is associated with...
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
osteosarcomas (Rb mutation is in all cells)
Proprionyl coA; through methylmalonyl coA; B12
Codons may encode same amino acid since there are only ~20 aa

6. What is folic acid? What is its function?
Carnitine shuttle
Water soluble vitamin stored in liver; converted to THF by dihydrofolate reductase and is a coenzyme in 1 methyl transfers - impt for DNA synth
Progressive neurodegeneration (spasticity d/t UMN disease) - NO hepatosplenomegaly - cherry red spot on macula - lysosomes with onion skin - muscle weakness
Telomerase; an RNA polymerase that uses a RNA template to add DNA to the end of shortening 3' ends

7. What aa is most accumulated in hartnup disease? What vitamin becomes deficient?
Tryptophan; niacin
glycogen
NADPH oxidase - superoxide dismutase - myeloperoxidase; in the phagolysosome; HOCl (bleach); kills!
Dermatitis - glossitis - diarrhea

8. What is the other name of Vitamine B2? In What foods can it be found?
When child gets two chromosomes from one parent; imprinting dx
Riboflavin; dairy and meat
IMP
P53 and Rb; p53 activates BAX Which inhibits BCL2 When there is DNA damage - Rb phosphorylation allows progression into S phase

9. What aa becomes essential with phenylketonuria?
Tyrosine
Mitochondrial inheritance
Glutamine PRPP amidotransferase
Acetyl coA; gluconeogenesis

10. What is the most common cause of Osteogenesis Imperfecta? What is the inheritance pattern?
G6PD; provides malarial resistance; X linked recessive
Fabrys Disease and Hunters Syndrome
Lyonization; nondisjunction in mitotic divisions during embryonic period
Type I collagen; AD

11. How is the nitrogen of amino acids transferred to the liver for the urea cycle? explain this process.
Myoclonus epilepsy
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
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
Because cells of mononuclear phagocytic are esp rich in lysosomes

12. What does SAM turn into after it loses a methyl? How does it get turned back to SAM?
Colchine
NADH - NADPH - FADH2
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks

13. 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)
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
A cytoskeletal protein - apart of flagella - cilia - mitotic spindles and transport (in neurons for example)
Double strand breaks; nonhomologous end joining

14. What two substrates (other than pyruvate) does pyruvate carboxylase require?
Glutathione peroxidase (catalase) reduces H202 to H20 with glutathione; oxidized glutathione gets reduced by glutathione reductase with NADPH; NAP+ is regenerated to NADPH with Glucose 6 phosphate dehydrogenase; because H2O2 from the resp burts in th
Arginine
No; increased mutation rate!
ATP and biotin

15. What is lebers hereditary optic neuropathy? What is the main clinical symptom? and What is its mode of inheritance?
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
Constitutive - random insertion of gene into mouse genome; conditional - targeted through homologous recombination
Tyrosine
Increased orotic acid in urine - megaloblastic anemia - +/- hyperammonemia

16. Which is the most common lysosomal storage disease? What are its findings?
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
Autosomal Dominant; NF1 and mit. diseases (variable expression) - ?
Gauchers; hepatosplenomegaly - aseptic necrosis of the femur - bone crises - Gauchers cells - crumpled tissue paper looking (fibrillar appearing macrophages in liver - spleen - b.m.)
III

17. What does guanine have on it?
AAUAAA; Poly A polymerase; no
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase
A ketone
Oxygen!

18. What enzyme converts phenylalanine to tyrosine? What cofactor is used?
Cytosine to uracil
Phenylalanine hydroxylase; tetrahydrobiopterin factor
AR
'9+2' 9 pairs of microtubules connected by an axonemal dynein ATPase in a cylinder with 20 MTs in the center; the connecting ATPase allows bending and differential sliding

19. What is the deficient enzyme in Tay Sachs disease? accumulated Substrate?
Pyridoxine; converted to pyridoxal phsophate used in transamination reaction - decarboxylations - heme synthesis - niacin synthesis from trp - glycogen phosphorylase - cystathionine synthesis - NT synthesis
2 oxidative and nonoxidative; cytoplasm (both); none (none produced either - you are simply switching carriers of energy- from glucose 6 phosphate to NADPH)
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST); alanine to pyruvate and aspartate to oxaxloacetate
Hexosaminidase A; GM2 ganglioside

20. What are the four fates of pyruvate metabolism?
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
Acetazolamide to alkalinize the urine
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA
osteosarcomas (Rb mutation is in all cells)

21. What aa is the precursor for NO?
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)
Arginine
low Vmax
AST (mit damage) - and GGT (d/t SER hyperplasia d/t induction of p450 generation)

22. What are permanent cells? Which cells are permanent?
Phosphorylation of glucose
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
Ouabain
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs

23. How many RNA polymerases do prokaryotes have? What drug blocks prokaryotic RNA polymerase only? What is it used for?
1 for all three classes of RNA; Rifampin; TB
Schilling test
30S + 50S= 70S
Citrate - Isocitrate - alpha ketoglutarate - Succinyl CoA - Succinate - Fumarate - Malate - Oxaloacetate

24. Insulin receptor is not cAMP second receptor but rater tyrosine kinase...
Which dimerizes and activates a protein phosphatase Which de phosphorylates glycogen phosphorylase
to ions
GALT
activator

25. What are three ways to poison Ox phos?
Vit C; Vit E
The lack of lysosomal proteins (are secreted - default from RER!) results in macromolecules accumulating in lysosomes - inclusions cause pathology
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Tetrahydrofolates

26. What toxins prevent elongation in translation in humans (by binding 40s)?
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
Increased concentration of Cl ions in sweat test
Shiga toxin and ricin (protein in castor beans)
Gets converted to pyruvate via the Cori cycle in the liver

27. What is the mode of inheritance for the multiple endocrine neoplasias?
AD
Becomes methionine
to ions
Gain of function gene after expansion results in overproduction of huntingtin protein that in overproduction results in toxicity to neurons in the caudate --> caudate atrophy and decreased levels of GABA and Ach

28. What is the rate determining enzyme in glycolysis?
4 -4 -9
RER in nucleus (synthesize NTs)
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)
Phosphofructokinase 1

29. Which intermediate filament is found connective tissue? muscle? epithelial cells? neuroglia? neurons? What can intermediate filaments be used for?
A lysosomal storage disease; mannose 6 phosphate can not be added to lysosomal proteins
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products
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)
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells

30. What enzyme is deficient in maple syrup urine disease?
H1
Fructose 1 -6 bisphosphatase
Biotin; rate limiting step in fA synthesis - acetyl coA into malonyl coA
Branched alpha ketoacid dehydrogenase

31. deamination of What pyrimidine makes what?
Either salvage by HGPRT and PRPP or converstion to xanthine and then to uric acid by xanthine oxidase
Pellagra
Cytosine to uracil
dimers

32. Which syndrome results from microtubule polymerization defect? What are the clinical features?
Abetalipoproteinemia and increased G3P production from all the carbs!
Chediak Higashi Syndrome; decreased phagocytosis results in recurrent pyogenic infections - partial albinism - and peripheral neuropathy
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)
Hereditary defect of renal tubular amino acid transporter for cysteine - ornithine - lysine and arginine in the PCT of kidnyes

33. What is the primary source of nitrogen in the urea cycle
Adrenal medulla
Lys and arg
AD
Glutamate

34. in What disease are there antibodies to snRNPs?
osteosarcomas (Rb mutation is in all cells)
Lupus
Its When two acrocentric chromosomes combine their long arms (and short arms disappear); When the translocation is functional
Alpha ketoglutarate; no TCA cycle intermediates

35. How are fa oxidation and gluconeogenesis linked - ie What compound makes sure that they occur together and that glycolysis doesnt occur?
Glucagon - insulin and epinephrine
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
Acetyl coA and NADH inhibit pyruvate dehyrogenase
Tyrosine

36. How is CF treated? How does it work?
No; increased mutation rate!
Electron transport inhibitors - ATPase inhibitors - Uncoupling agents (increase permeability of the membrane - loss of hydrogen gradient)
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
With glucose 6 phosphatase; Von Gierkes disease

37. When are there low levels of cAMP? What does this result in?
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!!
Isocitrate dehydrogenase
Hepatic glycogenolysis (mostly) - also hepatic gluconeogenesis - adipose release of FFA (minor)
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

38. If a cell has a hyperchromatic or condensed nucleus is it undergoing transcription etc?
Robertsonian translocations
degradation
No
osteosarcomas (Rb mutation is in all cells)

39. What is osmotic damage and What molecules cause it? give some examples of osmotic damage - in What disease state do you see these?
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes
G6PD; provides malarial resistance; X linked recessive
Sulfonamides - primaquine - dapsone - antiTB drugs - fava beans - LDL; RBCs

40. Which two lysosomal storage disease are XR and not AR?
Cu; hydroxylation
Cardiomegaly and systemic findings leading to early death; lysosomal alpha -1 -4- glucosidase (acid maltase)
Galactokinase; galactose appears in blood and urine - sometimes infantile cataracts may present as failure to track objects or lack of a social smile
Fabrys Disease and Hunters Syndrome

41. Which vitamin keeps Fh4 in its reduced form?
Vit C
Gets converted to glycerol 3 P Which is then converted to DHAP Which then can enter glycolyis
dimers
Phenylalanine

42. What do enhancers/silencers bind?
TFs
Alcohol dehyrogenase (alcohol to acetaldehyde); acetaldehyde dehydrogenase (acetaldehyde to acetate); NAD+; B3 (niacin)
hexokinase - glucokinase (its a glutton)
HMP shunt (to NADPH) and glycogenolysis (as glucose 1 phosphate)

43. After the PCR reaction - How do you actually visualize the DNA?
ATP and dATP
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)
Flourescent DNA/RNA In SITU hybridization; for visualization of anomalies (like microdeletions) that are too small to be seen on karyotype; Prader - Willi syndrome
Cartilage (including hyaline) - vitreous body - nucleus pulposus

44. What disease manifestations result in AngelMan syndrome? is the deletion a result from the father or mothers chromosome?

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45. In x linked - in males - prevalence is...
= q
HMG CoA synthetase; HMG CoA reductase
liver and muscle
is asymmetric

46. What causes the lysosomal storage diseases?
Deficiencies in one of the many lysosomal enzymes results in accumulation of abnormal metabolic products
Vit C; Vit E
DOPA
Oxygen!

47. What is rickets? What is it caused by?
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary
NADH - Acetyl CoA - ATP; NAD+ - ADP and Ca2+
1 day; adipose release of FFA - hepatic gluconeogenesis from peripheral tissue lactate and alanine and from adipose tissue glycerol and propionyl coA
Adenosine deaminase deficiency; Severe combined immunodeficiency disease

48. What are the GI clinical symptoms of CF?
3'--> 5'; prokaryote only
Glycolysis and aerobic respiration
Glycogen synthase
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis

49. What causes cataracts in a galactose metabolism deficiency?
Oxygen!
Malnutrition - edema (hypoalbuminemia) - anemia liver (fatty change) - diarrhea d/t no brush border enzymes (no protein!)
Pantothenate; essential comp of CoA (needed for acyl transfers) and fatty acid synthase
Aldose reductase in the lens converts built up galactose to galactitol Which is osmotically active

50. What is the source of Biotin?
(cofactor for 100+ enzymes!) needed for zinc fingers transcription motif - collagen synthesis - alcohol metabolism
Acetyl CoA carboxylase
Bacterial synthesis in the intestine
Depression - progressive dementia - choreiform movements; between the ages of 20 and 50