USMLE Step 1 Biochemistry

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. why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased production of orotic acid - the pyrimidine precursor?
By activating protein phosphatase Which both dephosphorylates glycogen phosphorylase (deactivating) and dephosphorylates glycogen synthase (activating it )
Dactinomycin - doxorubicin
Because carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
Prevents neural tube defects in 1st trimester


2. Is PRPP added later or first in pyrimidine synthesis?
Later
Degeneration of retinal ganglion cells and axons; acute loss of central vision; mit. inheritance
Block Electron transport
Do agarose gel eletrophoresis to separate the different sizes of the PCR products (sizes id the the fragment)


3. What builds up in the blood of patients with maple syrup urine disease? urine/poo? What does this cause?
AD
Blood group types
Glutamate
Alpha ketoacids in the blood; alpha keto acids in the urine/poo smells of maple syrup/burnt sugar


4. 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
Ile - Phe - Thr - Trp
plasma membrane
Carnitine shuttle


5. Hexokinase has a HIGH affinity (low Km) and...
No
Inability to taste
low Vmax
only L form


6. From What aa does GABA come from? with What enzyme? and What vit cofactor? What does GABA stand for?
Glutamate; glutamate decarboxylase; B6; gamma aminobutyrate
In the reproductive tract and in the resp tract; infertility - bronchiectasis and recurrent sinusitis - associated with situs inversus
Alkaptonuria; ochronosis
Make it stiffer/more tensile; continues as we get older so gets less and less elastic


7. What disease is associated with chromosome 15?


8. Which amino acid is the most basic?
FA coA synthetase combines coA with FA to make acyl coA
Bendy bones seen in children due to decreased Vit D; craniotabes - skeletal deformities - growth retardation - rosary
Phenylethanolamine N- methyl transferase; SAM
Arg


9. Phosphorylated glycogen phosphorylase...
UV nonionizing radiation causing thymidine dimers
Avidin in raw egg whites prevents their reabsorption; also antibiotic use; dermatitis - alopecia - glossitis - lactic acidosis - enteritis
Glutamine PRPP amidotransferase
is active


10. How do hepatocytes release glucose 6 P from glycogenolysis into the blood ? in What disease is this enzyme deficient?
Zero order; increased production of NADH no matter the amount of end products produced
When there is glucagon; protein kinase A activation and thus phosphorylation of the complex resulting in the activation of fructose 2 -6 BPase and no PFK2
1) glycogenolysis (liver only contributes to plasma - muscle keeps it for itself) 2) gluconeogenesis
With glucose 6 phosphatase; Von Gierkes disease


11. What is alternative splicing? in What disease state does alt splicing occur unintentionally d/t early stop codons?
Glutamine - aspartate - glycine; aspartate
Formation of a proton gradient that is then used to create ATP through ATP synthase
When different combination of exons get combined together; in Beta thallasemia
Release alk phos


12. The mode of inheritance for both NF disease is...
Developmental delay - gargoylism - airway obstruction (d/t short neck) - corneal clouding - CAD - hepatosplenomegaly
Pyruvate to lactate (results in lactic acidosis and decreased pyruvate for oxaloacetate for gluconeogenesis) - increased production of glycerol 3 phosphate from DHAP for TG synthesis; and increased betahydroxybutyrate ketoacid production from increas
Both AD
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis


13. What is the rate determining enzyme in glycolysis?
Nucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond
By turning it into fructose - but sorbitol first; glucose to sorbitol via aldose reductase and sorbitol to fructose sorbital dehydrogenase
Phosphofructokinase 1
Vit K; neonates have sterile intestine and can not synthesize vit K


14. What does guanine have on it?
Carbidopa
Phenylalanine build up
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
A ketone


15. By What products are gluconeogenesis and glycolysis linked so that they are not both occurirng at the same time?
only L form
A topoisomerase II inhibitor; because cancer cells use II more so than healthy cells
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
Acidic


16. What is heteroplasmy?
1 gene has greater than 1 phenotypic effect; locus heterogeneity (many different mutations can cause the same phenotype); PKU; albinism - deafness - Marfanoid habitus (Marfans - MEN 2B - homocystinuria)
Progressive neurodegeneration (spasticity d/t UMN disease) - hepatosplenomegaly - cherry red spot on macula - foam cells
Presence of normal and mutated mtDNA (like mosaicism except in mit)
Renal calculi of uric acid (replaces uric acid in excretion)


17. How is CF treated? How does it work?
Nacetyl cysteine; breaks disulfide bridges of mucus plugs
5' end; 3' end
Consumption of bear liver - and overtreatment isoretinoin; constitutional symptoms and alopecia and skin changes
Neimann pick and tay sachs


18. What does SAM turn into after it loses a methyl? How does it get turned back to SAM?
Tyrosinase
Homocysteine; homocysteine methyltransferase with B12 and tetrahydrofolate
Hemolytic anemia due to heinz bodies Which are oxidized hemoglobin precipitating within the RBCs
= q


19. Name a few intermediate filament defects. hints: alcohol - 2 neuro dx


20. What is the findings in Fabrys disease?
Alkaptonuria; ochronosis
Peripheral neuropathies of hands/feet; angiokeratomas between umbilicus and knees; cardiovascular/renal disease
Amniocentesis karyotyping
Lactose (glucose and galactose)


21. What are the four trinucleotide expansion disease and What are the expansions?
Myotonic Dystrophy (CTG) - Huntington (CAG) Friedricks Ataxia (GAA) - Fragile (CGG)
In the charging reaction! for this reason aminoacyl tRNA synthetase double checks
Because cells of mononuclear phagocytic are esp rich in lysosomes
Tyrosine


22. What is a telangiectasia? Where can they be found?and What is a common result of them?
Acetyl coA; gluconeogenesis
So that excess glucose can be stored (phosphorylated glucose cant leave) in the liver and saved as a buffer
Dilated capillaries and veins; skin - mucous membranes - GI - resp - urinary tracts; rupture and can cause epistaxis - hemorrhage - GI bleeding
TPP (from B1- pyrophosphate) - FAD (B2) - NADH2 (B3) - CoA (B5 - panothenate) - Lipoic Acid


23. What fungicide/rat repellent inhibits the peptidyltransferase activity in humans?
The products of FA oxidation (Acetyl CoA and NADH) phosphorylate pyruvate deyhdrogenase Which inactivates it
Niemann pick and Tay Sachs; hepatosplenomegaly or not
Cycloheximide
low Vmax


24. Which histone ties DNA- histone nucleosome beads into more condensed structures?
Cystine kidney stones (cystine staghorn calculi)
H1
ATP synthase - ATP- ADP translocase - ETC
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


25. What are the clinical symptoms of Marfans?
Because that is Where the products will be consumed; acetyl coA; TCA cycle of ketone body production
Fructose bisphosphatase 2
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
Thinned cornea from lack of connective tissue so see underlying choroidal lens


26. Findings for Cori's disease? deficient enzyme?
Hungtington
Milder form of type 1 with normal blood lactate levels; debranching enzyme (alpha -1 -6 glucosidase)
Some viruses
HMG CoA synthetase; HMG CoA reductase


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


28. What are cilia made of ?
Uric acidemia; HGPRT; purine salvage
MT!
Mental retardation - obesity - short stature - hypogonadism - hypotonia - hyperphagia; father
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles


29. What drug blocks ribonucleotide reductase? What does this enzyme do?
Hydroxurea; deoxygenates ribose
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA
Uracil; thymine
SER and mitochondria; NADPH


30. polymerase chain reaction (PCR)- Name an application for it
Genotyping for mutant alleles (different sizes)
Alcohol dehydrogenase; cytosol; mitochondria; NAD+
Mental and growth retardation; seizures - fair skin - eczema - musty body odor; neurotoxic ketones and acids stimulate vomitting - can lead to pyloric stenosis
3; 5; mit matrix; alpha ketoglutarate dehydrogenase complex


31. What is retinoic acid used for in treatments?
Chronic pancreatitis leads to type 1 diabetes - pancreatic insufficiency leads to ADEK deficiencies - malabsorption and steatorrhea - blocked gallbladder ducts leads to secondary biliary cirrhosis
Vit C deficiency; glossitis - anemia - weakened vessels - hemorrahges - hemarthroses - swollen gums - bruises -
NADH - NADPH - FADH2
Wrinkles - acne and psoriasis topically - promyelocytic anemia - measles


32. Which intermediate filament is found connective tissue? muscle? epithelial cells? neuroglia? neurons? What can intermediate filaments be used for?
Vimentin - desmin - cytokeratin - glial fibrillary acidic protein (GFAP) - neurofilaments; to differentiate between different cells
2/3; LCAT (lecithin cholesterol acyltransferase)
Hurlers Syndromes;Iduronate sulfatase deficiency; alpha L iduronidase def; heparan sulfate - dermatan sulfate
Cells that stay in G0 and cannot replicate - instead they regenerate from stem cells; neurons - skeletal and cardiac muscles - and RBCs


33. What three enzyme def. can cause homocysteinura? What is their mode of inheritance?
Preprocollagen - procollagen - tropocollagen - collagen
adrenal medulla
Homocysteine methyltransferase - cystathionine reductase - decreased affinity of cystathionine reductase for pyridoxal phosphate; AR
Indirect inhibtion of the Na+/Ca+2 exchange resulting in increased intracellular calcium Which increases cardiac contractility


34. What is a positive allosteric regulator on pyruvate carboxylase? for What reaction is this?
NF2 - DiGeorge/Velocardiofacial syndromes
Acetyl coA; gluconeogenesis
Southern - DNA - Northern - RNA - Western - Protein; blotting procedures are used to detect a specific nucleic acid or aa sequence; the sample is electrophoresed and then transferred to a filter Which is then soaked in a denaturant and then a radiola
All and Alzheimers


35. What are the symptoms involved in lactase deficiency?
Osmotic diarrhea - bloating - cramps
Glycogen synthase
Cleft palate - Abnormal faces - thymic aplasia (t cell def) - Cardiac defects - hypocalcemia (secondary to hypoparathyroid); 3rd and 4th branchial pouch aberrant development
Arg


36. What explains the fatty change in kwashiokor?
Abetalipoproteinemia and increased G3P production from all the carbs!
Induction in chronic alcoholism (increases tolerance); inhibition in acute alcoholism
A 16C FA! the product in FA synthesis; saturated; no double bonds
G6PD; provides malarial resistance; X linked recessive


37. What is osmotic damage and What molecules cause it? give some examples of osmotic damage - in What disease state do you see these?
1 gene has greater than 1 phenotypic effect; locus heterogeneity (many different mutations can cause the same phenotype); PKU; albinism - deafness - Marfanoid habitus (Marfans - MEN 2B - homocystinuria)
Excess ATP and dATP has negative feedback inhibition on ribonucleotide reductase Which results in decreased DNA synthesis and thus decreased lymphocyte count
Carbidopa
When water is pulled in; polyols (sugar alcohols); cataracts - retinopathy - peripheral neuropathy all seen in chronic hyperglycemia in diabetes


38. What is given to prevent side effects When methotrexate is given? why does this work?
Increased concentration of Cl ions in sweat test
Folinic acid; does not require DHF
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
Chorea and spastic paresis


39. Cysteine is one of two _____ connected by disulfide bond
Abnormal glycogen metabolism results in accumulation of glycogen in cells
show disease
Alpha galactosidase A; ceramide trihexoside
dimers


40. What does ultrasound show in Downs?
Arg and his
Uric acidemia; HGPRT; purine salvage
Mental retardation - rocker bottom feet - micrognathia (small jaw) - low set ears - clenched hands with polydactyly - congenital heart disease
Increased nuchal translucency


41. Kwashiokor patients are not...
Alanine (through pyruvate) - aspartate (through OXA) - glutamate (alpha ketoglutarase); TCA
Fructose gets phosphorylated into fructose 1- P by fructokinase and then fructose 1P gets metabolized into DHAP or glyceraldehyde by aldolase B
calorie deficient
Positive on phosphofructokinase 1 and negative on fructose 1 -6 bisphosphatase


42. Which oxidant is most attributable to damage of RBCs?
LDL; LDL oxidize atherosclerosis
SER and mitochondria; NADPH
DHF
By epoxide reductase in the liver; coumadin; intestinal flora and leafy vegetables


43. Which glycogen storage disease causes hypertriglyceridemia?
fatty acid degradation
Von gierkes
G6PD dehydrogenase; transketolases
HnRNA; mRNA


44. What is methotrexate?
Triacylglycerols in adipose tissue broken down into glycerol and free fatty acids; growth hormone and epinephrine
B12 and folate; B12; washed out
Sphingomyelinase; sphingomyelin
A folic acid analogue


45. in What organ is the storage form of D3 converted to its active form? What enzyme? What hormone stimulates this action?
Zero order; increased production of NADH no matter the amount of end products produced
Kidney; 1 alpha hydroxylase; Parathyroid hormone
Carnitine shuttle
AD; disorder of blood vessels that results in telangietasias - skin discolorations - and AVMs (arteriovenous malformations)


46. What is the rate determining enzyme in fatty acid oxidation?
Cytoplasmic side; active
Ortinithine transcarbamoylase - orotic acid phosphoribosyltransferase - and orotidine 5 phosphate decarboxylase; urea cycle; last two are involved in conversion of orotic acid to UMP (adding PRPP to orotic acid)
Carnitine acyltransferase I
Kidney and intestinal epithelium


47. In the genetic code - What does unambiguous mean?
Each codon specifies only one amino acid
Steroid - cholesterol FA synthesis; HMP shunt
Creates nicks in DNA to prevent supercoiling; both!
Cell signaling defect in FGF receptor resulting in dwarfism and short limbs with normal head and trunk; AD


48. What is the rate determining enzyme in galactose metabolism?
GALT
Mannose
D2 (ergocalciferol) - D3 (cholecalciferol) - D3 (cholecalciferol)
Robertsonian translocations


49. What is main lab finding in Lesch Nyhan syndrome? due to What enzyme deficiency? What reaction is deficient?
AD
Respiratory burst - P450 - and glutathione reductase
Uric acidemia; HGPRT; purine salvage
Using serine (which turns into glycine after reaction)


50. What is the first step in glycolysis?
Matrix; intermembrane space
Bilateral acoustic schwannomas - juvenile cataracts
Phosphorylation of glucose
in hepatocytes