BIOCHEMISTRY USMLE STEP 1 STUDY KIT WITH COMPLETE QUESTIONS AND ANSWERS

BIOCHEMISTRY USMLE STEP 1 STUDY KIT
WITH COMPLETE QUESTIONS AND
ANSWERS
MOA of 6-MP and Azathioprine (and use) Correct Answer inhibit
de novo purine synthesis
tx: leukemia, immune dzs (RA, IBD)

prevents PRPP --> IMP

inhibits thymidylate synthase Correct Answer 5-fluorouracil

prevents dUMP --> dTMP, decreases thymine

inhibit dihydrofolate reductase (DHFR) Correct Answer
methotrexate (MTX) in humans
trimethoprim (TMP) in bacteria
pyrimethamine in protozoa

prevents pyrimidine synthesis (dUMP --> dTMP)

folate is supplemental for which drugs and why? Correct Answer
5-FU > increase potency
MTX > reduce side effects

major cause of AR SCID and why Correct Answer adenosine
deaminase (ADA) deficiency

prevents purine salvage, affects cells that need to divide a lot
(immune cells > reduces lymphocyte count)

pathophys of adenosine deaminase (ADA) deficiency Correct
Answer buildup of adenosine (bc low ADA prevents degradation)
> feedback inhibition of ribonucleotide reductase --I DNA

,synthesis --> dec lymphocyte count and causes
immunodeficiency (SCID)

purine salvage enzyme & function Correct Answer HGPRT

Recycles:
hypoxanthine > IMP
guanine > GMP

key enzymes of purine degradation Correct Answer ADA
(adenosine > inosine)
XO (hypoxanthine > xanthine > uric acid)

purine degradation overview Correct Answer 1. removal of
phosphate from nucleotide
2. removal of sugar from nucleoside
3. convert free bases (guanine & hypoxanthine) to xanthine by
xanthine oxidase (XO)
4. xanthine > uric acid by xanthine oxidase (XO)
5. uric acid to urine

defective purine salvage due to absent HGPRT (dz name) Correct
Answer Lesch-Nyhan syndrome

symptoms of Lesch-Nyhan syndrome Correct Answer HGPRT
deficiency:
Hyperuricemia
Gout
Pissed off (aggression, self-mutilation)
Retardation
dysTonia

young male
Gout
aggression, self-mutilation

,intellectual disability
dystonia Correct Answer Lesch-Nyhan syndrome

defective purine salvage due to absent HGPRT --> excessive uric
acid production and de novo purine synthesis

function of HGPRT Correct Answer purine salvage (adds sugar/P
to free base)
converts hypoxanthine + PRPP --> IMP
converts guanine + PRPP --> GMP

inheritance of Lesch-Nyhan syndrome Correct Answer X-linked
recessive

treatment of Lesch-Nyhan syndrome Correct Answer XO-
inhibitors: allopurinol or febuxostat (2nd line)

reduce uric acid causing hyperuricemia and sxs

XO-inhibitors reduces the efficacy of which drugs Correct Answer
drugs that look like purines (6-MP, leukemia tx)

AA encoded by only 1 codon Correct Answer AUG -->
methionine
UGG --> tryptophan

create single or double stranded breaks in DNA helix to add or
remove supercoils Correct Answer DNA topoisomerases

unwinds DNA template at replication fork Correct Answer
helicase

"helicase has the most fun bc it spends all day unzipping genes!"

bonds helicase breaks Correct Answer H-bonds only

, inhibit prokaryotic enzymes topoisomerase II (DNA gyrase) and
topoisomerase IV Correct Answer fluoroquinolones

direction of DNA and RNA synthesis & protein synthesis &
implications Correct Answer 5' --> 3'
N' --> C'

drugs (AZT) can target triphosphate bond @ 3' hydoxyl

found in prokaryotes only, has 5' --> 3' synthesis and 3' --> 5'
exonuclease proofreading Correct Answer DNA pol III

makes RNA primer on which DNA polymerase III can initiate
replication Correct Answer Primase

found in prokaryotes only, degrades RNA primer with 5' --> 3'
exonuclease and replaces in with 5' --> 3' DNA synthesis Correct
Answer DNA pol I

also has has 5' --> 3' synthesis and 3' --> 5' exonuclease
proofreading like DNA pol III

joins Okazaki fragments Correct Answer DNA ligase
catalyzes phosphodiester bond formation within a strand of
dsDNA

telomerase Correct Answer RNA-dependent DNA polymerase
that adds DNA to 3' ends of chromosomes to avoid loss of genetic
material with every duplication

euks only
only in cells that divide a lot (stem cells, immune cells)

transition point mutation Correct Answer purine to purine