Page 81 - First Aid for the USMLE Step 1 2020, Thirtieth edition [MedicalBooksVN.com]_Neat
P. 81
BIOCHEmISTRY ``BIOCHEMISTRY—MOlECUlAR BIOCHEmISTRY ``BIOCHEMISTRY—MOlECUlAR SECTION II 37
Purine salvage deficiencies
Nucleic acids Ribose-5-phosphate Nucleic acids
PRPP synthetase De novo synthesis
Nucleotides GMP IMP AMP
ADA APRT
Nucleosides Guanosine HGPRT Inosine Adenosine
PRPP
Free bases Guanine PRPP Hypoxanthine Adenine
XO
–
Xanthine Allopurinol
– Febuxostat Degradation and salvage
XO
Uric acid
Urate oxidase (rasburicase)
Allantoin Excretion
ADA, adenosine deaminase; APRT, adenine phosphoribosyltransferase;
HGPRT, hypoxanthine guanine phosphoribosyltransferase; XO, xanthine oxidase.
Adenosine deaminase ADA is required for degradation of adenosine One of the major causes of autosomal recessive
deficiency and deoxyadenosine. ADA dATP SCID.
ribonucleotide reductase activity
lymphotoxicity.
Lesch-Nyhan Defective purine salvage due to absent HGPRT, HGPRT:
syndrome which converts hypoxanthine to IMP and Hyperuricemia
guanine to GMP. Results in excess uric acid Gout
production and de novo purine synthesis. Pissed off (aggression, self-mutilation)
X-linked recessive. Retardation (intellectual disability)
Findings: intellectual disability, self-mutilation, DysTonia
aggression, hyperuricemia (orange “sand”
[sodium urate crystals] in diaper), gout,
dystonia, macrocytosis.
Treatment: allopurinol or febuxostat (2nd line).
Genetic code features
Unambiguous Each codon specifies only 1 amino acid.
Degenerate/ Most amino acids are coded by multiple codons. Exceptions: methionine (AUG) and tryptophan
redundant Wobble—codons that differ in 3rd (“wobble”) (UGG) encoded by only 1 codon.
position may code for the same tRNA/amino
acid. Specific base pairing is usually required
only in the first 2 nucleotide positions of
mRNA codon.
Commaless, Read from a fixed starting point as a continuous Exceptions: some viruses.
nonoverlapping sequence of bases.
Universal Genetic code is conserved throughout Exception in humans: mitochondria.
evolution.
FAS1_2019_01-Biochem.indd 37 11/7/19 3:16 PM
