Amino Acids, Proteins and Porphyrins
Which of the following is true? (AIPG 2011)
|A||Inhibition of acetylcholine esterase by Malathion can be reversed by increasing acetylcholine levels.|
|B||Sulphonilamides reduce folate synthesis irreversibly|
|C||Fluoroacetate competitively inhibits aconitase|
|D||Ethanol inhibits aldehyde dehydrogenase when used in methanol poisoning|
(Ref. Harper Biochemistry 28th/ pg. 143)
a. Malathion is an organophosphate parasympathomimetic which binds irreversibly to cholinesterase.
b. Sulfonamides competitively inhibit dihydropteroate synthase which acts to incorporate para-aminobenzoic acid (PABA) into dihydropteroic acid, the precursor to folic acid. Trimethoprim and pyrimethamine inhibit dihydrofolatereductase, which is responsible for the conversion of dihydrofolic acid to the active tetrahydrafolic acid.
c. Thus, the combination of sulfonamides with trimethoprim or pyrimethamine acts synergistically to reduce intracellular folate and subsequently inhibit nucleotide synthesis and bacterial cell growth. Sulfonamides and trimethoprim are active against gram-positive and gram-negative bacteria, Actinomyces, Nocardiaspp., Chlamydia, Plasmodium, and Toxoplasma.
d. Fluoroacetate is metabolized by cells to form fluorocitrate, a potent and specific competitive and reversible inhibitor of aconitase.
e. Ethanol Protects from Methanol Poisoning, Ethanol competes with Methanol for binding sites on alcohol dehydrogenase. MOA: competitively inhibits alcohol dehydrogenase (8000 times greater affinity) .