Gene/Protein
Disease
Symptom
Drug
Enzyme
Compound
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Gene/Protein
Disease
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Drug
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Target Concepts:
Gene/Protein
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Enzyme
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Query: EC:2.3.1.28 (
chloramphenicol acetyltransferase
)
5,100
document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)
A catalytically essential histidine residue (His-195) of
chloramphenicol acetyltransferase
(
CAT
) acts as a general base in catalysis, abstracting a proton from the primary hydroxy group of chloramphenicol. The pKa of His-195 has been determined from the pH-dependence of chemical modification. Both methyl 4-nitrobenzenesulphonate and iodoacetamide inactivate
CAT
by irreversible modification of His-195. The kinetics of inactivation by methyl 4-nitrobenzenesulphonate are pseudo-first-order, and the pH-dependence of inactivation yields a pKa value of 6.60.
Iodoacetamide
inactivation proceeds with second-order kinetics and a pKa value of 6.80. An alternative site of modification at the active site of
CAT
is the thiol group of Cys-31, a residue which has no catalytic role. On replacement of Cys-31 with alanine (Ala-31
CAT
), the pH-dependence of iodoacetamide inactivation gives a pKa value of 6.66. The pKa values derived from chemical-modification experiments directed at His-195 are in agreement with the pKa values of 6.62 and 6.61 determined for wild-type and Ala-31
CAT
respectively from the pH-dependence of kcat/Km.
...
PMID:The pKa of the catalytic histidine residue of chloramphenicol acetyltransferase. 843 83
