A step towards understanding of the molecular basis of ligand promiscuity in the aminoglycoside modifying enzymes

Aminoglycosides have proven very useful in the treatment of infections; lately their effectiveness has been greatly reduced due to increasing resistance. Among many known mechanisms of resistance to aminoglycosides, enzymatic modification is the most prevailing. More than 14 aminoglycoside -N3-acetyltransferases- a class of aminoglycoside modifying enzymes, are known today. This study focuses on a pair of acetyl transferases: The aminoglycoside-N3- acetyltransferase IIIb (AAC-IIIb) and the aminoglycoside-N3- acetyltransferase IIa (AAC-IIa). AAC-IIa and AAC-IIIb are very similar in their amino acid sequence and structure – yet they have a strong difference in their substrate selectivity, kinetic and thermodynamic properties. This work represents a comparative study of these two enzymes in an effort to determine thermodynamic basis of the differential substrate profiles of AAC-IIa to AAC-IIIb.