Comparison of Activation Enthalpies for Aminoglycoside Modification Reactions

At highly elevated temperatures, many biological reactions can proceed spontaneously from the ground state to the transition state. However, due to the long half-life of these reactions, catalysts are required to catalyze these reactions at modern day temperatures by lowering the activation energy. Wolfenden et al. has previously shown that catalysts enhance the rate of the reaction by reducing the enthalpy of activation. Therefore, the activation energies have been determined for three aminoglycoside modifying enzymes, APH(3’)-IIIa, AAC(3)-IIIb, and AAC(3)-VIa, to determine whether these three enzymes distinguish between the two classes of aminoglycoside antibiotics by reducing the enthalpy of activation during catalysis. Aminoglycosides are broad-spectrum antibiotics active against Gram-positive and Gram-negative organisms. These antibiotics contain a 2-deoxystreptamine ring and are classified into two classes: the 4,6-di-substituted kanamycin-like aminoglycosides and the 4,5-di-substituted neomycin-like aminoglycosides. Since their discovery in the 1940s, bacteria have shown a high level of resistance to these antimicrobial agents mainly due to the emergence of aminoglycoside modifying enzymes (AGMEs). AGMEs are classified into three main families based on the type of modification reaction: (a) O-Nucleotidyltransferases (ANT) catalyze the adenylation reaction of hydroxyl groups in the presence of ATP; (b) O-Phosphotransferases (APH) phosphorylate hydroxyl groups in the presence of ATP; (c) and N-Acetyltransferases (AAC) which acetylate an amino group on the 2-deoxystreptamine ring or aminohexose sugar rings in the presence of acetyl coenzyme A as the acetyl donor. APH(3’)-IIIa and AAC(3)-IIIb are two very promiscuous AGMEs while AAC(3)-VIa has a limited substrate profile. In general, there were no observed trends in the activation energies that distinguished between the two classes of aminoglycoside antibiotics for APH(3’)-IIIa and AAC(3)-IIIb. When studying the activation energies for a more hydrophobic aminoglycoside, gentamicin C₁ [C1] and C₂ [C2], AAC(3)-IIIb appeared to favor the more methylated gentamicin component, gentamicin C₁ [C1], whereas AAC(3)-VIa was shown to have a lower activation energy for gentamicin C₂ [C2].