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Chapter 3 : Structural Aspects of Aminoglycoside-Modifying Enzymes
Category: Bacterial Pathogenesis
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Three structurally and functionally unrelated classes of inactivation enzymes are known: the aminoglycoside kinases (APH), the adenylyltransferases (ANT), and the acetyltransferases (AAC). The past decade has seen a dramatic increase in our understanding of the structure and function of members of each of these classes, and each is discussed separately in this chapter. It was subsequently demonstrated that APHs maintain vestigial protein kinases activity. In fact, since the structure determination of APH(3’)-IIIa, which represented the first distant relative of the protein kinases, this superfamily has further expanded and includes now lipid kinases and choline kinases. One of the most intriguing differences when comparing Ser/Thr and Tyr kinases with aminoglycoside kinases is the region of the enzymes involved in substrate binding. In fact, the activation loop is completely lacking in APH(3’)-IIIa, and vice versa there are no remnants of an aminoglycoside-binding loop segment in protein kinases. Aminoglycoside adenylyltransferases (ANTs) catalyze the transfer of AMP to aminoglycoside hydroxyl groups. Other members of the GNAT family include other small-molecule acyltransferases such as serotonin acetyltransferase and protein acetyltransferases such as the histone acetyltransferases. The realization following the determination of enzyme three-dimensional structure that aminoglycoside resistance enzymes are members of larger families of proteins that share similar structures and mechanisms has permitted insight into the origins and evolution of resistance. This information will arm us in the continuing efforts to meet the challenge of resistance at the molecular level and apply this work to the management of infectious disease.
Structures and nomenclature of aminoglycoside antibiotics. For a more complete list, see reference 69 .
Interaction of paromomycin with the A-site 16S rRNA. Paromomycin is shown in black. Nucleotides A1408, A1492, and A1493 are shown in gray. Upon binding of paromomycin, nucleotides A1492 and A1493 are flipped out in translation-competent mode.
Three-dimensional structures of APH(3′)-IIIa and related kinases. Conserved secondary structure elements are shown in the bottom half of the figure.
Aminoglycoside binding region of APH(3′)-IIIa. The 6-aminohexose (p ring) and 2-deoxystreptamine rings of neomycin (black) and kanamycin (gray) bind in identical fashion while the double prime (q) rings of both substrates bind in different sub-sites ( 26 ).
Domain structure of ANT(4′). Each monomer in the active site is shown in a different shade of gray. The substrates kanamycin and the nonhydrolyzable ATP analogue AMPCPP are shown in black in the active sites situated at the dimer interface.
Aminoglycoside acetyltransferases are members of the GNAT superfamily.
Aminoglycoside antibiotics in clinical use