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Aminoglycoside-3'-phosphotransferase
Aminoglycoside-3'-phosphotransferase (APH(3')), also known as aminoglycoside kinase, is an enzyme that primarily catalyzes the addition of phosphate from ATP to the 3'-hydroxyl group of a 4,6-disubstituted aminoglycoside, such as kanamycin. However, APH(3') has also been found to phosphorylate at the 5'-hydroxyl group in 4,5-disubstituted aminoglycosides, which lack a 3'-hydroxyl group, and to diphosphorylate hydroxyl groups in aminoglycosides that have both 3'- and 5'-hydroxyl groups.〔 Primarily positively charged at biological conditions, aminoglycosides bind to the negatively charged backbone of nucleic acids to disrupt protein synthesis, effectively inhibiting bacterial cell growth. APH(3') mediated phosphorylation of aminoglycosides effectively disrupts their mechanism of action, introducing a phosphate group that reduces their binding affinity due to steric hindrances and unfavorable electrostatic interactions. APH(3') is primarily found in certain species of gram-positive bacteria.
==Structure==
APH(3’) thermodynamically favors a dimer form of two identical APH(3’) monomers that are connected by two disulfide bonds between Cys19 and Cys156, with the active sites facing each other.〔 However, the large distance between the two monomers’ active sites suggests that they are independent of each other, and do not operate in a cooperative fashion. Additionally, dimerization of APH(3’) does not affect the activity of the enzyme.〔〔
Each monomer consists of two lobes, the beta-sheet rich N-terminus and alpha-helix rich C-terminus, with a twelve amino acid region connecting the two. The N-terminal lobe is composed of 5 antiparallel ß-sheets, with an α-helix between sheets 3 and 4. The C-terminal lobe is divided into a central core region (two α-helices and a hairpin-loop followed by four ß-sheets), an insert region (two α-helices connected by a loop structure), and a C-terminal region (two α-helices).〔 The resulting pocket that is encapsulated by the two lobes make up the enzyme active site.〔 This pocket is largely composed of negatively charged amino acid residues, which stabilize the positive charge of and orient the substrate in the active site. Additionally, this pocket is thought to contribute to the promiscuity of the enzyme, allowing it to take in and stabilize several different kinds of aminoglycosides.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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