Discovery of a cytokinin deaminase

Abstract

An enzyme of unknown function within the amidohydrolase superfamily was discovered to catalyze the hydrolysis of N-6-substituted adenine derivatives, several of which are cytokinins. Cytokinins are a common type of plant hormone and N-6-substituted adenines are also found as modifications to tRNA. Patl2390, from Pseudoalteromonas atlantica T6c, was shown to hydrolytically deaminate N-6-isopentenyladenine to hypoxanthine and isopentenylamine with a k(cat)/K(m) of 1.2 × 10(7) M(-1) s(-1). Additional substrates include N-6-benzyl adenine, cis- and trans-zeatin, kinetin, O-6-methylguanine, N-6-butyladenine, N-6-methyladenine, N,N-dimethyladenine, 6-methoxypurine, 6-chloropurine, and 6-thiomethylpurine. This enzyme does not catalyze the deamination of adenine or adenosine. A comparative model of Patl2390 was computed using the three-dimensional crystal structure of Pa0148 (PDB code 3PAO ) as a structural template, and docking was used to refine the model to accommodate experimentally identified substrates. This is the first identification of an enzyme that will hydrolyze an N-6-substituted side chain larger than methylamine from adenine.

Figure 1

Sequence similarity network created using Cytoscape(6) of cog1816 from the amidohydrolase superfamily. Each node in the network represents a single sequence, and each edge (depicted as lines) represents the pairwise connection between two sequences at a BLAST E-value of better than 1 × 10⁻⁷⁰. Lengths of edges are not significant, except for tightly clustered groups which are more closely related than sequences with only a few connections. Group 5 contains the E. coli adenosine deaminase. The yellow dots in Group 3 represent proteins that will deaminate adenine. The proteins represented by the pink dots at the periphery of Group 3 are characterized in this paper.

Figure 2

A structural model of the active site of Patl2390 with N-6-benzyladenine.

Scheme 1

Scheme 2