Effects of interface mutations on the dimerization of alanine glyoxylate aminotransferase and implications in the mistargeting of the pathogenic variants F152I and I244T

In this work the dimerization process of the minor allelic form of human alanine glyoxylate aminotransferase, a pyridoxal 5'-phosphate enzyme, was investigated. Bioinformatic analyses followed by site-directed mutagenesis, size exclusion chromatography and catalytic activity experiments allowed us to identify Arg118, Phe238 and Phe240 as interfacial residues not essential for transaminase activity but important for dimer-monomer dissociation. The apo and the holo forms of the triple mutant R118A-Mi/F238S-Mi/F240S-Mi display a dimer-monomer equilibrium dissociation constant value at least ̴ 260- and 31-fold larger, respectively, than the corresponding ones of AGT-Mi. In the presence of PLP, the apomonomer of the triple mutant undergoes a biphasic process: the fast phase represents the formation of an inactive PLP-bound monomer, while the slow phase depicts the monomer-monomer association that parallels the regain of transaminase activity. The latter events occur with a rate constant of ̴ 0.02 μM(-1)min(-1). In the absence of PLP, the apomonomer is also able to dimerize but with a rate constant value ̴ 2700-fold lower. Thereafter, the possible interference with the dimerization process of AGT-Mi exerted by the mutated residues in the I244T-Mi and F152I-Mi variants associated with Primary Hyperoxaluria type 1 was investigated by molecular dynamics simulations. On the basis of the present and previous studies, a model for the dimerization process of AGT-Mi, I244T-Mi and F152I-Mi, which outlines the structural defects responsible for the complete or partial mistargeting of the pathogenic variants, was proposed and discussed.