Crystal structure analysis of recombinant rat kidney long chain hydroxy acid oxidase.

Long chain hydroxy acid oxidase (LCHAO) is a member of an FMN-dependent enzyme family that oxidizes L-2-hydroxy acids to ketoacids. LCHAO is a peroxisomal enzyme, and the identity of its physiological substrate is unclear. Mandelate is the most efficient substrate known and is commonly used in the test tube. LCHAO differs from most family members in that one of the otherwise invariant active site residues is a phenylalanine (Phe23) instead of a tyrosine. We now report the crystal structure of LCHAO. It shows the same beta8alpha8 TIM barrel structure as other structurally characterized family members, e.g., spinach glycolate oxidase (GOX) and the electron transferases yeast flavocytochrome b2 (FCB2) and Pseudomonas putida mandelate dehydrogenase (MDH). Loop 4, which is mobile in other family members, is visible in part. An acetate ion is present in the active site. The flavin interacts with the protein in the same way as in the electron transferases, and not as in GOX, an unexpected observation. An interpretation is proposed to explain this difference between GOX on one hand and FCB2 and LCHAO on the other hand, which had been proposed to arise from the differences between family members in their reactivity with oxygen. A comparison of models of the substrate bound to various published structures suggests that the very different reactivity with mandelate of LCHAO, GOX, FCB2, and MDH cannot be rationalized by a hydride transfer mechanism.

p-Cresol methylhydroxylase: alteration of the structure of the flavoprotein subunit upon its binding to the cytochrome subunit.

The structures of two forms of a recombinant flavoprotein have been determined at high resolution and compared. These proteins are (1) the flavocytochrome c p-cresol methylhydroxylase (rPCMH, 1.85 A resolution) and (2) the cytochrome-free flavoprotein subunit of rPCMH (PchF, 1.30 A resolution). A significant conformational difference is observed in a protein segment that is in contact with the re face of the isoalloxazine ring of FAD when the structure of PchF is compared to the subunit in the intact flavocytochrome. This structural change is important for optimum catalytic function of the flavoprotein, which has been shown to be dependent on the presence of the cytochrome subunit. This change results in different protein-flavin and apparently different protein-substrate interactions that have a "tuning effect" on the electronic and redox properties of bound p-cresol and the covalently bound FAD. The conformational change in the segment in the cofactor-binding site is induced by a small rearrangement in the flavoprotein-cytochrome interface region of the flavoprotein.