Bioinformatic analysis of fold-type III PLP-dependent enzymes discovers multimeric racemases

Maintenance of cellular vitamin B6 levels and mitochondrial oxidative function depend on pyridoxal 5'-phosphate homeostasis protein

Recently, biallelic variants in PLPBP coding for pyridoxal 5'-phosphate homeostasis protein (PLPHP) were identified as a novel cause of early-onset vitamin B6-dependent epilepsy. The molecular function and precise role of PLPHP in vitamin B6 metabolism are not well understood. To address these questions, we used PLPHP-deficient patient skin fibroblasts and HEK293 cells and YBL036C (PLPHP ortholog)-deficient yeast. We showed that independent of extracellular B6 vitamer type (pyridoxine, pyridoxamine, or pyridoxal), intracellular pyridoxal 5'-phosphate (PLP) was lower in PLPHP-deficient fibroblasts and HEK293 cells than controls. Culturing cells with pyridoxine or pyridoxamine led to the concentration-dependent accumulation of pyridoxine 5'-phosphate and pyridoxamine 5'-phosphate (PMP), respectively, suggesting insufficient pyridox(am)ine 5'-phosphate oxidase activity. Experiments utilizing 13C4-pyridoxine confirmed lower pyridox(am)ine 5'-phosphate oxidase activity and revealed increased fractional turnovers of PLP and pyridoxal, indicating increased PLP hydrolysis to pyridoxal in PLPHP-deficient cells. This effect could be partly counteracted by inactivation of pyridoxal phosphatase. PLPHP deficiency had a distinct effect on mitochondrial PLP and PMP, suggesting impaired activity of mitochondrial transaminases. Moreover, in YBL036C-deficient yeast, PLP was depleted and PMP accumulated only with carbon sources requiring mitochondrial metabolism. Lactate and pyruvate accumulation along with the decrease of tricarboxylic acid cycle intermediates downstream of α-ketoglutarate suggested impaired mitochondrial oxidative metabolism in PLPHP-deficient HEK293 cells. We hypothesize that impaired activity of mitochondrial transaminases may contribute to this depletion. Taken together, our study provides new insights into the pathomechanisms of PLPBP deficiency and reinforces the link between PLPHP function, vitamin B6 metabolism, and mitochondrial oxidative metabolism.

Characteristics of alanine racemase in Lactobacillus sakeiZH-2 strain

Some d-amino acid functions for food production are widely known: d-alanine improves sensory evaluations of sake, beer, and fermented foods. Therefore, for the application of d-amino acids, alanine racemase (ALRase) in Lactobacillus sakei ZH-2, which has strong racemization, was analyzed using molecular biological methods. It had been hypothesized that ALRase coding DNA, alr, in ZH-2 strain differs from those of other Lactobacillus sakei strains. However, complete genome sequencing by the National Center for Biotechnology (NCBI) revealed the amino acid sequence of alr in ZH-2 strain to have homology of 99.4% similarity with the alr in Lactobacillus sakei 23K strain. However, it is considered that the sequence of alr was a unique amino acid sequence in the lactic acid bacteria group. DNA "alr" of ZH-2 strain has a 1140 bp DNA base with 41 kDa molecular mass. Its molecular mass was inferred as approximately 38.0 kDa using SDS-PAGE. Its optimum conditions are pH 9.0 at 30-40°C, showing stability at pH 9.0-10.0 and 4-40°C. Its cofactor is pyridoxal phosphate. Its activity is activated more by copper and zinc ions than by the lack of a metal ion. Additionally, its K m is 1.32 × 10-3 (mol), with V max of 4.27 × 10-5 (μmol-1 min-1). ALRase reacted against alanine most strongly in other substrates such as amino acids. The enzyme against serine was found to have 40% activity against alanine. The enzyme converted up to 54.5% of d-alanine from l-alanine ZH-2 strain.

Structural and Functional Analysis of the Pyridoxal Phosphate Homeostasis Protein YggS from Fusobacterium nucleatum

Pyridoxal 5′-phosphate (PLP) is the active form of vitamin B6, but it is highly reactive and poisonous in its free form. YggS is a PLP-binding protein found in bacteria and humans that mediates PLP homeostasis by delivering PLP to target enzymes or by performing a protective function. Several biochemical and structural studies of YggS have been reported, but the mechanism by which YggS recognizes PLP has not been fully elucidated. Here, we report a functional and structural analysis of YggS from Fusobacterium nucleatum (FnYggS). The PLP molecule could bind to native FnYggS, but no PLP binding was observed for selenomethionine (SeMet)-derivatized FnYggS. The crystal structure of FnYggS showed a type III TIM barrel fold, exhibiting structural homology with several other PLP-dependent enzymes. Although FnYggS exhibited low (<35%) amino acid sequence similarity with previously studied YggS proteins, its overall structure and PLP-binding site were highly conserved. In the PLP-binding site of FnYggS, the sulfate ion was coordinated by the conserved residues Ser201, Gly218, and Thr219, which were positioned to provide the binding moiety for the phosphate group of PLP. The mutagenesis study showed that the conserved Ser201 residue in FnYggS was the key residue for PLP binding. These results will expand the knowledge of the molecular properties and function of the YggS family.