Purification, crystallization and preliminary crystallographic analysis of the glycine-cleavage system component T-protein from Pyrococcus horikoshii OT3

Structure of aryl O-demethylase offers molecular insight into a catalytic tyrosine-dependent mechanism

Some strains of soil and marine bacteria have evolved intricate metabolic pathways for using environmentally derived aromatics as a carbon source. Many of these metabolic pathways go through intermediates such as vanillate, 3-O-methylgallate, and syringate. Demethylation of these compounds is essential for downstream aryl modification, ring opening, and subsequent assimilation of these compounds into the tricarboxylic acid (TCA) cycle, and, correspondingly, there are a variety of associated aryl demethylase systems that vary in complexity. Intriguingly, only a basic understanding of the least complex system, the tetrahydrofolate-dependent aryl demethylase LigM from Sphingomonas paucimobilis, a bacterial strain that metabolizes lignin-derived aromatics, was previously available. LigM-catalyzed demethylation enables further modification and ring opening of the single-ring aromatics vanillate and 3-O-methylgallate, which are common byproducts of biofuel production. Here, we characterize aryl O-demethylation by LigM and report its 1.81-Å crystal structure, revealing a unique demethylase fold and a canonical folate-binding domain. Structural homology and geometry optimization calculations enabled the identification of LigM's tetrahydrofolate-binding site and protein-folate interactions. Computationally guided mutagenesis and kinetic analyses allowed the identification of the enzyme's aryl-binding site location and determination of its unique, catalytic tyrosine-dependent reaction mechanism. This work defines LigM as a distinct demethylase, both structurally and functionally, and provides insight into demethylation and its reaction requirements. These results afford the mechanistic details required for efficient utilization of LigM as a tool for aryl O-demethylation and as a component of synthetic biology efforts to valorize previously underused aromatic compounds.

Single cells within the Puerto Rico trench suggest hadal adaptation of microbial lineages

Hadal ecosystems are found at a depth of 6,000 m below sea level and below, occupying less than 1% of the total area of the ocean. The microbial communities and metabolic potential in these ecosystems are largely uncharacterized. Here, we present four single amplified genomes (SAGs) obtained from 8,219 m below the sea surface within the hadal ecosystem of the Puerto Rico Trench (PRT). These SAGs are derived from members of deep-sea clades, including the Thaumarchaeota and SAR11 clade, and two are related to previously isolated piezophilic (high-pressure-adapted) microorganisms. In order to identify genes that might play a role in adaptation to deep-sea environments, comparative analyses were performed with genomes from closely related shallow-water microbes. The archaeal SAG possesses genes associated with mixotrophy, including lipoylation and the glycine cleavage pathway. The SAR11 SAG encodes glycolytic enzymes previously reported to be missing from this abundant and cosmopolitan group. The other SAGs, which are related to piezophilic isolates, possess genes that may supplement energy demands through the oxidation of hydrogen or the reduction of nitrous oxide. We found evidence for potential trench-specific gene distributions, as several SAG genes were observed only in a PRT metagenome and not in shallower deep-sea metagenomes. These results illustrate new ecotype features that might perform important roles in the adaptation of microorganisms to life in hadal environments.

Crystal structure of T-protein of the glycine cleavage system. Cofactor binding, insights into H-protein recognition, and molecular basis for understanding nonketotic hyperglycinemia

The glycine cleavage system catalyzes the oxidative decarboxylation of glycine in bacteria and in mitochondria of animals and plants. Its deficiency in human causes nonketotic hyperglycinemia, an inborn error of glycine metabolism. T-protein, one of the four components of the glycine cleavage system,is a tetrahydrofolate dependent aminomethyltransferase. It catalyzes the transfer of the methylene carbon unit to tetrahydrofolate from the methylamine group covalently attached to the lipoamide arm of H-protein. To gain insight into the T-protein function at the molecular level, we have determined the first crystal structure of T-protein from Thermotoga maritima by the multiwavelength anomalous diffraction method of x-ray crystallography and refined four structures: the apoform; the tetrahydrofolate complex; the folinic acid complex; and the lipoic acid complex. The overall fold of T-protein is similar to that of the C-terminal tetrahydrofolate-binding region (residues 421-830) of Arthrobacter globiformis dimethylglycine oxidase. Tetrahydrofolate (or folinic acid) is bound near the center of the tripartite T-protein. Lipoic acid is bound adjacent to the tetrahydrofolate binding pocket, thus defining the interaction surface for H-protein binding. A homology model of the human T-protein provides the structural framework for understanding the molecular mechanisms underlying the development of nonketotic hyperglycinemia due to missense mutations of the human T-protein.