The aspartate aminotransferase-catalysed exchange of the alpha-protons of aspartate and glutamate: the effects of the R386A and R292V mutations on this exchange reaction

Vitamin B12 Auxotrophy in Isolates from the Deep Subsurface of the Iberian Pyrite Belt

Vitamin B₁₂ is an enzymatic cofactor that is essential for both eukaryotes and prokaryotes. The development of life in extreme environments depends on cofactors such as vitamin B₁₂ as well. The genomes of twelve microorganisms isolated from the deep subsurface of the Iberian Pyrite Belt have been analyzed in search of enzymatic activities that require vitamin B₁₂ or are involved in its synthesis and import. Results have revealed that vitamin B₁₂ is needed by these microorganisms for several essential enzymes such as ribonucleotide reductase, methionine synthase and epoxyqueosine reductase. Isolate Desulfosporosinus sp. DEEP is the only analyzed genome that holds a set core of proteins that could lead to the production of vitamin B₁₂. The rest are dependent on obtaining it from the subsurface oligotrophic environment in which they grow. Sought proteins involved in the import of vitamin B₁₂ are not widespread in the sample. The dependence found in the genomes of these microorganisms is supported by the production of vitamin B₁₂ by microorganisms such as Desulfosporosinus sp. DEEP, showing that the operation of deep subsurface biogeochemical cycles is dependent on cofactors such as vitamin B₁₂.

Labeling of membrane proteins by cell-free expression

The particular advantage of the cell-free reaction is that it allows a plethora of supplementation during protein expression and offers complete control over the available amino acid pool in view of concentration and composition. In combination with the fast and reliable production efficiencies of cell-free systems, the labeling and subsequent structural evaluation of very challenging targets, such as membrane proteins, comes into focus. We describe current methods for the isotopic labeling of cell-free synthesized membrane proteins and we review techniques available to the practitioner pursuing structural studies by nuclear magnetic resonance spectroscopy. Though isotopic labeling of individual amino acid types appears to be relatively straightforward, an ongoing critical issue in most labeling schemes for structural approaches is the selective substitution of deuterons for protons. While few options are available, the continuous refinement of labeling schemes in combination with improved pulse sequences and optimized instrumentation gives promising perspectives for extended applications in the structural evaluation of cell-free synthesized membrane.

It takes two to tango: defining an essential second active site in pyridoxal 5'-phosphate synthase

The prevalent de novo biosynthetic pathway of vitamin B6 involves only two enzymes (Pdx1 and Pdx2) that form an ornate multisubunit complex functioning as a glutamine amidotransferase. The synthase subunit, Pdx1, utilizes ribose 5-phosphate and glyceraldehyde 3-phosphate, as well as ammonia derived from the glutaminase activity of Pdx2 to directly form the cofactor vitamer, pyridoxal 5′-phosphate. Given the fact that a single enzyme performs the majority of the chemistry behind this reaction, a complicated mechanism is anticipated. Recently, the individual steps along the reaction co-ordinate are beginning to be unraveled. In particular, the binding of the pentose substrate and the first steps of the reaction have been elucidated but it is not known if the latter part of the chemistry, involving the triose sugar, takes place in the same or a disparate site. Here, we demonstrate through the use of enzyme assays, enzyme kinetics, and mutagenesis studies that indeed a second site is involved in binding the triose sugar and moreover, is the location of the final vitamin product, pyridoxal 5′-phosphate. Furthermore, we show that product release is triggered by the presence of a PLP-dependent enzyme. Finally, we provide evidence that a single arginine residue of the C terminus of Pdx1 is responsible for coordinating co-operativity in this elaborate protein machinery.

The stereospecificity and catalytic efficiency of the tryptophan synthase-catalysed exchange of the alpha-protons of amino acids

13C-NMR has been used to follow the tryptophan synthase (EC 4.2.1.20) catalysed hydrogen-deuterium exchange of the pro-2R and pro-2S protons of [2-13C]glycine at pH 7.8. 1H-NMR has also been used to follow the tryptophan-synthase-catalysed hydrogen-deuterium exchange of the alpha-protons of a range of L- and D-amino acids at pH 7.8. The pK(a) values of the alpha-protons of these amino acids have been estimated and we have determined whether or not their exchange rates can be predicted from their pK(a) values. With the exception of tryptophan and norleucine, the stereospecificities of the first-order alpha-proton exchange rates are independent of the size and electronegativity of the amino acid R-group. Similar results are obtained with the second-order alpha-proton exchange rates, except that both L-tryptophan and L-serine have much higher stereospecificities than all the other amino acids studied.

Stereospecificity of alpha-proton exchange reactions catalysed by pyridoxal-5'-phosphate-dependent enzymes

A NMR method for quantifying the catalytic efficiency and stereospecificity of the exchange of the alpha-protons of glycine is described. It is used to determine how the binding of the alpha-carboxylate group of amino acids contributes to the stereospecificity of exchange reactions catalysed by tryptophan synthase, serine hydroxymethyltransferase and a catalytic antibody utilising pyridoxal-5'-phosphate (PLP) as a cofactor. Using larger substrates, it is shown how the size of the amino acid side chain contributes to the stereospecificity of exchange. Mutants of aspartate aminotransferase are used to determine how substrate binding controls the catalytic efficiency and stereospecificity of the exchange of the alpha-protons of aspartate and glutamate. Evidence is presented which shows that with serine hydroxymethyltransferase, L-norleucine is not bound at the same catalytic site as glycine. Finally the catalytic efficiency and stereospecificity of the alpha-proton exchange reactions catalysed by all the PLP-dependent catalysts examined are compared.