Amine cations promote concurrent conversion of prohistidine decarboxylase from Lactobacillus 30a to active enzyme and a modified proenzyme

On the origin of biochemistry at an alkaline hydrothermal vent

A model for the origin of biochemistry at an alkaline hydrothermal vent has been developed that focuses on the acetyl-CoA (Wood-Ljungdahl) pathway of CO2 fixation and central intermediary metabolism leading to the synthesis of the constituents of purines and pyrimidines. The idea that acetogenesis and methanogenesis were the ancestral forms of energy metabolism among the first free-living eubacteria and archaebacteria, respectively, stands in the foreground. The synthesis of formyl pterins, which are essential intermediates of the Wood-Ljungdahl pathway and purine biosynthesis, is found to confront early metabolic systems with steep bioenergetic demands that would appear to link some, but not all, steps of CO2 reduction to geochemical processes in or on the Earth's crust. Inorganically catalysed prebiotic analogues of the core biochemical reactions involved in pterin-dependent methyl synthesis of the modern acetyl-CoA pathway are considered. The following compounds appear as probable candidates for central involvement in prebiotic chemistry: metal sulphides, formate, carbon monoxide, methyl sulphide, acetate, formyl phosphate, carboxy phosphate, carbamate, carbamoyl phosphate, acetyl thioesters, acetyl phosphate, possibly carbonyl sulphide and eventually pterins. Carbon might have entered early metabolism via reactions hardly different from those in the modern Wood-Ljungdahl pathway, the pyruvate synthase reaction and the incomplete reverse citric acid cycle. The key energy-rich intermediates were perhaps acetyl thioesters, with acetyl phosphate possibly serving as the universal metabolic energy currency prior to the origin of genes. Nitrogen might have entered metabolism as geochemical NH3 via two routes: the synthesis of carbamoyl phosphate and reductive transaminations of alpha-keto acids. Together with intermediates of methyl synthesis, these two routes of nitrogen assimilation would directly supply all intermediates of modern purine and pyrimidine biosynthesis. Thermodynamic considerations related to formyl pterin synthesis suggest that the ability to harness a naturally pre-existing proton gradient at the vent-ocean interface via an ATPase is older than the ability to generate a proton gradient with chemistry that is specified by genes.

Molecular evolution of proteasomes

Proteasomes are large, multisubunit proteases that are found, in one form or another, in all domains of life and play a critical role in intracellular protein degradation. Although they have substantial structural similarity, the proteasomes of bacteria, archaea, and eukaryotes show many differences in architecture and subunit composition. This article discusses possible paths by which proteasomes may have evolved from simple precursors to the highly complicated and diverse complexes observed today.

Cloning, sequencing, expression, and site-directed mutagenesis of the gene from Clostridium perfringens encoding pyruvoyl-dependent histidine decarboxylase

The DNA encoding pyruvoyl-dependent histidine decarboxylase (HisDCase) of Clostridium perfringens was cloned, sequenced, and overexpressed in Escherichia coli. The gene encodes a single polypeptide of 320 amino acids, Mr 35,526, demonstrating that clostridial HisDCase, which has an (alpha beta)6 structure, is synthesized as a precursor (proHisDCase, pi 6). No pi subunits of proHisDCase were observed in crude or purified preparations of the cloned HisDCase; they appear to undergo rapid cleavage in vivo to the alpha (Mr 24,887) and beta (Mr 10,526) subunits characteristic of this HisDCase. This cleavage occurs between Ser-96 and Ser-97; Ser-97 gives rise to the catalytically essential pyruvoyl group blocking the N-termini of the alpha subunits of the active enzyme. When Ser-97 was converted to an alanyl residue by site-specific mutagenesis, the expressed, inactive protein (pi' 6) contained a single peptide species (pi', Mr 35,510) that was not cleaved either in vivo or in vitro. These results support previous conclusions that activation of the wild-type clostridial proenzyme occurs via nonhydrolytic serinolysis. Although clostridial HisDCase has only a 47% sequence similarity to HisDCase from Lactobacillus 30a, all of the residues known to be important for substrate binding and catalytic action of the Lactobacillus HisDCase are conserved in the C. perfringens enzyme. While the encoded N-terminal Met of clostridial HisDCase is removed by E. coli, the cloned enzyme retains a 10-residue presequence (NKNLEANRNR) not present in the mature enzyme isolated from C. perfringens.