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Mitochondrial succinate CoA ligase (GDP-forming) catalyzes the reversible conversion of succinyl CoA to succinate plus Coenzyme A, coupled to the conversion of GDP and orthophosphate to GTP. The enzyme is a heterodimer containing SUCLG1 and SUCLG2 monomers.

The enzyme catalyzing the reaction in vertebrates is a heterodimer that occurs in two isoforms. The enzymes have been purified from pigeon and rat tissue and characterized in detail. Both isoforms, an alpha:betaA heterodimer and an alpha:betaG heterodimer, catalyze the reversible conversion of succinyl CoA to succinate plus Coenzyme A. The alpha:betaA heterodimer couples this conversion to the synthesis of ATP from ADP and orthophosphate, while the alpha:betaG heterodimer couples it to the synthesis of GTP from GDP and orthophosphate (Johnson et al. 1998a,b; Lambeth et al. 2004). Consistent with these results in model systems, patients homozygous for a mutant allele of the gene encoding the ADP enzyme beta subunit, SUCLA2, are deficient in succinyl CoA ligase activity (Elpeleg et al. 2005).

Both isoforms are found in vivo, and appear to be expressed at different levels in various tissues. Their relative contributions to the flux of carbon atoms through the TCA cycle are unknown. Genetic and biochemical data suggest that the alpha:betaA isoform may be required to catalyze the reverse reaction, conversion of succinate, Coenzyme A, and ATP to succinyl CoA, ADP, and orthophosphate for heme biosynthesis (Furuyama and Sassa 2000).

Mutations in SUCLG1 are the cause of the infantile metabolic disease named mitochondrial DNA depletion syndrome 9 (MTDPS9; MIM:245400; reviewed by Molaei Ramsheh et al., 2020).

The P-loop guanosine triphosphatases (GTPases) control a multitude of biological processes, ranging from cell division, cell cycling, and signal transduction, to ribosome assembly and protein synthesis. GTPases exert their control by interchanging between an inactive GDP-bound state and an active GTP-bound state, thereby acting as molecular switches. The common denominator of GTPases is the highly conserved guanine nucleotide-binding (G) domain that is responsible for binding and hydrolysis of guanine nucleotides.Translational GTPases (trGTPases) are a family of proteins in which GTPase activity is stimulated by the large ribosomal subunit. This family includes translation initiation, elongation, and release factors and contains four subfamilies that are widespread, if not ubiquitous, in all three superkingdoms :Prokaryotic initiation factor 2 (IF2) and the related eukaryotic initiation factor 5B (eIF5B), catalyze ribosomal subunit joining to form elongation- competent ribosomes .Bacterial SelB and eukaryotic/archaeal gamma subunit of initiation factor 2 (eIF-2gamma), specifically recognise noncanonical tRNAs. SelB specifically recognises selenocysteylated tRNA(Sec) and eIF-2gamma initiator tRNA (Met-tRNA(i)) .Bacterial elongation factor Tu (EF-Tu) and its archaeal and eukaryotic counterpart elongation factor 1 (EF-1 alpha), bring the aminoacyl-tRNA into the A site of the ribosome .Bacterial peptide elongation factor G (EF-G) and its counterpart in Eukarya and Archaea, EF-2, catalyse the translocation step of translation .The basic topology of the tr-type G domain consists of a six-stranded central β-sheet surrounded by five α-helices. Helices alpha2, alpha3 and alpha4 are on one side of the sheet, whereas alpha1 and alpha5 are on the other . GTP is bound by the CTF-type G domain in a way common for G domains involving five conserved sequence motifs termed G1-G5. The base is in contact with the NKxD (G4) and SAx (G5) motifs, and the phosphates of the nucleotide are stabilised by main- and side-chain interactions with the P loop GxxxxGKT (G1). The most severe conformational changes are observed for the two switch regions which contain the xT/Sx (G2) and DxxG (G3) motifs that function as sensors for the presence of the gamma-phosphate. A Mg(2+) ion is coordinated by six oxygen ligands with octahedral coordination geometry; two of the ligands are water molecules, two come from the beta- and gamma-phosphates, and two are provided by the side chains of G1 and G2 threonines.This entry represents a conserved site in the tr-type G domain. It is on a G2-containing region that has been shown to be involved in a conformational change mediated by the hydrolysis of GTP to GDP.