Amino acid substitutions in the subunit interface enhancing thermostability of Thermoplasma acidophilum citrate synthase

Functional conservatism in mitochondrial evolution: insight from hybridization of arctic and brook charrs

To assess the potential adaptive value of mtDNA, we evaluated functional properties and thermal sensitivity of key mitochondrial enzymes in two species that have originally evolved in different thermal environments (arctic charr, Salvelinus alpinus, and brook charr, S. fontinalis), as well as in their hybrids. We measured the activity of two enzymes of the electron transport system (cytochrome c oxidase and NADH-ubiquinone oxidoreductase), one enzyme of the mitochondrial matrix (citrate synthase), and one enzyme of the anaerobic glycolysis (lactate dehydrogenase) in the red muscle at three temperatures (6 degrees C, 12 degrees C and 18 degrees C). Surprisingly, the species presented no significant differences in enzyme activity, thermal sensitivity or thermostability of key metabolic enzymes even though they evolved in different thermal environments and present important differences in amino acid sequences. It seems that amino acid substitutions between those species have minor impact on the functional properties of mitochondrial enzymes studied. The thermal sensitivity results (Q(10)) obtained for inner-membrane mitochondrial enzymes differed somewhat from those of mitochondrial matrix or cytosolic enzymes. This result indicates the modulation of thermal sensitivity of all mitochondrial inner-membrane processes by a common parameter, which could be the structural and functional properties of membrane phospholipids.

Increasing the thermostability of Flavobacterium meningosepticum glycerol kinase by changing Ser329 to Asp in the subunit interface region

The thermostability enhancement of Flavobacterium meningosepticum glycerol kinase (FGK) by random mutagenesis in the subunit interface region was investigated. A single Escherichia coli transformant, which produced a more thermostable glycerol kinase than the parent enzyme, was obtained. The nucleotide sequence of the gene of the mutant enzyme (FGK2615) was determined, and the four amino acid replacements were identified as Glu327 to Asp, Ser329 to Asp, Thr330 to Ala and Ser334 to Lys. Although the properties of FGK2615 were fundamentally similar to those of the parent enzyme, the thermostability and Km for ATP had changed. The thermostability of FGK2615 was apparently increased; the temperature at which the enzyme activity is inactivated by 50% for a 30-min incubation of FGK2615 was determined to be 72.1 degrees C which was 3.1 degrees C higher than that of the parent FGK. Four additional mutants each having a single amino acid replacement (Glu327 to Asp, Ser329 to Asp, Thr330 to Ala and Ser334 to Lys) were prepared and their thermostability and Km for substrates were evaluated. The effect of the substitution of Ser329 to Asp is discussed.

Review: Protein function at thermal extremes: balancing stability and flexibility

No organism can survive across the entire temperature range found in the biosphere, and a given species can rarely support active metabolism across more than a few tens of degrees C. Nevertheless, life can be maintained at surprisingly extreme temperatures, from below -50 to over 110 degrees C. That proteins, which are assembled with the same 20 amino acids in all species, can function well at both extremes of this range illustrates the plasticity available in the construction of these macromolecules. In studying proteins from extremophiles, researchers have found no new amino acids, covalent modifications or structural motifs that explain the ability of these molecules to function in such harsh environments. Rather, subtle redistributions of the same intramolecular interactions required for protein stabilization at moderate temperatures are sufficient to maintain structural integrity at hot or cold extremes. The key to protein function, whether in polar seas or hot springs, is the maintenance of an appropriate balance between molecular stability on the one hand and structural flexibility on the other. Stability is needed to ensure the appropriate geometry for ligand binding, as well as to avoid denaturation, while flexibility is necessary to allow catalysis at a metabolically appropriate rate. Comparisons of homologous proteins from organisms spanning a wide range of thermal habitats show that adaptive mutations, as well as stabilizing solutes, maintain a balance between these two attributes, regardless of the temperature at which the protein functions.

The effect of valine substitution for glycine in the dimer interface of citrate synthase from Thermoplasma acidophilum on stability and activity

To determine the role of hydrophobic interactions in the dimer interface of citrate synthase (CS) from Thermoplasma (Tp) acidophilum in thermostabilization, we have used site-directed mutagenesis to replace Gly 196 by Val on the helix L of the subunit interface. Recombinant wild-type and Gly 196 mutant TpCS enzymes were largely identical in terms of substrate specificities (K(m) for oxaloacetate and acetyl CoA). However, the mutation not only reduced catalytic activity (about 10-fold) (i.e., V(max), k(cat) and specific activity) of the TpCS, but also decreased its thermal and chemical stability. Archaeal citrate synthase is active as a dimer, since residues from both monomers participate in the active site. Our results suggest that Gly196 --> Val mutation interferes with dimerization, so that improper dimerization or dissociation of the dimer would have a profound affect on the activity as well as the conformational stability of TpCS.