Genome-wide identification and gene expression analysis of GHMP kinase gene family in banana cv. Rasthali.

BACKGROUND: The GHMP kinase gene family encompasses ATP-dependent kinases, significantly involved in the biosynthesis of isoprenes, amino acids, and metabolism of carbohydrates. Banana is a staple tropical crop that is globally consumed but known for high sensitivity to salt, cold, and drought stresses. The GHMP kinases are known to play a significant role during abiotic stresses in plants. The present study emphasizes the role of GHMP kinases in various abiotic stress conditions in banana. METHODS AND RESULTS: We identified 12 GHMP kinase (MaGHMP kinase) genes in the banana genome database and witnessed the presence of the conserved Pro-X-X-X-Gly-Leu-X-Ser-Ser-Ala domain in their protein sequences. All genes were found to be involved in ATP-binding and carried kinase activity confronting their biological roles in the isoprene (27%) and amino acid (20%) biosyntheses. The expression analysis of genes during cold, drought, and salt stress conditions in tissue culture grown banana cultivar Rasthali plants showed a significant involvement of MaGHMP kinase genes in these stress conditions. The highest expression of MaGHMP kinase3 (8.5 fold) was noted during cold stress, while MaGHMP kinase1 (25 fold and 40.01 fold) showed maximum expression during drought and salt stress conditions in leaf tissue of Rasthali. CONCLUSION: Our findings suggested that MaGHMP kinase1 (MaHSK) and MaGHMP kinase3 (MaGlcAK) could be considered promising candidates for thwarting the abiotic stresses in banana.

Improving the Activity and Stability of Human Galactokinase for Therapeutic and Biotechnological Applications.

Galactokinase catalyses the site- and stereospecific phosphorylation of α-d-galactose. As such it has attracted interest as a biocatalyst for the introduction of phosphate groups into monosaccharides. However, attempts to broaden the substrate range of human galactokinase have generally resulted in substantially reduced activity. The enzyme also has biotechnological potential in enzyme replacement therapy (ERT) for type II galactosaemia. The return-to-consensus approach can be used to identify residues that can be altered to increase protein stability and enzyme activity. This approach identified six residues of potential interest in human galactokinase. Some of the single consensus variants (M60V, D268E, A334S and G373S) increased the catalytic turnover of the enzyme, but none resulted in improved stability. When all six changes were introduced into the protein (M60V/M180V/D268E/A334S/R366Q/G373S), thermal stability was increased. Molecular dynamics simulations suggested that these changes altered the protein's conformation at key sites. The number of salt bridges and hydrogen bonds was also increased. Combining the six consensus variations with Y379W (a variant with greater substrate promiscuity) increased the stability of this variant and its turnover towards some substrates. Thus, the six consensus variants can be used to stabilise catalytically interesting variants of human galactokinase and might also be useful if the protein were to be used in ERT.

Modulation of the mobility of a key region in human galactokinase: Impacts on catalysis and stability.

Galactokinase catalyses the phosphorylation of α-d-galactose and some structurally related monosaccharides. The enzyme is of interest due to its potential as a biocatalyst for the production of sugar 1-phosphates and due to its involvement in the inherited metabolic disease type II galactosemia. It has been previously shown that a region (residues 231-245) in human galactokinase often has altered mobility when active site residues are varied. We hypothesised that the reverse may be true and that designing changes to this region might affect the functioning of the active site of the enzyme. Focussing on four residues (Leu-231, Gln-242, Glu-244 and Glu-245) we conducted molecular dynamics simulations to explore the effects of changing these residues to glycine or serine. In most cases the variations resulted in local changes to the 231-245 region and global changes to the root mean squared fluctuation (RMSF) of the protein. The four serine variants were expressed as recombinant proteins. All had altered steady state enzyme kinetic parameters with α-d-galactose as a substrate. However, these changes were generally less than ten-fold in magnitude. Changes were also observed with 2-deoxy-α-d-galactose, α-d-galactosamine and α-d-talose as substrates, including (in some cases) loss of detectable activity, suggesting that these variations can tune the specificity of the enzyme. This study demonstrates that activity and specificity of human galactokinase can be modulated by variations designed to affect active site flexibility. It is likely that this principle can be generalised to other enzymes.

A galactokinase-like protein from the liver fluke Fasciola hepatica.

Galactokinase catalyses the ATP-dependent phosphorylation of galactose. A galactokinase-like sequence was identified in a Fasciola hepatica EST library. Recombinant expression of the corresponding protein in Escherichia coli resulted in a protein of approximately 50 kDa. The protein is monomeric, like galactokinases from higher animals, yeasts and some bacteria. The protein has no detectable enzymatic activity with galactose or N-acetylgalactosamine as a substrate. However, it does bind to ATP. Molecular modelling predicted that the protein adopts a similar fold to galactokinase and other GHMP kinases. However, a key loop in the active site was identified which may influence the lack of activity. Sequence analysis strongly suggested that this protein (and other proteins annotated as "galactokinase" in the trematodes Schistosoma mansoni and Clonorchis sinensis) are closer to N-acetylgalactosamine kinases. No other galactokinase-like sequences appear to be present in the genomes of these three species. This raises the intriguing possibility that these (and possibly other) trematodes are unable to catabolise galactose through the Leloir pathway due to the lack of a functional galactokinase.

The role of arabinokinase in arabinose toxicity in plants.

Plant cell wall polymers are synthesized by glycosyltransferases using nucleotide sugars as substrates. Most UDP-sugars are synthesized from UDP-glucose via de novo pathways but salvage pathways work in parallel to recycle sugars, which have been released during cell wall polymer and glycoprotein turnover. Here we report on the cloning and biochemical analysis of two arabinokinases in Arabidopsis. Arabinokinase is a 100 kDa protein located in the cytosol with a putative N-terminal glycosyltransferase domain and a C-terminal sugar-1-kinase domain. This unique structure is highly conserved in the plant kingdom. Arabinokinase has a high affinity for l-arabinose, which is the only sugar substrate of this GHMP (galactose; homoserine; mevalonate; phosphomevalonate) kinase. Plants that were knocked-out for arabinokinase and the previously described ara1-1 mutant were characterized. The ARA1-1 mutant form of the enzyme carries a point mutation in an α-helix. The mutation is close to the substrate binding site and changes the Km value for arabinose from 80 µm in the wild type to 17 000 µm in ARA1-1. The previous arabinose toxicity explanation is challenged by knockout plants in arabinokinase that accumulate higher levels of arabinose but do not show signs of arabinose toxicity. Analysis of marker genes from sugar signalling pathways (SnRK1 and Tor) suggest that ara1-1 misinterprets its carbon energy status. Although glucose is present in ara1-1 similar to wild type levels, it constitutively changes gene expression as typically found in wild type plants only under starvation conditions. Furthermore, ara1-1 shows increased expression of marker genes for programmed cell death as found in other lesion mimic mutants.

Galactokinase promiscuity: a question of flexibility?

Galactokinase catalyses the first committed step of the Leloir pathway, i.e. the ATP-dependent phosphorylation of α-D-galactose at C1-OH. Reduced galactokinase activity results in the inherited metabolic disease type II galactosaemia. However, inhibition of galactokinase is considered a viable approach to treating more severe forms of galactosaemia (types I and III). Considerable progress has been made in the identification of high affinity, selective inhibitors. Although the structure of galactokinase from a variety of species is known, its catalytic mechanism remains uncertain. Although the bulk of evidence suggests that the reaction proceeds via an active site base mechanism, some experimental and theoretical studies contradict this. The enzyme has potential as a biocatalyst in the production of sugar 1-phosphates. This potential is limited by its high specificity. A variety of approaches have been taken to identify galactokinase variants which are more promiscuous. These have broadened galactokinase's specificity to include a wide range of D- and L-sugars. Initial studies suggest that some of these alterations result in increased flexibility at the active site. It is suggested that modulation of protein flexibility is at least as important as structural modifications in determining the success or failure of enzyme engineering.

Dynamic origins of substrate promiscuity in bacterial galactokinases.

Galactokinase catalyses the ATP-dependent phosphorylation of galactose and structurally related sugars. The enzyme has attracted interest as a potential biocatalyst for the production of sugar 1-phosphates and several attempts have been made to broaden its specificity. In general, bacterial galactokinases have wider substrate ranges than mammalian ones. The enzymes from Escherichia coli and Lactococcus lactis have received particular attention and a number of variants with increased promiscuity have been identified. Here, we present a molecular dynamics study designed to investigate the molecular causes of the wider substrate ranges of these enzymes and their variants with particular reference to protein mobility. Some regions close to the active site of the enzyme have different structures in the bacterial enzymes compared to the human one. Alterations known to increase the substrate range (e.g. Y371H in the E. coli enzyme), tend to alter the conformation of a key α-helical region (residues 216-232 in the E. coli enzyme). The equivalent helix in the human enzyme has previously been predicted to be altered in variants which affect catalytic activity or protein stability. This helix appears to be a key region in galactokinases from a range of species and may represent an interesting target for future attempts to broaden the specificity of galactokinases.

Structural analysis of mevalonate-3-kinase provides insight into the mechanisms of isoprenoid pathway decarboxylases.

In animals, cholesterol is made from 5-carbon building blocks produced by the mevalonate pathway. Drugs that inhibit the mevalonate pathway such as atorvastatin (lipitor) have led to successful treatments for high cholesterol in humans. Another potential target for the inhibition of cholesterol synthesis is mevalonate diphosphate decarboxylase (MDD), which catalyzes the phosphorylation of (R)-mevalonate diphosphate, followed by decarboxylation to yield isopentenyl pyrophosphate. We recently discovered an MDD homolog, mevalonate-3-kinase (M3K) from Thermoplasma acidophilum, which catalyzes the identical phosphorylation of (R)-mevalonate, but without concomitant decarboxylation. Thus, M3K catalyzes half the reaction of the decarboxylase, allowing us to separate features of the active site that are required for decarboxylation from features required for phosphorylation. Here we determine the crystal structure of M3K in the apo form, and with bound substrates, and compare it to MDD structures. Structural and mutagenic analysis reveals modifications that allow M3K to bind mevalonate rather than mevalonate diphosphate. Comparison to homologous MDD structures show that both enzymes employ analogous Arg or Lys residues to catalyze phosphate transfer. However, an invariant active site Asp/Lys pair of MDD previously thought to play a role in phosphorylation is missing in M3K with no functional replacement. Thus, we suggest that the invariant Asp/Lys pair in MDD may be critical for decarboxylation rather than phosphorylation.