Promiscuity comes at a price: catalytic versatility vs efficiency in different metal ion derivatives of the potential bioremediator GpdQ.

The glycerophosphodiesterase from Enterobacter aerogenes (GpdQ) is a highly promiscuous dinuclear metallohydrolase with respect to both substrate specificity and metal ion composition. While this promiscuity may adversely affect the enzyme's catalytic efficiency its ability to hydrolyse some organophosphates (OPs) and by-products of OP degradation have turned GpdQ into a promising candidate for bioremedial applications. Here, we investigated both metal ion binding and the effect of the metal ion composition on catalysis. The prevalent in vivo metal ion composition for GpdQ is proposed to be of the type Fe(II)Zn(II), a reflection of natural abundance rather than catalytic optimisation. The Fe(II) appears to have lower binding affinity than other divalent metal ions, and the catalytic efficiency of this mixed metal center is considerably smaller than that of Mn(II), Co(II) or Cd(II)-containing derivatives of GpdQ. Interestingly, metal ion replacements do not only affect catalytic efficiency but also the optimal pH range for the reaction, suggesting that different metal ion combinations may employ different mechanistic strategies. These metal ion-triggered modulations are likely to be mediated via an extensive hydrogen bond network that links the two metal ion binding sites via residues in the substrate binding pocket. The observed functional diversity may be the cause for the modest catalytic efficiency of wild-type GpdQ but may also be essential to enable the enzyme to evolve rapidly to alter substrate specificity and enhance k(cat) values, as has recently been demonstrated in a directed evolution experiment. This article is part of a Special Issue entitled: Chemistry and mechanism of phosphatases, diesterases and triesterases.

Glutathione transferase kappa deficiency causes glomerular nephropathy without overt oxidative stress.

Glutathione transferase kappa (GSTK1-1) is a highly conserved, mitochondrial enzyme potentially involved in redox reactions. GSTK1-1-deficient mice were generated to further study the enzyme's biological role. Reduced and total glutathione levels in liver and kidney were unchanged by GSTK1-1 deficiency and NADPH quinone oxidoreductase 1 expression was not elevated indicating that there is no general underlying oxidative stress in Gstk1(-/-) mice. Electron microscopy of liver and kidney showed no changes in mitochondrial morphology with GSTK1-1 deficiency. The death of a number of Gstk1(-/-) males with urinary tract problems prompted close examination of the kidneys. Electron microscopy revealed glomerular basement membrane changes at 3 months, accompanied by detectable microalbuminuria in male mice (albumin:creatinine ratio of 2.66±0.83 vs 1.13±0.20 mg/mmol for Gstk1(-/-) and wild-type (WT), respectively, P=0.001). This was followed by significant foot process effacement (40-55% vs 10% for Gstk1(-/-) and WT, respectively) at 6 months of age in all Gstk1(-/-) mice examined. Kidney tubules were ultrastructurally normal. Compared with human disease, the Gstk1(-/-) kidneys show changes seen in glomerulopathies causing nephrotic syndrome. Gstk1(-/-) mice may offer insights into the early development of glomerular nephropathies.

Polymorphisms in the human glutathione transferase Kappa (GSTK1) promoter alter gene expression.

The level of glutathione transferase Kappa (GSTK1-1) has been correlated with obesity (Liu et.al. 2008 PNAS 105: 18302-7) and a polymorphism in the hGSTK1 promoter has been associated with insulin secretion and fat deposition (Gao et al 2009 Endocr J 56: 487-94). We searched for additional polymorphisms that may influence GSTK1-1 function or expression. Two SNPs were identified in the 5' non-coding region. A SNP at -1308 that occurs in Chinese subjects is predicted to eliminate a FXR/RXR transcription factor-binding site and causes a 55% increase in transcription rate in HepG2 cells and a 59% decrease in HEK293 cells. These data suggest that the impact of this polymorphism is complex and tissue specific. A SNP at -1032 alters a methylation site and represses transcription by 38%. These observations provide the first functional insight into genetic factors that regulate hGSTK1 expression and may directly influence insulin secretion and fat deposition.

Directed evolution of new and improved enzyme functions using an evolutionary intermediate and multidirectional search.

The ease with which enzymes can be adapted from their native roles and engineered to function specifically for industrial or commercial applications is crucial to enabling enzyme technology to advance beyond its current state. Directed evolution is a powerful tool for engineering enzymes with improved physical and catalytic properties and can be used to evolve enzymes where lack of structural information may thwart the use of rational design. In this study, we take the versatile and diverse α/ß hydrolase fold framework, in the form of dienelactone hydrolase, and evolve it over three unique sequential evolutions with a total of 14 rounds of screening to generate a series of enzyme variants. The native enzyme has a low level of promiscuous activity toward p-nitrophenyl acetate but almost undetectable activity toward larger p-nitrophenyl esters. Using p-nitrophenyl acetate as an evolutionary intermediate, we have generated variants with altered specificity and catalytic activity up to 3 orders of magnitude higher than the native enzyme toward the larger nonphysiological p-nitrophenyl ester substrates. Several variants also possess increased stability resulting from the multidimensional approach to screening. Crystal structure analysis and substrate docking show how the enzyme active site changes over the course of the evolutions as either a direct or an indirect result of mutations.

Inositol 1,4,5-trisphosphate receptor subtypes are differentially distributed between smooth muscle and endothelial layers of rat arteries.

In blood vessels, the ability to control vascular tone depends on extracellular calcium entry and the release of calcium from inositol 1,4,5-trisphosphate receptor (IP3R)-gated stores located in both the endothelial and smooth muscle cells of the vascular wall. Therefore, we examined mRNA expression and protein distribution of IP3R subtypes in intact aorta, basilar and mesenteric arteries of the rat. IP3R1 mRNA was predominantly expressed in all three arteries. Immunohistochemistry showed that IP3R1 was present in both the muscle and endothelial cell layers, while IP3R2 and IP3R3 were largely restricted to the endothelium. Weak expression of IP3R2 was observed in the smooth muscle of the basilar artery. Co-localisation studies of IP3R subtypes with known cellular elements showed no association of any of the three subtypes with the endothelial cell plasma membrane, but a close association between the subtypes and actin filaments was observed in all cell layers. IP3R2 was found to be present near the endothelial cell nucleus. We are the first to demonstrate differential IP3R subtype distribution between the cell layers of the intact vascular wall and hypothesise that this may underlie the diversity of IP3R-dependent responses, such as vasoconstriction, vasodilation and vasomotion, displayed by arteries.

Functional characterisation of ganglioside-induced differentiation-associated protein 1 as a glutathione transferase.

Mutations in the ganglioside-induced differentiation-associated protein 1 (GDAP1) gene have been linked with Charcot-Marie-Tooth (CMT) disease. This protein, and its paralogue GDAP1L1, appear to be structurally related to the cytosolic glutathione S-transferases (GST) including an N-terminal thioredoxin fold domain with conserved active site residues. The specific function, of GDAP1 remains unknown. To further characterise their structure and function we purified recombinant human GDAP1 and GDAP1L1 proteins using bacterial expression and immobilised metal affinity chromatography. Like other cytosolic GSTs, GDAP1 protein has a dimeric structure. Although the full-length proteins were largely insoluble, the deletion of a proposed C-terminal transmembrane domain allowed the preparation of soluble protein. The purified proteins were assayed for glutathione-dependent activity against a library of 'prototypic' GST substrates. No evidence of glutathione-dependent activity or an ability to bind glutathione immobilised on agarose was found.