Keith McCall

Malic acid, a petroleum-derived C4-dicarboxylic acid that is used in the food and beverage industries, is also produced by a number of microorganisms that follow a variety of metabolic routes. Several members of the genus Aspergillus utilize a two-step cytosolic pathway from pyruvate to malate known as the reductive tricarboxylic acid (rTCA) pathway. This simple and efficient pathway has a maximum theoretical yield of 2 mol malate/mol glucose when the starting pyruvate originates from… Vis mere

Malic acid, a petroleum-derived C4-dicarboxylic acid that is used in the food and beverage industries, is also produced by a number of microorganisms that follow a variety of metabolic routes. Several members of the genus Aspergillus utilize a two-step cytosolic pathway from pyruvate to malate known as the reductive tricarboxylic acid (rTCA) pathway. This simple and efficient pathway has a maximum theoretical yield of 2 mol malate/mol glucose when the starting pyruvate originates from glycolysis. Production of malic acid by Aspergillus oryzae NRRL 3488 was first improved by overexpression of a native C4-dicarboxylate transporter, leading to a greater than twofold increase in the rate of malate production. Overexpression of the native cytosolic alleles of pyruvate carboxylase and malate dehydrogenase, comprising the rTCA pathway, in conjunction with the transporter resulted in an additional 27 % increase in malate production rate. A strain overexpressing all three genes achieved a malate titer of 154 g/L in 164 h, corresponding to a production rate of 0.94 g/L/h, with an associated yield on glucose of 1.38 mol/mol (69 % of the theoretical maximum). This rate of malate production is the highest reported for any microbial system. Vis mindre

The Zap1 transcription factor is a central player in zinc homeostasis in yeast. This protein regulates the expression of genes involved in zinc accumulation and storage. For most of its target genes, Zap1 activates expression in zinc-limited cells and this function is inhibited in replete cells. Zap1 has two activation domains, AD1 and AD2, which are independently regulated by zinc status. In this study, we characterized AD1 and its regulation by zinc. AD1 was mapped using deletions to residues… Vis mere

The Zap1 transcription factor is a central player in zinc homeostasis in yeast. This protein regulates the expression of genes involved in zinc accumulation and storage. For most of its target genes, Zap1 activates expression in zinc-limited cells and this function is inhibited in replete cells. Zap1 has two activation domains, AD1 and AD2, which are independently regulated by zinc status. In this study, we characterized AD1 and its regulation by zinc. AD1 was mapped using deletions to residues 332-402 of Zap1. The region required for the zinc responsiveness of this activation domain, designated 'ZRD(AD1), was mapped to residues 182-502. Thus, AD1 is embedded within its larger zinc-responsive domain. Using a combination of in silico analysis, random mutagenesis and site-directed mutagenesis, we identified key residues within ZRD(AD1) required for its regulation by zinc. Most of these residues are cysteines and histidines that could potentially serve as Zn(II) ligands. These results suggest that ZRD(AD1) senses zinc by direct Zn(II) binding. Consistent with this hypothesis, purified ZRD(AD1) bound multiple Zn(II) ions. Finally, our results indicate that, in the context of the full-length Zap1 protein, AD1 and AD2 are both critical to the full control of gene expression in response to zinc. Vis mindre

Using Carbonic Anhydrase as a model enzyme, various hypotheses regarding metal ion selectivity of metalloproteins were tested through site-directed mutagenesis and binding kinetics studies.

An investigation into the zinc mediation of the yeast Zap1 transcription factor.

This chapter contains sections titled:
Introduction
Mitochondrial Structure
Mitochondrial Transport
Assembly of Mitochondrial Cytochrome c Oxidase
Copper Ion Delivery to Targets other than the Mitochondrion
Copper Ion Transport to the Mitochondrion by Cox17
Co-metallochaperones in Cu Metallation of Cytochrome c Oxidase
Terminal Oxidases in Prokaryotes
Metallation of Prokaryotic Terminal Oxidases
Postulated Model

The ability to construct molecular motifs with predictable properties in aqueous solution requires an extensive knowledge of the relationships between structure and energetics. The design of metal binding motifs is currently an area of intense interest in the bioorganic community. To date synthetic motifs designed to bind metal ions lack the remarkable affinities observed in biological systems. To better understand the structural basis of metal ion affinity, we report here the thermodynamics of… Vis mere

The ability to construct molecular motifs with predictable properties in aqueous solution requires an extensive knowledge of the relationships between structure and energetics. The design of metal binding motifs is currently an area of intense interest in the bioorganic community. To date synthetic motifs designed to bind metal ions lack the remarkable affinities observed in biological systems. To better understand the structural basis of metal ion affinity, we report here the thermodynamics of binding of divalent zinc ions to wild-type and mutant carbonic anhydrases and the interpretation of these parameters in terms of structure. Mutations were made both to the direct His ligand at position 94 and to indirect, or second-shell, ligands Gln-92, Glu-117, and Thr-199. The thermodynamics of ligand binding by several mutant proteins is complicated by the development of a second zinc binding site on mutation; such effects must be considered carefully in the interpretation of thermodynamic data. In all instances modification of the protein produces a complex series of changes in both the enthalpy and entropy of ligand binding. In most cases these effects are most readily rationalized in terms of ligand and protein desolvation, rather than in terms of changes in the direct interactions of ligand and protein. Alteration of second-shell ligands, thought to function primarily by orienting the direct ligands, produces profoundly different effects on the enthalpy of binding, depending on the nature of the residue. These results suggest a range of activities for these ligands, contributing both enthalpic and entropic effects to the overall thermodynamics of binding. Together, our results demonstrate the importance of understanding relationships between structure and hydration in the construction of novel ligands and biological polymers. Vis mindre

Understanding the energetic consequences of molecular structure in aqueous solution is a prerequisite to the rational design of synthetic motifs with predictable properties. Such properties include ligand binding and the collapse of polymer chains into discrete three-dimensional structures. Despite advances in macromolecular structure determination, correlations of structure with high-resolution thermodynamic data remain limited. Here we compare thermodynamic parameters for the binding of… Vis mere

Understanding the energetic consequences of molecular structure in aqueous solution is a prerequisite to the rational design of synthetic motifs with predictable properties. Such properties include ligand binding and the collapse of polymer chains into discrete three-dimensional structures. Despite advances in macromolecular structure determination, correlations of structure with high-resolution thermodynamic data remain limited. Here we compare thermodynamic parameters for the binding of Zn(II), Cu(II), and Co(II) to human carbonic anhydrase II. These calorimetrically determined values are interpreted in terms of high-resolution X-ray crystallographic data. While both zinc and cobalt are bound with a 1:1 stoichiometry, CAII binds two copper ions. Considering only the high-affinity site, there is a diminution in the enthalpy of binding through the series Co(II) → Zn(II) → Cu(II) that mirrors the enthalpy of hydration; this observation reinforces the notion that the thermodynamics of solute association with water is at least as important as the thermodynamics of solute−solute interaction and that these effects must be considered when interpreting association in aqueous solution. Additionally, ΔCp data suggest that zinc binding to CAII proceeds with a greater contribution from desolvation than does binding of either copper or cobalt, suggesting Nature optimizes binding by optimizing desolvation. Vis mindre

Transition metal ions, although maintained at low concentrations, play diverse important roles in many biological processes. Two assays useful for the rapid quantification of a range of first-row transition metal ions have been developed. The colorimetric assay extends the 4-(2-pyridylazo)resorcinol assay of Hunt et al. (J. Biol. Chem. 255, 14793 (1984)) to measure nanomole quantities of Co2+, Ni2+, and Cu2+ as well as Zn2+. The fluorimetric assay takes advantage of the coordination of a number… Vis mere

Transition metal ions, although maintained at low concentrations, play diverse important roles in many biological processes. Two assays useful for the rapid quantification of a range of first-row transition metal ions have been developed. The colorimetric assay extends the 4-(2-pyridylazo)resorcinol assay of Hunt et al. (J. Biol. Chem. 255, 14793 (1984)) to measure nanomole quantities of Co2+, Ni2+, and Cu2+ as well as Zn2+. The fluorimetric assay takes advantage of the coordination of a number of metal ions (Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+) by Fura-2 and can also be used to measure nanomole quantities of these ions. The assays developed here have the advantage of not requiring the extensive sample preparation necessary for other methodologies, such as atomic absorption spectroscopy and inductively coupled plasma emission spectroscopy (ICPES), while being comparable in accuracy to the detection limits of ICPES for the first-row transition metal ions. To demonstrate the effectiveness of these assays, we determined the affinity of carbonic anhydrase II (CA II), a prototypical zinc enzyme, for Ni2+ and Cd2+. These data indicate that CA II binds transition metals with high affinity and is much more selective for Zn2+ over Ni2+ or Cd2+ than most small-molecule chelators or other metalloenzymes. Vis mindre

Zinc is required for the activity of > 300 enzymes, covering all six classes of enzymes. Zinc binding sites in proteins are often distorted tetrahedral or trigonal bipyramidal geometry, made up of the sulfur of cysteine, the nitrogen of histidine or the oxygen of aspartate and glutamate, or a combination. Zinc in proteins can either participate directly in chemical catalysis or be important for maintaining protein structure and stability. In all catalytic sites, the zinc ion functions as a… Vis mere

Zinc is required for the activity of > 300 enzymes, covering all six classes of enzymes. Zinc binding sites in proteins are often distorted tetrahedral or trigonal bipyramidal geometry, made up of the sulfur of cysteine, the nitrogen of histidine or the oxygen of aspartate and glutamate, or a combination. Zinc in proteins can either participate directly in chemical catalysis or be important for maintaining protein structure and stability. In all catalytic sites, the zinc ion functions as a Lewis acid. Researchers in our laboratory are dissecting the determinants of molecular recognition and catalysis in the zinc-binding site of carbonic anhydrase. These studies demonstrate that the chemical nature of the direct ligands and the structure of the surrounding hydrogen bond network are crucial for both the activity of carbonic anhydrase and the metal ion affinity of the zinc-binding site. An understanding of naturally occurring zinc-binding sites will aid in creating de novo zinc-binding proteins and in designing new metal sites in existing proteins for novel purposes such as to serve as metal ion biosensors. Vis mindre

Because of their high affinity and selectivity, metalloproteins can be used as transducers in novel sensors, i.e., biosensors, for the determination of trace levels of metal ions in solution. Here, we exploit carbonic anhydrase to determine picomolar to nanomolar concentrations of free transition metal ions by fluorescence anisotropy (polarization) in a reagentless format. Carbonic anhydrase variants engineered with a cysteine replacing a residue chosen near the active site (F131C and H64C)… Vis mere

Because of their high affinity and selectivity, metalloproteins can be used as transducers in novel sensors, i.e., biosensors, for the determination of trace levels of metal ions in solution. Here, we exploit carbonic anhydrase to determine picomolar to nanomolar concentrations of free transition metal ions by fluorescence anisotropy (polarization) in a reagentless format. Carbonic anhydrase variants engineered with a cysteine replacing a residue chosen near the active site (F131C and H64C) were covalently labeled with derivatives of benzoxadiazole sulfonamide. These labeled variants exhibited changes in anisotropy up to 0.07 upon binding free Cu(II), Co(II), and Zn(II) with apparent Kd's close to the values observed with wild-type apocarbonic anhydrase. The covalent attachment of the label has significant advantages over noncovalent labels we have described previously. Furthermore, the metal ion-dependent anisotropy changes were predictable using simple theory. The results demonstrate that free transition metal ions can be determined at trace levels in aqueous solution using inexpensive instruments. Vis mindre

Recently, we have developed a biosensor for zinc based on the very tight binding of this metal by the enzyme carbonic anhydrase, which requires Zn(II) for catalysis. We were able to transduce the binding of the metal as a change in fluorescence intensity or lifetime by use of a colored inhibitor whose metal-dependent binding permits fluorescence resonance energy transfer (Forster transfer) to occur. We have extended this concept to include other metals and other analytes which may be bound in… Vis mere

Recently, we have developed a biosensor for zinc based on the very tight binding of this metal by the enzyme carbonic anhydrase, which requires Zn(II) for catalysis. We were able to transduce the binding of the metal as a change in fluorescence intensity or lifetime by use of a colored inhibitor whose metal-dependent binding permits fluorescence resonance energy transfer (Forster transfer) to occur. We have extended this concept to include other metals and other analytes which may be bound in the native (or mutant) enzyme active site with a concomitant color change; the color change is transduced as a change in energy transfer efficiency. We have also recently demonstrated a similar approach, wherein the presence of a metal ion in the binding site is transduced as a change in fluorescence anisotropy. Results in cuvettes and with fiber optic sensors are shown. Vis mindre