Complementation of the yeast deletion mutant DeltaNCE103 by members of the beta class of carbonic anhydrases is dependent on carbonic anhydrase activity rather than on antioxidant activity.

In recent years, members of the beta class of CAs (carbonic anhydrases) have been shown to complement Delta NCE103, a yeast strain unable to grow under aerobic conditions. The activity required for complementation of Delta NCE103 by tobacco chloroplast CA was studied by site-directed mutagenesis. E196A (Glu196-->Ala), a mutated tobacco CA with low levels of CA activity, complemented Delta NCE103. To determine whether restoration of Delta NCE103 was due to residual levels of CA activity or whether it was related to previously proposed antioxidant activity of CAs [Götz, Gnann and Zimmermann (1999) Yeast 15, 855-864], additional complementation analysis was performed using human CAII, an alpha CA structurally unrelated to the beta class of CAs to which the tobacco protein belongs. Human CAII complemented Delta NCE103, strongly arguing that CA activity is responsible for the complementation of Delta NCE103. Consistent with this conclusion, recombinant NCE103 synthesized in Escherichia coli shows CA activity, and Delta NCE103 expressing the tobacco chloroplast CA exhibits the same sensitivity to H2O2 as the wild-type strain

Identification and characterization of a carboxysomal y-carbonic anhydrase from the cyanobacterium Nostoc sp. PCC 7120

Carboxysomes are proteinaceous microcompartments that encapsulate carbonic anhydrase (CA) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco); carboxysomes, therefore, catalyze reversible HCO3 - dehydration and the subsequent fixation of CO2. The N- and C-terminal domains of the β-carboxysome scaffold protein CcmM participate in a network of protein-protein interactions that are essential for carboxysome biogenesis, organization, and function. The N-terminal domain of CcmM in the thermophile Thermosynechococcus elongatus BP-1 is also a catalytically active, redox regulated γ-CA. To experimentally determine if CcmM from a mesophilic cyanobacterium is active, we cloned, expressed and purified recombinant, full-length CcmM from Nostoc sp. PCC 7120 as well as the N-terminal 209 amino acid γ-CA-like domain. Both recombinant proteins displayed ethoxyzolamide-sensitive CA activity in mass spectrometric assays, as did the carboxysome-enriched TP fraction. NstCcmM209 was characterized as a moderately active and efficient γ-CA with a k cat of 2.0 × 10 4 s-1 and k cat/K m of 4.1 × 106 M-1 s-1 at 25 °C and pH 8, a pH optimum between 8 and 9.5 and a temperature optimum spanning 25-35 °C. NstCcmM209 also catalyzed the hydrolysis of the CO2 analog carbonyl sulfide. Circular dichroism and intrinsic tryptophan fluorescence analysis demonstrated that NstCcmM209 was progressively and irreversibly denatured above 50 °C. NstCcmM209 activity was inhibited by the reducing agent tris(hydroxymethyl) phosphine, an effect that was fully reversed by a molar excess of diamide, a thiol oxidizing agent, consistent with oxidative activation being a universal regulatory mechanism of CcmM orthologs. Immunogold electron microscopy and Western blot analysis of TP pellets indicated that Rubisco and CcmM co-localize and are concentrated in Nostoc sp. PCC 7120 carboxysomes

Structure and Catalytic Mechanism of Nicotinate (Vitamin B₃) Degradative Enzyme Maleamate Amidohydrolase from <i>Bordetella bronchiseptica</i> RB50

The penultimate reaction in the oxidative degradation of nicotinate (vitamin B₃) to fumarate in several species of aerobic bacteria is the hydrolytic deamination of maleamate to maleate, catalyzed by maleamate amidohydrolase (NicF). Although it has been considered a model system for bacterial degradation of N-heterocyclic compounds, only recently have gene clusters that encode the enzymes of this catabolic pathway been identified to allow detailed investigations concerning the structural basis of their mechanisms. Here, the <i>Bb</i>1774 gene from <i>Bordetella bronchiseptica</i> RB50, putatively annotated as <i>nicF</i>, has been cloned, and the recombinant enzyme, overexpressed and purified from <i>Escherichia coli</i>, is shown to catalyze efficiently the hydrolysis of maleamate to maleate and ammonium ion. Steady-state kinetic analysis of the reaction by isothermal titration calorimetry (ITC) established <i>k</i>_cat and <i>K</i>_M values (pH 7.5 and 25 °C) of 11.7 ± 0.2 s^–1 and 128 ± 6 μM, respectively. The observed <i>K</i>_D of the NicF·maleate (E·P) complex, also measured by ITC, is approximated to be 3.8 ± 0.4 mM. The crystal structure of NicF, determined at 2.4 Å using molecular replacement, shows that the enzyme belongs to the cysteine hydrolase superfamily. The structure provides insight concerning the roles of potential catalytically important residues, most notably a conserved catalytic triad (Asp29, Lys117, and Cys150) observed in the proximity of a conserved non-proline <i>cis</i>-peptide bond within a small cavity that is likely the active site. On the basis of this structural information, the hydrolysis of maleamate is proposed to proceed by a nucleophilic addition–elimination sequence involving the thiolate side chain of Cys150

Calmodulin complexes with brain and muscle creatine kinase peptides

Calmodulin (CaM) is a ubiquitous Ca2+ sensing protein that binds to and modulates numerous target proteins and enzymes during cellular signaling processes. A large number of CaM-target complexes have been identified and structurally characterized, revealing a wide diversity of CaM-binding modes. A newly identified target is creatine kinase (CK), a central enzyme in cellular energy homeostasis. This study reports two high-resolution X-ray structures, determined to 1.24 ​Å and 1.43 ​Å resolution, of calmodulin in complex with peptides from human brain and muscle CK, respectively. Both complexes adopt a rare extended binding mode with an observed stoichiometry of 1:2 CaM:peptide, confirmed by isothermal titration calorimetry, suggesting that each CaM domain independently binds one CK peptide in a Ca2+-depended manner. While the overall binding mode is similar between the structures with muscle or brain-type CK peptides, the most significant difference is the opposite binding orientation of the peptides in the N-terminal domain. This may extrapolate into distinct binding modes and regulation of the full-length CK isoforms. The structural insights gained in this study strengthen the link between cellular energy homeostasis and Ca2+-mediated cell signaling and may shed light on ways by which cells can ‘fine tune’ their energy levels to match the spatial and temporal demands