Matrix-assisted laser desorption/ionization mass spectrometry analysis and thiol-group determination of isoforms of bovine cytochrome c oxidase, a hydrophobic multisubunit membrane protein

Individual biochemical behaviour versus biological robustness: spotlight on the regulation of cytochrome c oxidase

During evolution from prokaryotes to eukaryotes, the main function of cytochrome c oxidase (COX), i.e., the coupling of oxygen reduction to proton translocation without the production of ROS (reactive oxygen species) remained unchanged demonstrating its robustness. A new regulation of respiration by the ATP/ADP ratio was introduced in eukaryotes based on nucleotide interaction with the added COX subunit IV. This allosteric ATP-inhibition was proposed to keep the mitochondrial membrane potential (ΔΨ(m)) at low healthy values and thus prevents the formation of ROS at complexes I and III. ROS have been implicated in various degenerative diseases. The allosteric ATP-inhibition of COX is reversibly switched on and off by phosphorylation of COX at a serine or threonine. In more than 100 individual preparations of rat heart and liver mitochondria, prepared under identical conditions, the extent of allosteric ATP-inhibition varied. This variability correlates with the variable inhibition of uncoupled respiration in intact isolated mitochondria by ATP. It is concluded that in higher organisms the allosteric ATP-inhibition is continually switched on and off by neuronal signalling in order to change oxidative phosphorylation from optimal efficiency with lower rate of ATP synthesis under resting conditions (low ΔΨ(m) and ROS production) to maximal rate of ATP synthesis under active (working, stress) conditions (elevated ΔΨ(m) and ROS production).

Phosphorylation and kinetics of mammalian cytochrome c oxidase

The influence of protein phosphorylation on the kinetics of cytochrome c oxidase was investigated by applying Western blotting, mass spectrometry, and kinetic measurements with an oxygen electrode. The isolated enzyme from bovine heart exhibited serine, threonine, and/or tyrosine phosphorylation in various subunits, except subunit I, by using phosphoamino acid-specific antibodies. The kinetics revealed slight inhibition of oxygen uptake in the presence of ATP, as compared with the presence of ADP. Mass spectrometry identified the phosphorylation of Ser-34 at subunit IV and Ser-4 and Thr-35 at subunit Va. Incubation of the isolated enzyme with protein kinase A, cAMP, and ATP resulted in serine and threonine phosphorylation of subunit I, which was correlated with sigmoidal inhibition kinetics in the presence of ATP. This allosteric ATP-inhibition of cytochrome c oxidase was also found in rat heart mitochondria, which had been rapidly prepared in the presence of protein phosphatase inhibitors. The isolated rat heart enzyme, prepared from the mitochondria by blue native gel electrophoresis, showed serine, threonine, and tyrosine phosphorylation of subunit I. It is concluded that the allosteric ATP-inhibition of cytochrome c oxidase, previously suggested to keep the mitochondrial membrane potential and thus the reactive oxygen species production in cells at low levels, occurs in living cells and is based on phosphorylation of cytochrome c oxidase subunit I.

Phosphorylation of rat liver mitochondrial carnitine palmitoyltransferase-I: effect on the kinetic properties of the enzyme

Hepatic carnitine palmitoyltransferase-I (CPT-IL) isolated from mitochondrial outer membranes obtained in the presence of protein phosphatase inhibitors is readily recognized by phosphoamino acid antibodies. Mass spectrometric analysis of CPT-IL tryptic digests revealed the presence of three phosphopeptides including one with a protein kinase CKII (CKII) consensus site. Incubation of dephosphorylated outer membranes with protein kinases and [gamma-32P]ATP resulted in radiolabeling of CPT-I only by CKII. Using mass spectrometry, only one region of phosphorylation was detected in CPT-I isolated from CKII-treated mitochondria. The sequence of the peptide and position of phosphorylated amino acids have been determined unequivocally as FpSSPETDpSHRFGK (residues 740-752). Furthermore, incubation of dephosphorylated outer membranes with CKII and unlabeled ATP led to increased catalytic activity and rendered malonyl-CoA inhibition of CPT-I from competitive to uncompetitive. These observations identify a new mechanism for regulation of hepatic CPT-I by phosphorylation.

Mass spectrometric detection of protein, lipid and heme components of cytochrome c oxidase from R. sphaeroides and the stabilization of non-covalent complexes from the enzyme

The cytochrome c oxidase enzyme from the Rhodobacter sphaeroides bacteria exists as a complex of four peptide subunits, two hemes, and a variety of lipids and metal ions held together by non-covalent forces. While the native enzyme functions as an associated unit, this complex usually dissociates during MALDI- TOF analysis. Through the use of matrix additives such as sucrose, the complete complex and partial complexes can be stabilized in the MALDI-TOF experiment. The dissociation of the complex allows for the detection of the components of the enzyme. The direct detection of associated lipids from an aqueous solution of the intact enzyme may eliminate the need for enzyme disruption and lipid extraction. The partial dissociation of multisubunit enzymes in such experiments may allow for the determination of subunit-subunit and subunit-lipid interactions

Peptide mapping and disulfide bond analysis of the cytoplasmic region of an intrinsic membrane protein by mass spectrometry

Intrinsic membrane proteins pose substantial obstacles to analysis by common analytical techniques due to their hydrophobic nature and solubilization requirements. This is the case for studies involving HPLC coupled to mass spectrometry. We have developed an HPLC/mass spectrometry approach to explore and map the peptide sequence of the SERCA1a Ca(2+)-ATPase from the sarcoplasmic reticulum an integral membrane protein of 110 kDa. After extensive proteolysis of the protein, the mass of the proteolytic fragments was analyzed by HPLC/mass spectrometry. Only part of the cytoplasmic fragments was recovered under nondenaturing conditions. On the other hand, peptide fragments obtained under denaturing conditions were found to cover nearly all the cytoplasmic region. Sarcoplasmic reticulum (SR) Ca(2+)-ATPase contains 24 cysteine residues, 18 of which are in the cytosolic or lumenal region of the protein. Peptides containing free cysteines were identified by a mass increase resulting from carboxyamidomethylation of the cysteines with iodoacetamide. Alkylation reactions were executed either before or after reduction of the peptide fragments by dithiothreitol. Analysis of the mass of the fragments indicates that no disulfide bonds exist in the cytoplasmic portion of SR Ca(2+)-ATPase.