Energy transfer inhibition induced by modification of membrane-bound chloroplast coupling factor 1 by pyridoxal phosphate

The transmembrane arrangement of the ADP/ATP carrier as elucidated by the lysine reagent pyridoxal 5-phosphate

The lysine reagent pyridoxal 5-phosphate was applied to the ADP/ATP carrier (AAC) in order to elucidate topological and functional properties of the numerous lysines within the primary structure. To establish appropriate labeling conditions, the influence of pyridoxal-P on transport and inhibitor binding to the AAC was examined. The ADP/ATP transport is sensitive to low concentrations of pyridoxal-P with a Ki = 0.4 mM. Binding of [3H]carboxyatracylate and [3H]bongkrekate is largely inhibited by pyridoxal-P treatment with Ki approximately 1 mM. [3H]Carboxyatractylate is not and [3H]bongkrekate weakly removed by pyridoxal-P, whereas [3H]atractylate is displaced to a large extent. Under optimized conditions of pyridoxal-P concentration, of pH and of time exposure, the AAC was exposed to [3H]pyridoxal-P in mitochondria, in submitochondrial particles and in the detergent-solubilized carrier. The [3H]pyridoxal-P-labeled AAC was isolated from mitochondria and particles. After citraconylation thermolysinolytic peptides were prepared. The pyridoxyl-lysine-containing peptides were purified and the pyridoxal-P incorporation to specific lysines was determined by sequencing. The pyridoxal-P incorporation into the AAC in various states was evaluated with regard to structural and functional aspects. First, by comparing pyridoxal-P incorporation in mitochondria and sonic particles, the segments of the polypeptide chain exposed to the cytosolic and matrix side of the membrane are detected. Second, the additional lysine incorporation into the isolated as compared to the membrane-bound carrier is attributed to the protein collar facing the phospholipid headgroups. Third, the difference between lysine incorporation into the carboxyatractylate-AAC and bongkrekate-AAC complexes reflect either conformational changes or lysines involved in the translocation channel through the protein. Fourth, the additional lysine labeled in the atractylate-carrier complex as compared to the carboxyatractylate-carrier complex is attributed to a cationic site in the binding center. These results are incorporated into a transmembrane folding model of the carrier.

Possible role of histidine in the L-proline transport system of Saccharomyces cerevisiae

The L-proline transport system of Saccharomyces cerevisiae is shown to be specifically inactivated upon incubation of intact yeast cells with the histidine modifier diethylpyrocarbonate. The extent of inactivation is half-maximum at 0.5 mM diethylpyrocarbonate for an incubation of 2 min at 30 degrees C and pH 6.0. Under the same conditions, the time dependence of inactivation is monophasic with the second-order rate constant of 5.5 M-1 X s-1 and the maximum rate Jmax of L-proline transport is lowered by about 50%, while the KT value remains unchanged. Moreover, L-proline afforded significant protection against diethylpyrocarbonate inactivation. The complete reactivation of a partially inactivated L-proline transport system by neutral hydroxylamine and the elimination of the possibility that the modification of other amino acid residues are responsible for the inactivation, suggested that the transport protein inactivation occurs solely by a modification of histidine residues.

Chemical modification of essential carboxyl group and histidine residue in the plasma-membrane 5'-nucleotidase

An investigation, using specific chemical reagents, of the amino acids involved in the catalytic activity of the purified 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) from bovine liver plasma membranes, was carried out. The enzyme was irreversibly inactivated by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The inhibition kinetics were of the first-order type and decreased partially in the presence of nucleotides and divalent cations. These results indicate for the first time that a carboxyl group is essential for the catalytic process of 5'-nucleotidase. Moreover, chemical modification by diethylpyrocarbonate also produced inactivation of the enzyme and showed a differential spectrum with a peak at 240 nm characteristic of N-carbethoxyhistidine residues. This inactivation was efficiently released upon decarbethoxylation by hydroxylamine only when the extent of inactivation, due to low concentration of diethylpyrocarbonate, was limited. The time-dependent inactivation followed first-order kinetics and nucleotides afforded significant protection against diethylpyrocarbonate modification. The results indicate the involvement of the histidine residue in catalysis.