Mutations in the tether region of the iron-sulfur protein affect the activity and assembly of the cytochrome bc(1) complex of yeast mitochondria.

Resolution of the crystal structure of the mitochondrial cytochrome bc(1) complex has indicated that the extra-membranous extrinsic domain of the iron-sulfur protein containing the 2Fe2S cluster is connected by a tether to the transmembrane helix that anchors the iron-sulfur protein to the complex. To investigate the role of this tether in the cytochrome bc(1) complex, we have mutated the conserved amino acid residues Ala-86, Ala-90, Ala-92, Lys-93 and Glu-95 and constructed deletion mutants DeltaVLA(88-90) and DeltaAMA(90-92) and an insertion mutant I87AAA88 in the iron-sulfur protein of the yeast, Saccharomyces cerevisiae. In cells grown at 30 degrees C, enzymatic activities of the bc(1) complex were reduced 22-56% in mutants A86L, A90I, A92C, A92R and E95R, and the deletion mutants, DeltaVLA(88-90) and DeltaAMA(90-92), while activity of the insertion mutant was reduced 90%. No loss of cytochromes b or c-c(1), detected spectrally, or the iron-sulfur protein, determined by quantitative immunoblotting, was observed in these mutants with the exception of the mutants of Ala-92 in which the loss of activity paralleled a loss in the amount of the iron-sulfur protein. EPR spectroscopy revealed no changes in the iron-sulfur cluster of mutants A86L, A90I, A92R or the deletion mutant DeltaVLA(88-90). Greater losses of both protein and activity were observed in all of the mutants of Ala-92 as well as in A90F grown at 37 degrees C. suggesting that these conserved alanine residues may be involved in maintaining the stability of the iron-sulfur protein and its assembly into the bc(1) complex. By contrast, no significant loss of iron-sulfur protein was observed in the mutants of Ala-86 in cells grown at either 30 degrees C or 37 degrees C despite the 50-70% loss of enzymatic activity suggesting that Ala-86 may play a critical role in catalysis in the bc(1) complex.

The role of various domains of the iron-sulfur protein in the assembly and activity of the cytochrome bc1 complex of yeast mitochondria.

Assembly studies in vitro of deletion mutants of the iron-sulfur protein into the cytochrome bc1 complex revealed that mutants localized in the extramembranous regions of the protein were not assembled into the complex in contrast to the efficient assembly of mutants in the membrane-spanning region. Charged amino acids located in the extramembranous alpha1-beta4 loop and the alpha1 helix were mutated and expressed in yeast cells lacking the gene for the iron-sulfur protein. Mutating the charged amino acid residues H124, E125, R146, K148, and D149 as well as V132 and W152 resulted in loss of enzymatic activity due to the loss of iron-sulfur protein suggesting that these amino acids are required to maintain protein stability. By contrast, no loss of iron-sulfur protein accompanied the 30-50% loss of bc1 complex activity in mutants of three conserved alanine residues, A86, A90, and A92, suggesting that these residues may be involved in the proposed movement of the flexible tether of the iron-sulfur protein during catalysis.

Catabolism of proline by procyclic culture forms of Trypanosoma congolense.

The effect of various metabolic inhibitors on the rate of oxygen consumption by procyclic culture forms of Trypanosoma congolense utilizing proline as substrate was investigated. Cyanide inhibited the rate of oxygen consumption by 81.0 +/- 6.7%, malonate inhibited the rate by 51.6 +/- 1.6% and Antimycin A by 73.1 +/- 5.9%. A combination of cyanide and malonate inhibited the rate of oxygen consumption by 84.9 +/- 6.7% while a combination of antimycin A and malonate inhibited the rate by 81.6 +/- 7.6%. Rotenone had no effect on the rate of respiration except when the intact cells were first permeabilized by digitonin after which rotenone decreased the rate of respiration by 20-30%. Salicylhydroxamate (SHAM) did not have any effect on the rate of oxygen consumption. Enzymes involved in the catabolism of proline with high activities were: proline dehydrogenase, alpha-ketoglutarate dehydrogenase, succinate dehydrogenase, fumarase, NADP-linked malic enzyme, alanine aminotransferase and malate dehydrogenase. Activities of 1-pyrroline-5 carboxylate dehydrogenase, glutamate dehydrogenase, aspartate aminotransferase and NAD-linked malic enzyme were detectable but lower. The end products of proline catabolism were alanine and glutamate. Unlike the case in Trypanosoma brucei brucei aspartate was not detected. Possible pathways of proline catabolism in procyclic culture forms of T. congolense and of electron transfer are proposed.

Pathways of glucose catabolism in procyclic Trypanosoma congolense.

Studies of respiration on glucose in procyclic Trypanosoma congolense in the presence of rotenone, antimycin, cyanide, salicylhydroxamic acid and malonate have indicated the presence of NADH dehydrogenase, cytochrome b-c1, cytochrome aa3, trypanosome alternate oxidase and NADH fumarate reductase/succinate dehydrogenase pathway that contributes electrons to coenzyme Q of the respiratory chain. The rotenone sensitive NADH dehydrogenase, the trypanosome alternate oxidase, and cytochrome aa3 accounted for 24.5 +/- 6.5, 36.2 +/- 4.2 and 54.1 +/- 5.5% respectively of the total respiration. Activities of lactate dehydrogenase, NAD(+)-linked malic enzyme and pyruvate kinase were less than 6 nanomoles/min/mg protein suggesting that they play a minor role in energy metabolism of the parasite. Phosphoenolpyruvate carboxykinase, pyruvate dehydrogenase, succinate dehydrogenase, NADP(+)-linked malic enzyme, NADH fumarate reductase, malate dehydrogenase, and alpha-ketoglutarate dehydrogenase and glycerol kinase on the other hand had specific activities greater than 60 nanomoles/min/mg protein. These enzyme activities could account for the production of pyruvate, acetate, succinate and glycerol. The results further show that the amount of glycerol produced was 35-48% of the combined total of pyruvate, acetate and succinate produced. It is apparent that some of the glycerol 3-phosphate produced in glycolysis in the presence of salicylhydroxamic acid is dephosphorylated to form glycerol while the rest is oxidised via cytochrome aa3 to form acetate, succinate and pyruvate.

The alpha 1-helix in the extra-membranous domain contributes to the stability of the iron-sulfur protein of the cytochrome bc1 complex.

Previously, we reported that several charged amino acids located in the alpha1-beta4 loop of the iron-sulfur protein are required to maintain the stability of the protein and its assembly into the cytochrome bc1 complex. The current study extends this analysis to several amino acids localized in the single alpha-helix present in the extra-membranous domain of the protein. Three charged and two uncharged residues in the alpha1-helix were mutated and used to transform yeast cells (JPJ1) lacking the iron-sulfur protein gene. Mutants V132L and H124L grew at half the rate of the wild type in medium containing glycerol/ethanol, while E125Q grew more slowly than the wild type. The rates of growth of T122A and E128Q were identical to that of the wild-type cells. Activity of the cytochrome bc1 complex was decreased 70, 40, and 80% in mutants H124L, E125Q, and V132L, respectively, while the activity of T122A and E128Q was decreased 30 and 20% relative to the wild type. Western blotting experiments revealed that the content of the iron-sulfur protein was decreased in mutants H124L and V132L; however, no decrease in the content of the iron-sulfur protein was observed in the other mutants. These results suggest that several amino acids located in the alpha1-helix of the protein are important in maintaining the stability and proper assembly of the iron-sulfur protein in the bc1 complex.

The role of charged amino acids in the alpha1-beta4 loop of the iron-sulfur protein of the cytochrome bc1 complex of yeast mitochondria.

Previous experiments using deletion mutants of the iron-sulfur protein had indicated that amino acid residues 138-153 might be involved in the assembly of this protein into the cytochrome bc1 complex. To determine which specific residues might be involved in the assembly process, charged amino acids located in the alpha1-beta4 loop of the iron-sulfur protein were mutated to uncharged residues and tryptophan 152 to phenylalanine. The mutant genes were used to transform yeast cells (JPJ1) lacking the iron-sulfur protein gene. Mutants R146I and W152F had almost undetectable growth in medium containing glycerol/ethanol, whereas mutants D143A, K148I, and D149A grew more slowly than the wild type. Activity of the cytochrome bc1 complex was decreased 50, 90, 67, 89, and 90% in mutants D143A, R146I, K148I, D149A, and W152F, respectively, but unchanged in mutants D139A, Q141I, D145L, and V147S. In all of these mutants except W152F, the cytochrome c1 content, determined by immunoblotting, was comparable with that of wild-type cells. However, immunoblotting revealed that the content of the iron-sulfur protein was decreased proportionately in the five mutants with lowered enzymatic activity and growth suggesting that these amino acids are critical for maintaining the stability of the iron-sulfur protein. The efficiency of assembly in vitro compared with the wild type determined by selective immunoprecipitation was unchanged in the mutants with the exception of R146I, D149A, and W152F where decreases of 80, 60, and 60%, respectively, were observed suggesting that these amino acids are critical for the proper assembly of the iron-sulfur protein into the bc1 complex.

Oxidation of NADH by a rotenone and antimycin-sensitive pathway in the mitochondrion of procyclic Trypanosoma brucei brucei.

The pathway of NADH oxidation in the procyclic Trypanosoma brucei brucei was investigated in a crude mitochondrial membrane fraction and in whole cells permeabilized with digitonin. NADH:cytochrome c reductase activity was 75% inhibited by concentrations of antimycin that inhibited 95% succinate:cytochrome c reductase activity suggesting that the major pathway for NADH oxidation in the mitochondria involved the cytochrome bc1 complex of the electron transfer chain. Both NADH:cytochrome c and NADH:ubiquinone reductase activities were inhibited 80-90% by rotenone indicating the presence of a complex I-like NADH dehydrogenase in the mitochondrion of trypanosomes. In whole cells permeabilized with low concentrations of digitonin, the oxidation of malate, proline and glucose (in the presence of salicylhydroxamic acid, the inhibitor of the alternate oxidase) was inhibited 30-50% by rotenone. The presence of an alternative pathway for NADH oxidation involving fumarate reductase was indicated by the observation that malonate, the specific inhibitor of succinate dehydrogenase, inhibited 30-35% the rate of oxygen uptake with malate and glucose as substrates in the digitonin-permeabilized cells. We conclude that in the mitochondrion of the procyclic form of T. brucei, NADH is preferentially oxidized by a rotenone-sensitive NADH:ubiquinone oxidoreductase; however, NADH can also be oxidized to some extent by the enzyme fumarate reductase present in the mitochondrion of T. brucei.