Biochimica et biophysica acta最新文献

Mobilization of the ribose moiety and of the amino group of guanosine may be realized in rat liver extract by the concerted action of purine nucleoside phosphorylase and guanase. Ribose 1-phosphate formed from guanosine through the action of purine nucleoside phosphorylase acts as ribose donor in the synthesis of xanthosine catalyzed by the same enzyme. The presence of guanase, which irreversibly converts guanine to xanthine, affects the overall process of guanosine transformation. As a result of this purine pathway, guanosine is converted into xanthosine, thus overcoming the lack of guanosine deaminase in mammals. Furthermore, in rat liver extract the activated ribose moiety stemming from the catabolism of purine nucleosides can be transferred to uracil and, in the presence of ATP, used for the synthesis of pyrimidine nucleotides; therefore, purine nucleosides can act as ribose donors for the salvage of pyrimidine bases.

The role of the acidic patches of spinach plastocyanin in the interaction with a soluble form of turnip cytochrome ƒ was studied by a combination of site-directed mutagenesis, NMR spectroscopy and kinetic analysis. The charge of the two 'eastern' patches, consisting of conserved acidic residues 42-45 and 59-61 respectively, was altered by incorporation of neutral or positively charged groups. Up to four negative charges were deleted in six different mutants and a further mutant, Q88E, provided an additional negative charge in the same region. Overall second-order rate constants (k₂) for reduction by cytochrome ƒ were determined by stopped-flow spectrophotometry. A 2- to 3-fold decrease in k₂ was observed for each negative charge abolished, regardless of its position, and in Q88E there was a 20% increase. From the ionic strength dependence similar values for k₂ at infinite ionic strength were predicted for the native and mutant proteins, while the electrostatic attraction term decreased with each negative charge removed. The equilibrium constant for association (K_A) was determined from the change in T₂ of ¹H resonances of plastocyanin. Loss of negative charges caused marked decreases in K_A roughly in parallel with the decreases in k₂, which suggests that the main effect was on binding rather than the rate of intracomplex electron transfer. Taken together, these results provide convincing evidence for participation of residues of both acidic patches in the interaction with cytochrome ƒ.

The whereabouts of the Ca²⁺ site in Photosystem II (PSII) was investigated by experiments in which Mn²⁺ was substituted for Ca²⁺. When stoichiometric amounts of Mn²⁺ ions were added to Ca²⁺-depleted PSII, the Mn²⁺ was not detected by EPR. The titration of Ca²⁺ back into Ca²⁺-depleted/Mn²⁺-containing PSII resulted in the simultaneous release of the Mn²⁺ and the loss of the two EPR signals which are characteristic of the Ca²⁺-depleted enzyme (i.e., the stable, modified S₂ multiline signal arising from the intrinsic Mn cluster and the split S₃ signal from an organic radical interacting with the Mn cluster). These results indicate that the Mn²⁺ occupies the functional Ca²⁺ site. The S₂ and S₃ EPR signal characteristic of this kind of Ca²⁺-depleted preparation were unaffected by the binding of the Mn²⁺ Since, from earlier results, it seems likely that the modification and stability of S₂ multiline signal in these PSII preparations is due to binding of chelator to or close to the Mn cluster, the present results indicate that the Ca²⁺ site (at least when occupied by Mn²⁺) does not overlap with the chelator binding site. Since Mn²⁺ binding does not effect the S₂ EPR signal from the Mn cluster, it can be concluded that the Mn²⁺ is not involved in detectable magnetic interactions with the cluster. This result indicates that the Mn²⁺-occupied Ca²⁺ binding site is outside the first co-ordination sphere of the Mn cluster. The relaxation properties of TyrD^. were enhanced by the presence of the Mn²⁺ when the Mn cluster was in the S₁ state.

An EPR signal at g = 6.8 attributed to the cytochrome (Cyt) b-559 heme Fe(III) ligated by OH^- (Fiege, R., Schrieber, U., Lubitz, W., Renger, G. and Shuvalov, V.A. (1995) FEBS Lett. 377, 325-329) was studied. This signal is observed in intact chloroplasts when oxidized by 10 mM 2,3-dicyano,5,6-dichloro-p-benzoquinone (DDQ), but not when 5 mM p-benzoquinone is added. Addition of KF (100 mM) or removal of Ca 21 for blocking the water-oxidizing complex considerably decreases the heme Fe(III)-OH^- EPR signal. In contrast, DCMU does not decrease this signal and does not influence its photochemical changes at 140 K. Thus, the EPR spectrum of Cyt b-559 Fe(III) ligated by OH^- is not affected by changes at the acceptor side of Photosystem 11, and its photochemical decrease is probably not due to reduction via the acceptor side. Comparison of the effect of KF on the model heme Fe(III) in myoglobin (Mb) at pH 10.5 shows that F^- replaces OH^- as a ligand at the sixth coordination position of the heme Fe(III) in both Mb and chloroplasts Cyt b-559. This replacement is accompanied by changes of the symmetry of the molecular field causing a disappearance of the EPR signals at g = 6.8 and 5.0. Our results provide further evidence for a possible participation of the Cyt b-559 heme Fe ligated by OH^- in photosynthetic water oxidation.

Both short-term and long-term adaptations of cyanobacterial metabolism to light and dark were studied in Nostoc sp. Mac. Long-term adaptations were induced by growing cells in the presence of glucose under (a) 30 μE m^-2 s^-1 continuous white light, (b) under a 14/10 h light/dark cycle, or (c) complete darkness. Short-term regulation of enzyme activities by light was then studied in cells rendered osmotically fragile with lysozyme. Cells were briefly illuminated then enzyme activities were measured following rapid lysis in a hypotonic assay medium. The following results were obtained. (1) Relative to fresh weight, dark-grown cells contained less chlorophyll, much less phycoerythrin, but similar amounts of phycocyanin compared to cells grown under either light regime. Relative to chlorophyll, the higher phycocyanin and much lower phycoerythrin in the dark-grown vs light-grown cells resembles long term changes in pigment content that occur during complementary chromatic adaptation to red vs orange light. (2) Both dark and light/dark grown cells displayed generally lowered photosynthetic activities compared to light-grown cells. The exception to this was the activity of fructose 1,6-bisphosphatase, which was higher in dark-grown cells. However, the photosynthetic induction period was markedly shorter in the light/dark-grown cells indicating an adaptation to changing illumination during growth. (3) Inhibitor studies using methyl viologen show that the fructose 1,6-bisphosphatase is reversibly light-activated in vivo by the cyanobacterial thioredoxin system under all growth conditions. Glucose-6-phosphate dehydrogenase activity was detected in cells grown in all conditions and this activity was reversibly deactivated by light or by dithiothreitol. In contrast, the protonmotive ATPase F₀F₁-type ATPase was fully active in both light and dark-adapted cells regardless of the light regime used for growth. (4) It is concluded that the Calvin cycle enzymes, their short-term regulatory system, including thioredoxin, glucose-6-phosphate dehydrogenase and an F₀F₁ ATPase not under thioredoxin control, are expressed in cells grown in complete darkness. Adaptation to heterotrophic growth in this cyanobacterium does not appear to involve synthesis of different enzyme forms lacking thioredoxin control sequences.

Molecular weights of the purified light-harvesting complexes B800-850 and B875 from purple bacterium Rubrivivax gelatinosus were determined in detergent solutions by analytical centrifugation. The precise density measurement of the antenna solutions provided the values of the buoyant factor of both complexes. Phospholipid content was measured in both antennae. Sedimentation velocity and equilibrium centrifugation indicated that the B800-850 sample is monodisperse and composed from heptamers (α/β/3 Bacteriochlorophyll α 1.3 Carotenoid)₇. B875 is not monodisperse but analysis of equilibrium centrifugation indicated that its smallest oligomer is a dodecamer (α/β/2 BChlα/2 hydroxyspheroidene)₁₂; larger oligomers probably coexist. The amount of bound detergent could be calculated. B800-850 binds about 0.4-0.5 g of lauryl-dimethyl-amine oxide per g of protein (i.e., 28-35 detergent molecules per α β ) while a large amount of detergent (2.3 g of decanoylsucrose per g of protein, i.e., 64 detergent molecules per α β ) is bound to 13875. The electron microscopy images of both antennae in detergent solutions after negative staining are presented and compared to the centrifugation results.