The crystal structure of Anabaena PCC 7119 ferredoxin-NADP+ reductase (FNR) suggests that the carboxylate group of Glu301 may be directly involved in the catalytic process of electron and proton transfer between the isoalloxazine moiety of FAD and FNR substrates (NADPH, ferredoxin, and flavodoxin). To assess this possibility, the carboxylate of Glu301 was removed by mutating the residue to an alanine. Various spectroscopic techniques (UV-vis absorption, fluorescence, and CD) indicate that the mutant protein folded properly and that significant protein structural rearrangements did not occur. Additionally, complex formation of the mutant FNR with its substrates was almost unaltered. Nevertheless, no semiquinone formation was seen during photoreduction of Glu301Ala FNR. Furthermore, steady-state activities in which FNR semiquinone formation was required during the electron-transfer processes to ferredoxin were appreciably affected by the mutation. Fast transient kinetic studies corroborated that removal of the carboxylate at position 301 decreases the rate constant approximately 40-fold for the electron transfer process with ferredoxin without appreciably affecting complex formation, and thus interferes with the stabilization of the transition state during electron-transfer between the FAD and the iron-sulfur cluster. Moreover, the mutation also altered the nonspecific reaction of FNR with 5'-deazariboflavin semiquinone, the electron-transfer reactions with flavodoxin, and the reoxidation properties of the enzyme. These results clearly establish Glu301 as a critical residue for electron transfer in FNR.

Our previous studies have shown that the rate constant for intramolecular electron transfer (IET) between the heme and molybdenum centers of chicken liver sulfite oxidase varies from approximately 20 to 1400 s(-1) depending upon reaction conditions [Pacheco, A., Hazzard, J. T., Tollin, G., and Enemark, J. H. (1999) J. Biol. Inorg. Chem. 4, 390-401]. These two centers are linked by a flexible polypeptide loop, suggesting that conformational changes, which alter the Mo-Fe distance, may play an important role in the observed IET rates. In this study, we have investigated IET in sulfite oxidase using laser flash photolysis as a function of solution viscosity. The solution viscosity was varied over the range of 1.0-2.0 cP by addition of either polyethylene glycol 400 or sucrose. In the presence of either viscosogen, an appreciable decrease in the IET rate constant value is observed with an increase in the solvent viscosity. The IET rate constant exhibits a linear dependence on the negative 0.7th power of the viscosity. Steady-state kinetics and EPR experiments are consistent with the interpretation that viscosity, and not other properties of the added viscosogens, is responsible for the dependence of IET rates on the solvent composition. The results are consistent with the role of conformational changes on IET in sulfite oxidase, which helps to clarify the inconsistency between the large rate constant for IET between the Mo and Fe centers and the long distance (approximately 32 A) between these two metal centers observed in the crystal structure [Kisker, C., Schindelin, H., Pacheco, A., Wehbi, W., Garnett, R. M., Rajagopalan, K. V., Enemark, J. H., and Rees, D. C. (1997) Cell 91, 973-983].

While the acquisition of invasiveness is a critical step in early stage breast carcinomas (DCIS), no established molecular markers reliably identify tumor progression. The metastasis gene osteopontin is subject to alternative splicing, which yields 3 messages, osteopontin-a, osteopontin-b and osteopontin-c. Osteopontin-c is selectively expressed in invasive, but not in noninvasive, breast tumor cell lines, and it effectively supports anchorage independence. We evaluated osteopontin-c as a biomarker. The RNA message for osteopontin-c was present in 16 of 20 breast cancers (80%), but was undetectable in 22 normal specimens obtained from reduction mammoplasty. In contrast, osteopontin-a RNA was expressed at various levels in all 20 breast cancers, 11 tumor-surrounding tissues and 21 normal samples. The splice variant osteopontin-b was present at barely detectable levels in 18 of 20 cancers and in 6 of 22 normal breasts. By immunohistochemistry, 66 of 69 normal breasts were negative, while 3 showed low level staining. Among the breast cancers, 43 of 56 cores (77%) stained positive for osteopontin-c. When correlated with tumor grade, the staining for osteopontin-c increased from grade 1 to grade 3. In a total of 178 breast specimens analyzed, osteopontin-c was present in 78% of cancers, 36% of surrounding tissues and 0% of normal tissues. Furthermore, osteopontin-c detects a higher fraction of breast cancers than estrogen receptor (ER), progesterone receptor or HER2. In conjunction, osteopontin-c, ER and HER2 reliably predict grade 2-3 breast cancer. Hence, osteopontin-c is a diagnostic and prognostic marker that may have value in a diagnostic panel together with conventional breast cancer markers. ' 2007 Wiley-Liss, Inc.

A combination of structural, thermodynamic, and transient kinetic data on wild-type and mutant Anabaena vegetative cell ferredoxins has been used to investigate the nature of the protein-protein interactions leading to electron transfer from reduced ferredoxin to oxidized ferredoxin:NADP+ reductase (FNR). We have determined the reduction potentials of wild-type vegetative ferredoxin, heterocyst ferredoxin, and 12 site-specific mutants at seven surface residues of vegetative ferredoxin, as well as the one- and two-electron reduction potentials of FNR, both alone and in complexes with wild-type and three mutant ferredoxins. X-ray crystallographic structure determinations have been carried out for six of the ferredoxin mutants. None of the mutants showed significant structural changes in the immediate vicinity of the [2Fe-2S] cluster, despite large decreases in electron-transfer reactivity (for E94K and S47A) and sizable increases in reduction potential (80 mV for E94K and 47 mV for S47A). Furthermore, the relatively small changes in Calpha backbone atom positions which were observed in these mutants do not correlate with the kinetic and thermodynamic properties. In sharp contrast to the S47A mutant, S47T retains electron-transfer activity, and its reduction potential is 100 mV more negative than that of the S47A mutant, implicating the importance of the hydrogen bond which exists between the side chain hydroxyl group of S47 and the side chain carboxyl oxygen of E94. Other ferredoxin mutations that alter both reduction potential and electron-transfer reactivity are E94Q, F65A, and F65I, whereas D62K, D68K, Q70K, E94D, and F65Y have reduction potentials and electron-transfer reactivity that are similar to those of wild-type ferredoxin. In electrostatic complexes with recombinant FNR, three of the kinetically impaired ferredoxin mutants, as did wild-type ferredoxin, induced large (approximately 40 mV) positive shifts in the reduction potential of the flavoprotein, thereby making electron transfer thermodynamically feasible. On the basis of these observations, we conclude that nonconservative mutations of three critical residues (S47, F65, and E94) on the surface of ferredoxin have large parallel effects on both the reduction potential and the electron-transfer reactivity of the [2Fe-2S] cluster and that the reduction potential changes are not the principal factor governing electron-transfer reactivity. Rather, the kinetic properties are most likely controlled by the specific orientations of the proteins within the transient electron-transfer complex.