Spectral characterization of the recombinant mouse tumor suppressor 101F6 protein

Spectral and Redox Properties of a Recombinant Mouse Cytochrome b561 Protein Suggest Transmembrane Electron Transfer Function

Cytochrome b561 proteins (CYB561s) are integral membrane proteins with six trans-membrane domains, two heme-b redox centers, one on each side of the host membrane. The major characteristics of these proteins are their ascorbate reducibility and trans-membrane electron transferring capability. More than one CYB561 can be found in a wide range of animal and plant phyla and they are localized in membranes different from the membranes participating in bioenergization. Two homologous proteins, both in humans and rodents, are thought to participate-via yet unidentified way-in cancer pathology. The recombinant forms of the human tumor suppressor 101F6 protein (Hs_CYB561D2) and its mouse ortholog (Mm_CYB561D2) have already been studied in some detail. However, nothing has yet been published about the physical-chemical properties of their homologues (Hs_CYB561D1 in humans and Mm_CYB561D1 in mice). In this paper we present optical, redox and structural properties of the recombinant Mm_CYB561D1, obtained based on various spectroscopic methods and homology modeling. The results are discussed in comparison to similar properties of the other members of the CYB561 protein family.

Proton-Coupled Electron Transport in Two Distinct CYBASC Paralogs of Arabidopsis thaliana: A Comparative Characterization of Highly Conserved Tyrosine and Lysine Residues

CYBASC proteins are ascorbate (AscH^-) reducible, diheme b-containing integral membrane cytochrome b₅₆₁ proteins (cytb₅₆₁), which are proposed to be involved in AscH^- recycling and facilitation of iron absorption. Two distinct CYBASC paralogs from the plant Arabidopsis thaliana, Atcytb₅₆₁-A (A-paralog) and Atcytb₅₆₁-B (B-paralog), have been found to differ in their visible-spectral characteristics and their interaction with AscH^- and ferric iron chelates. A previously determined crystal structure of the B-paralog provides the first insights into the structural organization of a CYBASC member and implies hydrogen bonding between the substrate AscH^- and the conserved lysine residues at positions 77 (B-K77) and 81 (B-K81). The function of the highly conserved tyrosine at position 70 (B-Y70) is not obvious in the crystal structure, but its localization indicates the possible involvement in proton-coupled electron transfer. Here we show that B-Y70 plays a major role in the modulation of the oxidation-reduction midpoint potential of the high-potential heme, E_M(b_H), as well as in AscH^- oxidation. Our results support the involvement of the functionally conserved B-K77 in the stabilization of the dianion Asc^2-. These findings are supported by the crystal structure of the B-paralog, but a comparative biochemical and biophysical characterization of the A- and B-paralogs implied distinct and more complex functions of the corresponding residues A-Y69 and A-K76 in the A-paralog. Our results emphasize the need for a high-resolution crystal structure of the A-paralog to illuminate the differences in functional organization between the two paralogs.

Tuning of Hemes b Equilibrium Redox Potential Is Not Required for Cross-Membrane Electron Transfer

In biological energy conversion, cross-membrane electron transfer often involves an assembly of two hemes b. The hemes display a large difference in redox midpoint potentials (ΔE_{m_}b), which in several proteins is assumed to facilitate cross-membrane electron transfer and overcome a barrier of membrane potential. Here we challenge this assumption reporting on heme b ligand mutants of cytochrome bc₁ in which, for the first time in transmembrane cytochrome, one natural histidine has been replaced by lysine without loss of the native low spin type of heme iron. With these mutants we show that ΔE_{m_}b can be markedly increased, and the redox potential of one of the hemes can stay above the level of quinone pool, or ΔE_{m_}b can be markedly decreased to the point that two hemes are almost isopotential, yet the enzyme retains catalytically competent electron transfer between quinone binding sites and remains functional in vivo. This reveals that cytochrome bc₁ can accommodate large changes in ΔE_{m_}b without hampering catalysis, as long as these changes do not impose overly endergonic steps on downhill electron transfer from substrate to product. We propose that hemes b in this cytochrome and in other membranous cytochromes b act as electronic connectors for the catalytic sites with no fine tuning in ΔE_{m_}b required for efficient cross-membrane electron transfer. We link this concept with a natural flexibility in occurrence of several thermodynamic configurations of the direction of electron flow and the direction of the gradient of potential in relation to the vector of the electric membrane potential.

Cytochromes b561: ascorbate-mediated trans-membrane electron transport

SIGNIFICANCE

Cytochromes b561 (CYB561s) constitute a family of trans-membrane (TM), di-heme proteins, occurring in a variety of organs and cell types, in plants and animals, and using ascorbate (ASC) as an electron donor. CYB561s function as monodehydroascorbate reductase, regenerating ASC, and as Fe³⁺-reductases, providing reduced iron for TM transport. A CYB561-core domain is also associated with dopamine β-monooxygenase redox domains (DOMON) in ubiquitous CYBDOM proteins. In plants, CYBDOMs form large protein families. Physiological functions supported by CYB561s and CYBDOMs include stress defense, cell wall modifications, iron metabolism, tumor suppression, and various neurological processes, including memory retention. CYB561s, therefore, significantly broaden our view on the physiological roles of ASC.

RECENT ADVANCES

The ubiquitous nature of CYB561s is only recently being recognized. Significant advances have been made through the study of recombinant CYB561s, revealing structural and functional properties of a unique "two-heme four-helix" protein configuration. In addition, the DOMON domains of CYBDOMs are suggested to contain another heme b.

CRITICAL ISSUES

New CYB561 proteins are still being identified, and there is a need to provide an insight and overview on the various roles of these proteins and their structural properties.

FUTURE DIRECTIONS

Mutant studies will reveal in greater detail the mechanisms by which CYB561s and CYBDOMs participate in cell metabolism in plants and animals. Moreover, the availability of efficient heterologous expression systems should allow protein crystallization, more detailed (atomic-level) structural information, and insights into the intra-molecular mechanism of electron transport.

Electron transfer reactions of candidate tumor suppressor 101F6 protein, a cytochrome b561 homologue, with ascorbate and monodehydroascorbate radical

The candidate tumor suppressor 101F6 protein is a homologue of adrenal chromaffin granule cytochrome b561, which is involved in the electron transfer from cytosolic ascorbate to intravesicular monodehydroascorbate radical. Since the tumor suppressor activity of 101F6 was enhanced in the presence of ascorbate, it was suggested that 101F6 might utilize a similar transmembrane electron transfer reaction. Detailed kinetic analyses were conducted on the detergent-solubilized recombinant human 101F6 for its electron transfer reactions with ascorbate and monodehydroascorbate radical by stopped-flow and pulse radiolysis techniques. The reduction of oxidized 101F6 with ascorbate was found to be independent of pH in contrast to those observed for chromaffin granule and Zea mays cytochromes b561 in which both cytochromes exhibited very slow rates at pH 5.0 but faster at pH 6.0 and 7.0. The absence of the inhibition for the electron acceptance from ascorbate upon the treatment with diethyl pyrocarbonate suggested that 101F6 might not utilize a "concerted proton/electron transfer mechanism". The second-order rate constant for the electron donation from the ascorbate-reduced 101F6 to the pulse-generated monodehydroascorbate radical was found to be 5.0 × 10(7) M(-1 )s(-1), about 2-fold faster than that of bovine chromaffin granule cytochrome b561 and about five times faster than that of Zea mays cytochrome b561, suggesting that human 101F6 is very effective for regenerating ascorbate from monodehydroascorbate radical in cells. Present observations suggest that 101F6 employs distinct electron transfer mechanisms on both sides of the membranes from those of other members of cytochrome b561 protein family.

Functional characterization of the recombinant human tumour suppressor 101F6 protein, a cytochrome b(561) homologue

Candidate human tumour suppressor gene product, 101F6 protein, is a highly hydrophobic transmembrane protein and a member of cytochrome b(561) family. Purified 101F6 protein expressed in Pichia pastoris cells showed visible absorption spectra similar but distinct from those of cytochrome b(561). Haem content analysis indicated presence of two haems B per molecule. Midpoint potentials of the purified protein were found as +109 and +26 mV for two haems, slightly lower than those for bovine chromaffin granule or plant Zea mays cytochromes b(561). Electron paramagnetic resonance (EPR) spectra in oxidized state at 5 K showed only a highly anisotropic low-spin (HALS) signal at g(z) = 3.75. However, at 15 and 20 K, another HALS-type signal appeared at g(z) = 3.65 being overlapped with that of g(z) = 3.75. The rhombic EPR signal at g(z) = 3.16 previously seen in other cytochromes b(561) was not observed, suggesting distinct haem environments. Absence of the inhibition in the electron transfer from ascorbate by a treatment of 101F6 protein with diethylpyrocarbonate showed a remarkable contrast from those of other cytochromes b(561) where the 'concerted H(+)/e(-) transfer mechanism' at the cytosolic haem centre was blocked by specific Nε-carbethoxylation of haem-coordinating imidazole, suggesting that 101F6 protein might accept electrons via a mechanism distinct from other cytochromes b(561).

High-yield production, purification and characterization of functional human duodenal cytochrome b in an Escherichia coli system

Human duodenal cytochrome b (Dcytb) is a transmembrane hemoprotein found in the duodenal brush border membrane and in erythrocytes. Dcytb has been linked to uptake of dietary iron and to ascorbate recycling in erythrocytes. Detailed biophysical and biochemical characterization of Dcytb has been limited by difficulties in expressing sufficient amounts of functional recombinant protein in yeast and insect cell systems. We have developed an Escherichia coli Rosetta-gami B(DE3) cell system for production of recombinant His-tagged human Dcytb with a yield of ∼26 mg of purified, ascorbate-reducible cytochrome per liter of culture. The recombinant protein is readily solubilized with n-dodecyl-β-D-maltoside and purified to electrophoretic homogeneity by one-step chromatography on cobalt affinity resin. The purified recombinant Dcytb has a heme to protein ratio very close to the theoretical value of 2 and retains functional reactivity with ascorbate, as assessed by spectroscopic and kinetic measurements. Ascorbate showed a marked kinetic selectivity for the high-potential heme center over the low-potential heme center in purified Dcytb. This new E. coli expression system for Dcytb offers ∼7-fold improvement in yield and other substantial advantages over existing expression systems for reliable production of functional Dcytb at levels suitable for biochemical, biophysical and structural characterization.

Functional and structural roles of residues in the third extramembrane segment of adrenal cytochrome b561

Several residues in the third extramembrane segment (EM3) of adrenal cytochrome b(561) have been proposed to be involved in this cytochrome's interaction with ascorbate, but there has been no systematic evaluation of residues in the segment. We used alanine scanning mutagenesis to assess the functional and structural roles of the EM3 residues and several adjacent residues (residues 70-85) in the bovine cytochrome. Each alanine mutant was expressed in a bacterial system, solubilized with detergent, and affinity-purified. The recombinant proteins contained approximately two hemes per monomer and, except for R74A, retained basic functionality (≥ 94% reduced by 20 mM ascorbate). Equilibrium spectrophotometric titrations with ascorbate were used to analyze the α-band line shape and amplitude during reduction of the high- and low-potential heme centers (b(H) and b(L), respectively) and the midpoint ascorbate concentrations for the b(H) and b(L) transitions (C(H) and C(L), respectively). Y73A and K85A markedly narrowed the b(H) α-band peak; other mutants had weaker effects or no effect on b(H) or b(L) spectra. Relative changes in C(H) for the mutants were larger than changes in C(L), with 1.5-2.9-fold increases in C(H) for L70A, L71A, Y73A, R74A, N78A, and K85A. The amounts of functional b(H) and b(L) centers in additional Arg74 mutants, assessed by ascorbate titration and EPR spectroscopy, declined in concert in the following order: wild type > R74K > R74Q > R74T and R74Y > R74E. The results of this first comprehensive experimental test of the proposed roles of EM3 residues have identified residues with a direct or indirect impact on ascorbate interactions, on the environment of the b(H) heme center, and on formation of the native b(H)-b(L) unit. Surprisingly, no individual EM3 residue was by itself indispensable for the interaction with ascorbate, and the role of the segment appears to be more subtle than previously thought. These results also support our topological model of the adrenal cytochrome, which positions b(H) near the cytoplasmic side of the membrane.