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.

中文翻译：

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拟南芥的两个不同的CYBASC旁系同源物中的质子耦合电子传输：高度保守的酪氨酸和赖氨酸残基的比较特征。

CYBASC蛋白是抗坏血酸（ASCH ^- ）还原，diheme b含完整的膜的细胞色素b _561克的蛋白质（细胞色素b ₅₆₁），其被提出参与ASCH ^-回收和铁的吸收提供便利。从植物两个不同CYBASC旁系同源物的拟南芥，在细胞色素b ₅₆₁ -A（A-旁系）和在细胞色素b ₅₆₁ -B（B-旁系同源），已经发现在它们的可见光谱特性及其与ASCH相互作用而不同^–和铁螯合物。的B-旁系同源物的先前确定的晶体结构提供了第一洞察CYBASC构件的结构组织和意味着衬底ASCH之间的氢键^-和在位置77（B-K77）和81（B-K81的保守的赖氨酸残基）。在位置70（B-Y70）上高度保守的酪氨酸的功能在晶体结构中并不明显，但其定位表明可能参与了质子耦合电子的转移。在这里，我们表明，B-Y70，起着高电位血红素氧化还原中点电位的调节中起主要作用ê_中号（b _{^ h}），以及在阿希^-氧化。我们的结果支持功能上保守的B-K77参与双阴离子Asc ^2–的稳定化。这些发现得到了B-旁系同源物的晶体结构的支持，但是A-和B-旁系同源物的比较生化和生物物理表征暗示了A-中相应残基A-Y69和A-K76的独特而又复杂的功能。旁白 我们的结果强调需要A旁系同源物的高分辨率晶体结构，以阐明两个旁系同源物之间功能性组织的差异。