Sato Emiko, Sagami Ikuko, Uchida Takeshi, Sato Akira, Kitagawa Teizo, Igarashi Jotaro, Shimizu Toru
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan.
Biochemistry. 2004 Nov 9;43(44):14189-98. doi: 10.1021/bi048742i.
SOUL is specifically expressed in the retina and pineal gland and displays more than 40% sequence homology with p22HBP, a heme protein ubiquitously expressed in numerous tissues. SOUL was purified as a dimer in the absence of heme from the Escherichia coli expression system but displayed a hexameric structure upon heme binding. Heme-bound SOUL displayed optical absorption and resonance Raman spectra typical of 6-coordinate low-spin heme protein, with one heme per monomeric unit for both the Fe(III) and Fe(II) complexes. Spectral data additionally suggest that one of the axial ligands of the Fe(III) heme complex is His. Mutation of His42 (the only His of SOUL) to Ala resulted in loss of heme binding, confirming that this residue is an axial ligand of SOUL. The K(d) value of heme for SOUL was estimated as 4.8 x 10(-9) M from the association and dissociation rate constants, suggesting high binding affinity. On the other hand, p22HBP was obtained as a monomer containing one heme per subunit, with a K(d) value of 2.1 x 10(-11) M. Spectra of heme-bound p22HBP were different from those of SOUL but similar to those of heme-bound bovine serum albumin in which heme bound to a hydrophobic cavity with no specific axial ligand coordination. Therefore, the heme-binding properties and coordination structure of SOUL are distinct from those of p22HBP, despite high sequence homology. The physiological role of the new heme-binding protein, SOUL, is further discussed in this report.
SOUL在视网膜和松果体中特异性表达，与p22HBP具有40%以上的序列同源性，p22HBP是一种在许多组织中普遍表达的血红素蛋白。在没有血红素的情况下，从大肠杆菌表达系统中纯化得到的SOUL为二聚体，但在结合血红素后呈现六聚体结构。结合血红素的SOUL显示出典型的六配位低自旋血红素蛋白的光吸收和共振拉曼光谱，Fe(III)和Fe(II)配合物的每个单体单元都有一个血红素。光谱数据还表明，Fe(III)血红素配合物的一个轴向配体是His。将His42（SOUL唯一的His）突变为Ala导致血红素结合丧失，证实该残基是SOUL的轴向配体。根据缔合和解离速率常数估计，SOUL对血红素的K(d)值为4.8×10(-9) M，表明具有高结合亲和力。另一方面，p22HBP以每个亚基含有一个血红素的单体形式获得，K(d)值为2.1×10(-11) M。结合血红素的p22HBP的光谱与SOUL的不同，但与结合血红素的牛血清白蛋白的光谱相似，其中血红素结合到一个没有特定轴向配体配位的疏水腔中。因此，尽管序列同源性高，但SOUL的血红素结合特性和配位结构与p22HBP不同。本报告进一步讨论了新的血红素结合蛋白SOUL的生理作用。
