Ganapathy Jagadeesan, Palayam Malathy, Pennathur Gautam, Sanmargam Aravindhan, Krishnasamy Gunasekaran
Center for Biotechnology and AU-KBC Research Center, Anna University, Chennai-600025, India.
Department of Physics, Presidency College, Chennai-600005, India.
Protein Pept Lett. 2018;25(8):748-756. doi: 10.2174/0929866525666180620163632.
Hemoglobin (Hb) subunits are composed of the specific functional prosthetic group "heme'' and a protein moiety "globin". Bird Hbs are functionally similar to mammalian Hbs but they are structurally dissimilar with mammalian. The insufficient structural studies on avian Hbs limit us to understand their degree of adaptation to such critical environments. The Great Cormorant (GCT) can fly and swim, the dual characteristic of GCT leads to study the sturcture of hemoglobin.
To determine the crystal structure of Great Cormorant Hemoglobin and to compare its three dimensional structure with other high and low oxygen affinity hemoglobin species to understand its characteristic features of high oxygen affinity.
The GCT hemoglobin has been purified, crystallized and data sets were processed using iMosflm. The integrated data has been solved using Molecular replacement method using Graylag hemoglobin (1FAW) as the template. The structure has been deposited in Protein Data Bank with PDB code: 3WR1.
In order to characterize the tertiary and quaternary structural differences, the structure of cormorant hemoglobin is compared with GLG, BHG and human Hb. The larger variation observed between GCT and human Hb indicates that GCT Hb differs remarkably from human. The α1β1 interface of Great cormorant Hb is similar to bar-headed goose Hb with few amino acid substitutions. It has been found that the interaction which is common among avian hemoglobins (α119 Pro- β55Leu) is altered by Ala 119 in GCT. This intra-dimer contact (α119 Pro - β 55 Leu) disruption leads to high oxygen affinity in BGH Hb. In cormorant, GLG and human the proline is unchanged but interestingly, in cormorant Hb, the β55 position was found to be Thr instead of Leu. Similar kind of substitutions (β 55 Leu - Ser) observed in Andean goose Hb structure leads to elevated oxygen affinity between Hb-O2. To our surprise, such type of substitution at β 55 (Thr) in cormorant Hb confirms that it is comparable with Andean goose Hb structure. Thus the sequence, structural differences at alpha, beta heme pocket and interface contacts confirms that GCT adopts high oxygen affinity conformation.
The three dimensional structure of Great cormorant hemoglobin has been investigated to understand its unique structural features to adopt during hypoxia condition. By comparing the sequence and overall structural similarities with high and low oxygen affinity species, it appears that GCT has more possibilities to subsist with low oxygen demand.
血红蛋白（Hb）亚基由特定的功能性辅基“血红素”和蛋白质部分“珠蛋白”组成。鸟类血红蛋白在功能上与哺乳动物血红蛋白相似，但在结构上与哺乳动物不同。对鸟类血红蛋白的结构研究不足，限制了我们对它们适应此类关键环境程度的理解。普通鸬鹚（GCT）既能飞行又能游泳，其双重特性促使人们研究其血红蛋白的结构。
确定普通鸬鹚血红蛋白的晶体结构，并将其三维结构与其他高、低氧亲和力血红蛋白物种进行比较，以了解其高氧亲和力的特征。
纯化普通鸬鹚血红蛋白，进行结晶，并使用iMosflm处理数据集。使用灰雁血红蛋白（1FAW）作为模板，通过分子置换法解析整合后的数据。该结构已存入蛋白质数据库，PDB代码为：3WR1。
为了表征三级和四级结构差异，将鸬鹚血红蛋白的结构与灰雁、斑头雁和人类血红蛋白进行比较。普通鸬鹚和人类血红蛋白之间观察到的较大差异表明，普通鸬鹚血红蛋白与人类血红蛋白明显不同。普通鸬鹚血红蛋白的α1β1界面与斑头雁血红蛋白相似，只有少数氨基酸取代。已发现鸟类血红蛋白中常见的相互作用（α119脯氨酸-β55亮氨酸）在普通鸬鹚中被丙氨酸119改变。这种二聚体内接触（α119脯氨酸-β55亮氨酸）的破坏导致斑头雁血红蛋白具有高氧亲和力。在鸬鹚、灰雁和人类中，脯氨酸未发生变化，但有趣的是，在鸬鹚血红蛋白中，β55位被发现是苏氨酸而不是亮氨酸。在安第斯雁血红蛋白结构中观察到类似的取代（β55亮氨酸-丝氨酸）导致血红蛋白与氧气之间的氧亲和力升高。令我们惊讶的是，鸬鹚血红蛋白中β55位的这种取代（苏氨酸）证实它与安第斯雁血红蛋白结构相当。因此，α、β血红素口袋和界面接触处的序列和结构差异证实普通鸬鹚采用高氧亲和力构象。
研究了普通鸬鹚血红蛋白的三维结构，以了解其在缺氧条件下采用的独特结构特征。通过比较与高、低氧亲和力物种的序列和整体结构相似性，似乎普通鸬鹚在低氧需求下生存的可能性更大。
