The γ-class carbonic anhydrases (γ-CAs) mainly come from methanogens methane-producing bacteria that grow in hot springs and catalyze the interconversion of carbon dioxide and water to bicarbonate and protons. Here, the γ-CA from Thermus thermophilus HB8 (γ-TtCA) was expressed and purified, its crystal structure was determined at 2.3 Å resolution in space group P1. The asymmetric unit contains two trimers and six catalytic Zn2+. In general, the fold of the protein is similar to those of homologous enzymes from Geobacillus Kaustophilus, Bacillus Cereus, Methanosarcina Thermophila and others. Each monomer comprises a triangular prism-like structure consisting of a left-handed β-helix and a C-terminal α-helix. The catalytic Zn2+ bound to three histidines and a phosphate radical in a tetrahedral fashion. It is located at the interface between the two monomers. Inductively coupled plasma mass spectrometry measurements further suggest that the molar ratio of zinc ions and protein molecules is 1:1. The structure revealed a novel different region situated between the left-handed β-helix and the C-terminal α-helix. Compared to previously reported structures, half of the C-terminal α-helix was replaced with a long loop in this structure. The purified γ-TtCA exhibits no significant carbonic anhydrase activity compared to α-class carbonic anhydrases. This study provides insight into the structural diversity of γ-CAs with potential function for γ-CAs.

中文翻译：

---

热稳定和锌离子结合的γ-类碳酸酐酶的分子结构。

γ-类碳酸酐酶（γ-CAs）主要来自产甲烷的甲烷菌，这些细菌在温泉中生长并催化二氧化碳和水相互转化为碳酸氢盐和质子。在这里，表达并纯化了嗜热栖热菌HB8（γ-TtCA）的γ-CA，在空间群P1中以2.3的分辨率确定了其晶体结构。不对称单元包含两个三聚体和六个催化性Zn2 +。通常，蛋白质的折叠与来自嗜碱芽孢杆菌，蜡状芽孢杆菌，嗜热甲烷菌等的同源酶的折叠相似。每个单体包括由左旋β-螺旋和C-末端α-螺旋组成的三角形棱柱状结构。催化的Zn2 +以四面体方式结合到三个组氨酸和一个磷酸基团上。它位于两种单体之间的界面。电感耦合等离子体质谱法测量进一步表明，锌离子与蛋白质分子的摩尔比为1：1。该结构揭示了位于左旋β螺旋和C端α螺旋之间的一个新颖的不同区域。与先前报道的结构相比，该结构中一半的C末端α-螺旋被长环取代。与α-类碳酸酐酶相比，纯化的γ-TtCA没有显着的碳酸酐酶活性。这项研究提供了对具有潜在功能的γ-CAs的γ-CAs结构多样性的见解。该结构揭示了位于左旋β螺旋和C端α螺旋之间的一个新颖的不同区域。与先前报道的结构相比，该结构中一半的C末端α-螺旋被长环取代。与α-类碳酸酐酶相比，纯化的γ-TtCA没有显着的碳酸酐酶活性。这项研究提供了对具有潜在功能的γ-CAs的γ-CAs结构多样性的见解。该结构揭示了位于左旋β螺旋和C端α螺旋之间的一个新颖的不同区域。与先前报道的结构相比，该结构中一半的C末端α-螺旋被长环取代。与α-类碳酸酐酶相比，纯化的γ-TtCA没有显着的碳酸酐酶活性。这项研究提供了对具有潜在功能的γ-CAs的γ-CAs结构多样性的见解。