小型核酶的结构和催化机理

自然界存在的小型核酶主要有锤头型核酶、发夹型核酶、肝炎δ病毒（HDV）核酶和VS核酶.锤头型核酶由3个短螺旋和1个广义保守的连接序列组成；发夹型核酶的催化中心由两个肩并肩挨着的区域构成；HDV核酶折叠成包含五个螺旋臂（P1～P4）的双结结构；VS核酶由五个螺旋结构组成，这些螺旋结构通过两个连接域连接起来.小型核酶的催化机理与其分子结构密切相关.金属离子或特定碱基都可作为催化反应的关键成分.锤头型核酶的催化必须有金属离子（尤其是二价金属离子）参与，而发夹型核酶则完全不需要金属离子.基因组HDV核酶进行催化时要有金属离子和特定碱基互相配合.

Structures and Catalytic Mechanisms of Small Ribozymes

Naturally existing small catalytic RNAs include hammerhead, hairpin, hepatitis delta virus (HDV) and Varkud satellite (VS) ribozymes. The structures of small ribozymes are simple but diversified. Hammerhead ribozyme consists of 3 short helices and a conserved joint chain. Hairpin ribozyme's deeply buried active region is made up of two side-by-side helices. HDV ribozyme folds to a double-knot structure which contains 5 helix arms. VS ribozyme is made up of 5 helix regions and two joint regions. These ribozymes appear to exploit different cleavage mechanisms which depend upon their individual architectures. Metal ions and nucleobases might be candidates for participants in acid/base catalysis. Hammerhead ribozyme has a basic requirement for divalent metal ions, such as Mg²⁺ ions, to complete it's function. However, hairpin ribozyme can be considered as a distinct one that does not require metal ions as cofactors. In the genomic HDV ribozyme, a metal ion functions as a general base and a cytosine residue functions as a general acid.