A heme (HEM) is a multi-atom, non-proteinaceous organic structure capable of positioning an iron (FE) ion in its center.
亚铁血红素 (HEM) 是一个多原子、非蛋白质的有机结构，能够将一个铁 (FE) 离子置于其中心。
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So, for example, if I have a sodium ion over here, and I have a chloride ion over here, where the distance from center to center r I'm denoting as r, this is nucleus to nucleus separation.
The metal ion may be the catalytic center of the cofactor, or it may serve to bind the enzyme and substrate together to hold the enzyme in a catalytically active conformation.