Heme containing proteins are involved in a broad range of cellular functions, from oxygen sensing and transport to catalyzing oxidoreductive reactions. The two major types of cytochrome (b-type and c-type) only differ in their mechanism of heme attachment, but this has major implications for their cellular roles in both localization and mechanism. The b-type cytochromes are commonly cytoplasmic, or are within the cytoplasmic membrane, while c-type cytochromes are always found outside of the cytoplasm. The mechanism of heme attachment allows for complex c-type multiheme complexes, having the capacity to hold multiple electrons, to be assembled. These are increasingly being identified as secreted into the extracellular environment. For organisms that respire using extracellular substrates, these large multiheme cytochromes allow for electron transfer networks from the cytoplasmic membrane to the cell exterior for the reduction of extracellular electron acceptors. In this review the structures and functions of these networks and the mechanisms by which electrons are transferred to extracellular substrates is described.

中文翻译：

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多血红素细胞色素在细菌细胞外厌氧呼吸中的作用。

含血红素的蛋白质涉及广泛的细胞功能，从氧感测和运输到催化氧化还原反应。细胞色素的两种主要类型（b型和c型）仅在血红素附着机理上有所不同，但这对它们在定位和机理中的细胞作用具有重要意义。b型细胞色素通常是细胞质的，或者在细胞质膜内，而c型细胞色素总是在细胞质之外。血红素附着的机制允许组装具有保持多个电子的能力的复杂的c型多血红素络合物。这些越来越多地被确定为分泌到细胞外环境中。对于使用细胞外底物呼吸的生物，这些大的多血红素细胞色素允许从细胞质膜到细胞外部的电子转移网络，以减少细胞外电子受体。在这篇综述中，描述了这些网络的结构和功能以及将电子转移到细胞外基质的机制。