The component of haemoglobin responsible for binding oxygen, consists of an iron ion that binds oxygen and a porphyrin ring that binds the globin molecules; one molecule binds one molecule of oxygen.
Haem is ubiquitous, abundant and necessary for energy metabolism. Most bacteria have a haem biosynthesis pathway, but nevertheless, since haem is a major source of iron (an essential metal), microbes take up exogenous haem to retrieve iron.
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