text: Microbes produce two reduced variants of chlorin, bacteriochlorins and isobacteriochlorins. Bacteriochlorins are found in some bacteriochlorophylls; the ring structure is produced by  Chlorophyllide a reductase (COR) reducing a chlorin ring at the C7-8 double boud. Isobacteriochlorins are found in nature mostly as sirohydrochlorin, a biosynthetic intermediate of vitamin B, produced without going through a chlorin. In living organisms, both are ultimately derived from uroporphyrinogen III, a near-universal intermediate in tetrapyrrole biosynthesis.