The structural chemistry and biosynthesis of chlorophylls

Abstract

Chlorophylls (Chls) harvest the solar energy that drives photosynthesis, which underpins most of the food chains on our planet. Starting from protoporphyrin IX, just seven biosynthetic reactions culminate in the synthesis of Chl a, the major light-absorbing pigment on Earth. Other such pigments, Chls b, c, d and f, widen the absorption range in the visible and red regions of the spectrum, and several bacteriochlorophylls (BChls), BChls a, b and g in particular, open new spectral windows allowing organisms to harvest near infra-red light. This perspective surveys the structural features of porphyrins, chlorins and bacteriochlorins that impart their characteristic absorption features, then presents a similar analysis of the biosynthetic intermediates leading to Chls a, b, c, d and f. The interlinked Chl and BChl biosynthetic pathways are summarised, then the rest of the perspective focusses on the enzymes that synthesise Chls a, b, c, d and f. AlphaFold 3 was used to model a complete set of structures for Chl biosynthesis enzymes, predicting intersubunit associations and the arrangements of cofactors and bound substrates, and providing insights into catalytic mechanisms. A new scheme for binding substrates and transferring products between pathway enzymes suggests how synthetic biology approaches can assemble hybrid Chl and BChl pathways to expand the spectral range for harvesting and using solar energy.