Figure 2.

A proposal for the origin of Chl-based phototrophy (see the text). In the primitive, ancestral pathway, a side activity of ferrochelatase produces Zn-PPIX as in modern cells although Zn²⁺ can also spontaneously insert into PPIX. Zn-PPIX could bind to an abundant soluble heme-binding protein, leading to a photoactive protein that could have reduced soluble Fd and replaced flavin-based electron bifurcation. Williamson et al. (2011) have discussed the possible role of Zn-tetrapryrroles as functional intermediates in Chl evolution.