Table 6 Enzymes of the BChl a biosynthesis pathway that are particularly sensitive to oxygen

bchI bchD bchH

Mg-chelatase

BchH contains the particular cystein motif CX₂CX₃CX₁₄C, which coordinates a [4Fe - 4S] - cluster. This iron - sulfur cluster is particularly sensitive to oxygen and is transformed to a [3Fe - 4S] cluster in its presence. [78].

bchM

Mg-PPIX-O-methyltransferase

The methyl group is supplied by S - adenosyl - methionine. The oxygen - sensitive BchH has been proposed to be the substrate - binding protein of BchM [79, 80]

bchE

Mg - PPIX - O - CH₃ oxidative cyclase

BchE contains a conserved CX₃CX₂C cystein motif, which coordinates an oxygen - sensitive [4Fe - 4S] - cluster [77].

bchL bchN bchB

Dark allowed protochlorophyllide oxidoreductase (DPOR)

The reductase component BchL contains two conserved cysteine residues and the BchL- dimer coordinates an oxygen - sensitive [4Fe - 4S]-cluster [89]. The catalytic component (BchN-BchB)₂ also contains conserved cysteines, which coordinate [4Fe - 4S]-clusters, however these are not oxygen-sensitive [129]

bchX bchY bchZ

Chlorin reductase (COR)

BchX, BchY and BchZ are enzyme subunits structurally and functionally similar to BchL, BchN, and BchB, respectively [90, 91]. It has been shown that the subunit dimers (BchL)₂ and (BchX)₂ of DPOR and COR, respectively, which drive electrons to the substrates of these enzymes, are exposed to the cytoplasm [91, 92], i.e. to selenite and to the ROS produced during selenite reduction.