Incomplete Hydrogenation by Geranylgeranyl Reductase from a Proteobacterial Phototroph Halorhodospira halochloris, Resulting in the Production of Bacteriochlorophyll with a Tetrahydrogeranylgeranyl Tail

Abstract

Light harvesting and charge separation are functions of chlorophyll and bacteriochlorophyll pigments. While most photosynthetic organisms use (bacterio)chlorophylls with a phytyl (2-phytenyl) group as the hydrophobic isoprenoid tail, Halorhodospira halochloris , an anoxygenic photosynthetic bacterium belonging to Gammaproteobacteria, produces bacteriochlorophylls with a unique 6,7,14,15-tetrahydrogeranylgeranyl (2,10-phytadienyl) tail. Geranylgeranyl reductase (GGR) encoded by the bchP gene, catalyzes hydrogenation at three unsaturated C=C bonds of a geranylgeranyl group, giving rise to the phytyl tail. In this study, we discovered that H. halochloris GGR exhibits only partial hydrogenation activities, resulting in the tetrahydrogeranylgeranyl tail formation. We hypothesized that the hydrogenation activity of H. halochloris GGR differed from that of Chlorobaculum tepidum GGR, which also produces a pigment with partially reduced hydrophobic tails (2,6-phytadienylated chlorophyll a ). An engineered GGR was also constructed and demonstrated to perform only single hydrogenation, resulting in the dihydrogeranylgeranyl tail formation. H. halochloris original and variant GGRs shed light on GGR catalytic mechanisms and offer prospective bioengineering tools in the microbial production of isoprenoid compounds. Importance Geranylgeranyl reductase (GGR) catalyzes the hydrogenation of carbon–carbon double bonds of unsaturated hydrocarbons of isoprenoid compounds, including α-tocopherols, phylloquinone, archaeal cell membranes, and (bacterio)chlorophyll pigments in various organisms. GGRs in photosynthetic organisms, including anoxygenic phototrophic bacteria, cyanobacteria, and plants perform successive triple hydrogenation to produce chlorophylls and bacteriochlorophylls with a phytyl chain. Here, we demonstrated that the GGR of a γ-proteobacterium Halorhodospira halochloris catalyzed unique double hydrogenation to produce bacteriochlorophylls with a tetrahydrogeranylgeranyl tail. We also constructed a variant enzyme derived from H. halochloris GGR that performs only single hydrogenation. The results of this study provide new insights into catalytic mechanisms of multi-position reductions by a single enzyme.

The terminal enzymes of (bacterio)chlorophyll biosynthesis

“…rubrum synthesizes unreduced BChl a GG , but also incorporates the de-metallated BPheo a with a phytyl tail into its reaction centre; the BchP enzyme from this organism is only able to reduce the tail of BPheo, indicating that BchG must esterify BChlide with GGPP, and that this pigment goes through a Mg-dechelation before the isoprenoid moiety is reduced [36,105]. An additional unique case is that of the anoxygenic phototrophic bacterium Halorhodospira halochloris , which accumulates BChl b carrying a tail with only two reductions (a 2,10-phytadienyl moiety), thought to be a result of an insertion mutation in the bchP gene [108,109]. Furthermore, Cba .…”

“…Furthermore, Cba. tepidum produce BChl a with a phytyl tail, but also Chl a with a unique TH-GG group with two reductions (a 2,6-phytadienyl moiety) [108,110].…”

Unraveling the multiplicity of geranylgeranyl reductases in Archaea: potential roles in saturation of terpenoids

“…The organism also produces species of respiratory quinones, such as caldariellaquinone (CQ) and sulfoquinone (SQ), membrane anchors, such as dolichol phosphate (DoIP), and C 20 -C 35 apolar polyisoprenoids (Salvador-Castell et al 2019 ; Holzer et al 1979 ; Elling et al 2016 ). Typically, CQ and SQs are found in their fully saturated forms, and their production is dependent on the amount of oxygen (Elling et al 2016 ; Trincone et al 1989 ). Fully saturated species of α- and ω-of C 40 ,C 45 , and C 50 DoIP species have been found in Sulfolobus (Guan et al 2011 ).…”