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ABSTRACT Nitrogenase iron (Fe) proteins reduce CO 2 to CO and/or hydrocarbons under ambient conditions. Here, we report a 2.4-Å crystal structure of the Fe protein from Methanosarcina acetivorans ( Ma NifH), which is generated in the presence of a reductant, dithionite, and an alternative CO 2 source, bicarbonate. Structural analysis of this methanogen Fe protein species suggests that CO 2 is possibly captured in an unactivated, linear conformation near the [Fe 4 S 4 ] cluster of Ma NifH by a conserved arginine (Arg) pair in a concerted and, possibly, asymmetric manner. Density functional theory calculations and mutational analyses provide further support for the capture of CO 2 on Ma NifH while suggesting a possible role of Arg in the initial coordination of CO 2 via hydrogen bonding and electrostatic interactions. These results provide a useful framework for further mechanistic investigations of CO 2 activation by a surface-exposed [Fe 4 S 4 ] cluster, which may facilitate future development of FeS catalysts for ambient conversion of CO 2 into valuable chemical commodities. IMPORTANCE This work reports the crystal structure of a previously uncharacterized Fe protein from a methanogenic organism, which provides important insights into the structural properties of the less-characterized, yet highly interesting archaeal nitrogenase enzymes. Moreover, the structure-derived implications for CO 2 capture by a surface-exposed [Fe 4 S 4 ] cluster point to the possibility of developing novel strategies for CO 2 sequestration while providing the initial insights into the unique mechanism of FeS-based CO 2 activation.more » « less
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Summary The GreenCut encompasses a suite of nucleus‐encoded proteins with orthologs among green lineage organisms (plants, green algae), but that are absent or poorly conserved in non‐photosynthetic/heterotrophic organisms. In
Chlamydomonas reinhardtii ,CPLD 49 (C onserved inP lantL ineage andD iatoms49 ) is an uncharacterized GreenCut protein that is critical for maintaining normal photosynthetic function. We demonstrate that acpld49 mutant has impaired photoautotrophic growth under high‐light conditions. The mutant exhibits a nearly 90% reduction in the level of the cytochromeb 6f complex (Cytb 6f ), which impacts linear and cyclic electron transport, but does not compromise the ability of the strain to perform state transitions. Furthermore,CPLD 49 strongly associates with thylakoid membranes where it may be part of a membrane protein complex with another GreenCut protein,CPLD 38; a mutant null forCPLD 38 also impacts Cytb 6f complex accumulation. We investigated several potential functions ofCPLD 49, with some suggested by protein homology. Our findings are congruent with the hypothesis thatCPLD 38 andCPLD 49 are part of a novel thylakoid membrane complex that primarily modulates accumulation, but also impacts the activity of the Cytb 6f complex. Based on motifs ofCPLD 49 and the activities of otherCPLD 49‐like proteins, we suggest a role for this putative dehydrogenase in the synthesis of a lipophilic thylakoid membrane molecule or cofactor that influences the assembly and activity of Cytb 6f . -
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Summary In response to high CO2environmental variability, green algae, such as
Chlamydomonas reinhardtii , have evolved multiple physiological states dictated by external CO2concentration. Genetic and physiological studies demonstrated that at least three CO2physiological states, a high CO2(0.5–5% CO2), a low CO2(0.03–0.4% CO2) and a very low CO2(< 0.02% CO2) state, exist inChlamydomonas . To acclimate in the low and very low CO2states,Chlamydomonas induces a sophisticated strategy known as a CO2‐concentrating mechanism (CCM) that enables proliferation and survival in these unfavorable CO2environments. Active uptake of Cifrom the environment is a fundamental aspect in theChlamydomonas CCM, and consists of CO2and HCO3–uptake systems that play distinct roles in low and very low CO2acclimation states. LCI1, a putative plasma membrane Citransporter, has been linked through conditional overexpression to active Ciuptake. However, both the role of LCI1 in various CO2acclimation states and the species of Ci, HCO3–or CO2, that LCI1 transports remain obscure. Here we report the impact of anLCI1 loss‐of‐function mutant on growth and photosynthesis in different genetic backgrounds at multiple pH values. These studies show that LCI1 appears to be associated with active CO2uptake in low CO2, especially above air‐level CO2, and that any LCI1 role in very low CO2is minimal.
