Both Forward and Reverse TCA Cycles Operate in Green Sulfur Bacteria

Kuo-Hsiang Tang,Robert E Blankenship

doi:10.1074/jbc.m110.157834

Kuo-Hsiang Tang, Robert E Blankenship

Open Access

https://doi.org/10.1074/jbc.m110.157834

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Abstract

The anoxygenic green sulfur bacteria (GSBs) assimilate CO(2) autotrophically through the reductive (reverse) tricarboxylic acid (RTCA) cycle. Some organic carbon sources, such as acetate and pyruvate, can be assimilated during the phototrophic growth of the GSBs, in the presence of CO(2) or HCO(3)(-). It has not been established why the inorganic carbonis required for incorporating organic carbon for growth and how the organic carbons are assimilated. In this report, we probed carbon flux during autotrophic and mixotrophic growth of the GSB Chlorobaculum tepidum. Our data indicate the following: (a) the RTCA cycle is active during autotrophic and mixotrophic growth; (b) the flux from pyruvate to acetyl-CoA is very low and acetyl-CoA is synthesized through the RTCA cycle and acetate assimilation; (c) pyruvate is largely assimilated through the RTCA cycle; and (d) acetate can be assimilated via both of the RTCA as well as the oxidative (forward) TCA (OTCA) cycle. The OTCA cycle revealed herein may explain better cell growth during mixotrophic growth with acetate, as energy is generated through the OTCA cycle. Furthermore, the genes specific for the OTCA cycle are either absent or down-regulated during phototrophic growth, implying that the OTCA cycle is not complete, and CO(2) is required for the RTCA cycle to produce metabolites in the TCA cycle. Moreover, CO(2) is essential for assimilating acetate and pyruvate through the CO(2)-anaplerotic pathway and pyruvate synthesis from acetyl-CoA.

Highlights

Introduction(Fd) oxidoreductase (acetyl-CoA ϩ CO2 ϩ 2Fdred ϩ 2Hϩ o pyruvate ϩ CoA ϩ 2Fdox), ATP citrate lyase (ACL, acetyl-CoA ϩ oxaloacetate ϩ ADP ϩ Pi o citrate ϩ CoA ϩ ATP), ␣-ketoglutarate:ferredoxin oxidoreductase (succinyl-CoA ϩ CO2 ϩ 2Fdred ϩ 2Hϩ o ␣-ketoglutartate ϩ CoA ϩ 2Fdox) and fumarare reductase (succinate ϩ acceptor o fumarate ϩ reduced acceptor)
(Fd) oxidoreductase, ATP citrate lyase (ACL, acetyl-CoA ϩ oxaloacetate ϩ ADP ϩ Pi o citrate ϩ CoA ϩ ATP), ␣-ketoglutarate:ferredoxin oxidoreductase and fumarare reductase
Pyruvate and Acetate Enhance Growth of C. tepidum—It has been well documented that the RTCA cycle is used for CO2 assimilation when C. tepidum grows photoautotrophically

Summary

Introduction

(Fd) oxidoreductase (acetyl-CoA ϩ CO2 ϩ 2Fdred ϩ 2Hϩ o pyruvate ϩ CoA ϩ 2Fdox), ATP citrate lyase (ACL, acetyl-CoA ϩ oxaloacetate ϩ ADP ϩ Pi o citrate ϩ CoA ϩ ATP), ␣-ketoglutarate:ferredoxin oxidoreductase (succinyl-CoA ϩ CO2 ϩ 2Fdred ϩ 2Hϩ o ␣-ketoglutartate ϩ CoA ϩ 2Fdox) and fumarare reductase (succinate ϩ acceptor o fumarate ϩ reduced acceptor). It is generally assumed that as (Ϫ)-erythro-2-FC is a strong inhibitor of aconitase [9, 10], the metabolic flow of the OTCA cycle is blocked and contributes to the toxicity of FAc. Sirevåg and Ormerod [11] reported decades ago that CO2 assimilation and carbon flux are affected upon addition of FAc during phototrophic growth of the GSB Chlorobium limicola. We address two essential questions: why organic carbon sources, in particular acetate, can enhance the growth of C. tepidum (and other GSBs) [5] and why CO2 (or HCO3Ϫ) is required for growth when pyruvate or acetate is assimilated. Throughout this report, an autotrophic culture supplied with acetate or pyruvate (acetate or pyruvate ϩ HCO3Ϫ) is defined as a “mixotrophic” culture, a classification used in a recent review by Kelly and Wood [13]

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Discussion

Conclusion