Heterocyst Thylakoid Bioenergetics

Abstract

Heterocysts are specialized cells that differentiate in the filaments of heterocystous cyanobacteria. Their role is to maintain a microoxic environment for the nitrogenase enzyme during diazotrophic growth. The lack of photosynthetic water oxidation in the heterocyst puts special constraints on the energetics for nitrogen fixation, and the electron transport pathways of heterocyst thylakoids are slightly different from those in vegetative cells. During recent years, there has been a growing interest in utilizing heterocysts as cell factories for the production of fuels and other chemical commodities. Optimization of these production systems requires some consideration of the bioenergetics behind nitrogen fixation. In this overview, we emphasize the role of photosynthetic electron transport in providing ATP and reductants to the nitrogenase enzyme, and provide some examples where heterocysts have been used as production facilities.

Keywords: biofuel; cyanobacteria; ferredoxin; photosynthesis nitrogen fixation; thylakoid.

Figure 1

Possible electron transfer pathways in the heterocyst, from carbohydrate breakdown to nitrogenase, involving the thylakoid membrane. Adapted from [7]. OPP = oxidative pentose phosphate pathway; FNR = ferredoxin/NADP+ reductase; Fd = ferredoxin; b₆/f = the cytochrome-b₆/f complex; PSI = Photosystem I; N₂ase = nitrogenase. The blue (light) Fd denotes Fd-I, and the red (dark) Fd is the heterocyst-specific FxdH. The light-colored FNR denotes the long form of FNR, the darker-colored denotes the truncated, or short, form of FNR.