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.

Metalloproteins in the Biology of Heterocysts

“…Heterocyst differentiation involves a wide range of metabolic and physiological changes to turn new differentiated cells into efficient N 2 -fixing factories with low O 2 levels [76,101]. These modifications include (i) the lack of PSII activity and, thus, the absence of O 2 production by photolysis of H 2 O to keep a low O 2 concentration [101,121,122,123], (ii) the absence of photosynthetic CO 2 fixation to avoid the use of energy and reducing equivalents in cellular processes other than N 2 fixation, (iii) the expression of an uptake hydrogenase to recover energy and reducing equivalents from H 2 produced as a byproduct during N 2 fixation, (iv) the expression of a suite of metalloenzymes that protect heterocysts against reactive O 2 species, (v) a high respiratory rate that provides energy for the N 2 fixation reaction and contributes to elimination of O 2 traces that could enter into the cells, and (vi) the synthesis of the nitrogenase complex and its auxiliary proteins [76,79,124,125].…”

“…In contrast, PSII is inactivated during heterocyst development (Table 3) [123]. By proteomic analysis of heterocysts isolated from Nostoc punctiforme ATCC 29133 or Anabaena sp.…”

“…Such proteases have been described to be involved in the degradation of, for example, the D1 protein of PSII [221] and, thus, are likely central metalloproteins for PSII inactivation and heterocyst development. Moreover, it has been suggested more recently that the PSII complex may remain intact in heterocysts and the inactivation process could involve the disruption of the supramolecular organization [123,222]. However, this mechanism of inactivation is not understood and remains largely unknown.…”

“…In the photosynthetic electron transport chain in heterocysts, electrons can follow three pathways [85,123]. Böhme and colleagues proposed a cyclic electron transport between PSI and cytochrome b 6 f via the enzyme FNR to generate ATP (Figure 3), which supplies energy to nitrogenase and other metabolic processes [85,224,225].…”

Cyanobacteria that Promote Plant Growth and Metabolites

“…Siyanobakteriler, bitki partnerinin büyümesi için azot sağlarlar. Siyanobakterilerin, atmosferik azotu nitrogenaz enzimi ile amonyum formuna dönüştürebildikleri (N2 + 8 H + + 8 e -+ 16 MgATP → 2 NH3 + H2 + 16 MgADP + 16 Pi) ve bu olayda ATP'nin kullanıldığı açıklanmıştır (Magnuson, 2019).…”