5.B.9 The Porin-Cytochrome c (Cyc2) Family Bacteria transport electrons across the cell envelope to external electron donors/acceptors. In Gram-negative bacteria, electron transfer across the outer membrane involves the use of an outer membrane β-barrel/cytochrome. These can be in the form of a porin-cytochrome protein, such as Cyc2 of the acidophilic bacterium, Acidithiobacillus ferrooxidans (White et al. 2016). CycC has 570 aas, an N-terminal α-TMS (leader sequence), and 18 predicted transmembrane β-strands. The iron respiratory chain of Acidithiobacillus ferrooxidans involves various metalloenzymes. Castelle et al. 2008 demonstrated that the oxygen reduction pathway from ferrous iron (the downhill pathway) is organized as a supercomplex constituted of proteins located in the outer and inner membranes as well as in the periplasm. The outer membrane-bound cytochrome c, Cyc2, was purified and proved to be responsible for iron oxidation; its redox potential was the highest measured to date for a cytochrome c. The organization of metalloproteins inside the supramolecular structure was specified by protein-protein interactions. The isolated complex spanning the two membranes had iron oxidase as well as oxygen reductase activities, indicating functional electron transfer between the first iron electron acceptor, Cyc2, and the Cu(A) center of cytochrome c oxidase aa₃. O₂ reduction from ferrous iron is coupled to the energy-consuming reduction of NAD⁺(P) from ferrous iron (uphill pathway) required for CO₂ fixation and other anabolic processes. Besides the proteins involved in O₂ reduction, there were additional proteins in the supercomplex involved in the uphill pathway (bc complex and cytochrome Cyc(42)), suggesting a possible physical link between these two pathways (Castelle et al. 2008; Liu et al. 2013; Kucera et al. 2016). The generalized reaction catalyzed by Cyc2 is: electron from Fe²⁺ (out) → Fe³⁺ (out) + electron (in a periplasmic protein)
References:
Castelle, C., M. Guiral, G. Malarte, F. Ledgham, G. Leroy, M. Brugna, and M.T. Giudici-Orticoni. (2008). A new iron-oxidizing/O2-reducing supercomplex spanning both inner and outer membranes, isolated from the extreme acidophile Acidithiobacillus ferrooxidans. J. Biol. Chem. 283: 25803-25811.
Jiang, V., S.D. Khare, and S. Banta. (2021). Computational Structure Prediction Provides a Plausible Mechanism for Electron Transfer by the Outer Membrane Protein Cyc2 from Acidithiobacillus ferrooxidans. Protein. Sci. [Epub: Ahead of Print]
Kucera, J., E. Pakostova, J. Lochman, O. Janiczek, and M. Mandl. (2016). Are there multiple mechanisms of anaerobic sulfur oxidation with ferric iron in Acidithiobacillus ferrooxidans? Res. Microbiol. 167: 357-366.
Liu, W., J. Lin, X. Pang, S. Mi, S. Cui, and J. Lin. (2013). Increases of ferrous iron oxidation activity and arsenic stressed cell growth by overexpression of Cyc2 in Acidithiobacillus ferrooxidans ATCC19859. Biotechnol Appl Biochem 60: 623-628.
White, G.F., M.J. Edwards, L. Gomez-Perez, D.J. Richardson, J.N. Butt, and T.A. Clarke. (2016). Mechanisms of Bacterial Extracellular Electron Exchange. Adv Microb Physiol 68: 87-138.
Examples:
TC#	Name	Organismal Type	Example
5.B.9.1.1	Outer transmembrane porin/cytochrome c protein of 488 aas with an N-terminal transmembrane α-helical signal peptide and 18 putative β-strands (White et al. 2016). The structure consists of three main parts: extracellular loops exposed to low-pH conditions, a transmembrane beta barrel, TMBB, and an N-terminal cytochrome-like region within the periplasmic space. The Cyc2 model was further refined by identifying likely iron and heme docking sites (Jiang et al. 2021).		Cyc2 of Acidithiobacillus ferrooxidans

5.B.9.1.2	Cytochrome₅₇₂ of 570 aas with 1 N-terminal α-TMSs and up to 18 transmembrane β-strands.		*Cyt₅₇₂ of Leptospirillum* sp. Group II 'C75'**

5.B.9.1.3	Putative cytochrome c1 protein of 483 aas with 1 N-terminal TMS.		Cyt c1 of Mariprofundus ferrooxydans

5.B.9.1.4	Uncharacterized protein of 445 aas and 1 N-terminal TMS		UP of Sideroxydans lithotrophicus

5.B.9.1.5	Cytochrome c, one heme binding site; 443 aas and 1 N-terminal TMS followed by 16 predicted transmembrane β-strands.		Cyt c of Geobacter bemidjiensis

5.B.9.1.6	Uncharacterized protein of 481 aas, an 1 N-terminal α-TMS, and 20 predicted transmembrane β-strands.		UP of Nitrospira defluvii

5.B.9.1.7	Uncharacterized protein of 423 aas with 1 N-terminal α-TMS and 18 putative transmembrane β-strands.		UP of Persephonella marina

5.B.9.1.8	Uncharacterized protein of 410 aas with 1 N-terminal TMS and 18 putative transmembrane β-strands.		UP of Chlorobium ferrooxidans