Speaker: Ms. Nikita Rao

Research Supervisor: Prof. Sheetal Kumar Jain

Topic: Structural basis of the Q-cycle in cytochrome b6f complex 

Date & Time: Thursday, 20th October 2022 at 4.00 pm

Venue: SSCU Auditorium (Old SSCU Building)



Photosynthesis is one of the most important processes in nature, which provides the assimilation of carbon dioxide and produces molecular oxygen using light energy. In an oxygenic photosynthetic system, light quanta are absorbed by the pigment-protein complexes of photosystem I (PSI) and photosystem II (PSII), followed by an electron transfer from PSII to PSI.

The cytochrome b6f (cytb6f) complex plays a critical role in the electron transfer process between PSII and PSI.1,2 The electron transfer within cytb6f occurs via the quinol (Q) cycle, which catalyzes the oxidation of plastoquinol (PQH2) and the reduction of both plastocyanin (PC) and plastoquinone (PQ) at two separate sites via electron bifurcation.3 A low potential pathway donates the electron to a PQ molecule bound at a site near the stromal side of the membrane, while a high potential pathway donates the electron to PC, which delivers the electron to PS I. However, a complete understanding of the Q-cycle mechanism is limited by a lack of knowledge of the binding of PQ/PQH2 molecules within the complex.4

In this talk, I will briefly discuss the processes taking place in photosynthesis, specifically focusing on the structure and role of cytochrome b6f complex in electron transfer via the Q-cycle mechanism. Genetic manipulation of photosynthetic regulation is now recognized as being key to increasing crop yields.5 The knowledge about the cytochrome b6f complex and other protein complexes can be implemented for the genetic manipulation of photosynthetic regulation and thus increase the photosynthetic efficiency.4



  1. Hill, R.; Bendall, F. Function of the Two Cytochrome Components in Chloroplasts: A Working Hypothesis. Nature1960, 186 (4719), 136–137., A. N. The Cytochrome B6f Complex at the Crossroad of Photosynthetic Electron Transport Pathways. Plant Physiology and Biochemistry2014, 81, 163–183.
  2. Mitchell, P. Possible Molecular Mechanisms of the Protonmotive Function of Cytochrome Systems. J TheorBiol1976, 62 (2), 327–367.

3. Malone, L. A.; Qian, P.; Mayneord, G. E.; Hitchcock, A.; Farmer, D. A.; Thompson, R. F.; Swainsbury, D. J. K.; Ranson, N. A.; Hunter, C. N.; Johnson, M. P. Cryo-EM Structure of the Spinach Cytochrome b 6 f Complex at 3.6 Å Resolution. Nature2019, 575 (7783), 535–539.

4. Zhu, X. G.; Long, S. P.; Ort, D. R. Improving Photosynthetic Efficiency for Greater Yield. Annu Rev Plant Biol2010, 61, 235–261.