Cytochrome c oxidase, also called Complex IV, is the final enzyme in the respiratory chain the mitochondria use to make ATP, the energy currency of the cell. It holds both heme and copper centers, and it absorbs light reaching into the near-infrared range (de Freitas & Hamblin, 2016).
That makes it one of the most important light-absorbing molecules, or chromophores, in photobiomodulation research. The leading hypothesis is that absorbed light frees the enzyme from nitric oxide, which had been slowing it down, so that electron transport, mitochondrial membrane potential, and ATP production can rise (de Freitas & Hamblin, 2016).
Why it matters
Only absorbed light can change anything, a principle known as the Grotthuss-Draper law. Cytochrome c oxidase gives that principle an address. It is where the Light signal is thought to begin.
Where the evidence stands
The enzyme and its absorption of light are well described. What happens next is still labeled a hypothesis by the researchers who study it, and a second hypothesis involving light-sensitive ion channels is also in play (de Freitas & Hamblin, 2016).
References
de Freitas, L. F., & Hamblin, M. R. (2016). Proposed mechanisms of photobiomodulation or low-level light therapy. IEEE Journal of Selected Topics in Quantum Electronics, 22(3), 348–364. https://doi.org/10.1109/JSTQE.2016.2561201