On top of that, vibrational spectroscopies (resonance Raman and infrared absorption ) had a strong contribution to the current understanding of the phytochrome photoreactions due to their high intrinsic time resolution, sensitivity to subtle changes in molecular geometries, and applicability to solution samples at physiological temperatures ( 19– 23). Over the last decades, many analytic techniques such as X-ray crystallography ( 13, 14), NMR spectroscopy ( 15), cryo-electron microscopy ( 16), time-resolved X-ray solution scattering ( 12, 17), and molecular dynamics simulations ( 18) have been used to probe the molecular details of this complex reaction cascade. Due to its magnitude, it is often viewed as the critical step in intramolecular signal amplification ( 12). The light-induced transition of the tongue is a rather dramatic structural change as it comprises the repositioning of all involved amino acids and the breaking and formation of tens of hydrogen bonds. 1), resulting in functional tertiary structure rearrangements of the catalytic domain ( 8). Due to this large distance, the light regulation of enzymatic activity involves an intricate mechanism: an initial light-triggered Z/E isomerization of BV causes refolding of a nearby secondary structure element (the “tongue” region, Fig. They photoswitch between a red-absorbing (Pr) and a far-red absorbing (Pfr) state, thereby regulating the according C-terminal catalytic domain, which is typically located several nanometers away from the chromophore. Bphs act as light-driven enzymes in bacterial two-component signaling, e.g., light-activated histidine kinases ( 9), phosphatases ( 10), or cyclases ( 11).
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