A chromophore that is a linear tetrapyrrolic prosthetic group covalently attached to a large soluble protein phytochrome. Light absorption by the phytochrome chromophore triggers photoconversion between two spectrally distinct forms of the photoreceptor: Pr, the red light absorbing form, and Pfr, the far-red light absorbing form.
An organic molecule or ion (usually a metal ion) that is required by an enzyme for its activity. It may be attached either loosely (coenzyme) or tightly (prosthetic group).