Molecular topography of phytochrome as deduced from the tritium-exchange method

The hydrogen-tritium-exchange measurements on phytochrome have been performed to detect the conformational differences between the red-absorbing (Pr) and the far-red absorbing (Pfr) forms of phytochrome. The large and small Pfr molecules revealed more exchangeable protons that did the corresponding Pr molecules by 96 and 70 protons, respectively. These results suggest that the Pr leads to Pfr phototransformation is accompanied by an additional exposure of the peptide chains in the Pfr molecule. Of 1682 theoretically exchangeable hydrogens in undegraded phytochrome, only 442 (26%) and 346 (21%) protons were found to be exchangeable (excluding instantaneously exchangeable protons that cannot be determined by the present method). Thus, the phytochrome protein appears to be compact and highly folded. The kinetic analyses of the tritium exchange-out curves indicate that two kinetically different groups are responsible for the conformational differences between the Pr and Pfr forms of phytochrome. These components are due to (1) the exposure of hydrogen-bonded peptide segments (alpha helix and/or beta-pleated sheet) in the chromophore vicinity of Pfr and (2) the exposure of hydrogen-bonded peptide segments on the chromophore peptide domain as well as on the chromophore-free tryptic domain of undegraded phytochrome.