On the Presence and Role of a Molecule of Chlorophylla in the Cytochromeb6 f Complex

Abstract

Highly purified preparations of cytochrome b 6 f complex from the unicellar freshwater alga Chlamydomonas reinhardtii contain about 1 molecule of chlorophyll a/ cytochrome f. Several lines of evidence indicate that the chlorophyll is an authentic component of the complex rather than a contaminant. In particular, (i) the stoichiometry is constant; (ii) the chlorophyll is associated with the complex at a specific binding site, as evidenced by resonance Raman spectroscopy; (iii) it does not originate from free chlorophyll released from thylakoid membranes upon solubilization; and (iv) its rate of exchange with free, radioactive chlorophyll a is extremely slow (weeks

On the Spatial Organization of Hemes and Chlorophyll in Cytochrome b 6 f

“…This orientation, taking into account theoretical experimental errors (Ϯ2.5°, see "Experimental Procedures"), is very different from that found for the majority of Chla molecules in the thylakoid membrane (36). A well defined orientation is consistent with the existence of a specific binding site for Chla on the b 6 f complex, as previously concluded from exchange experiments (20) and vibrational spectroscopy measurements (20,22). We further determined that the Q X transition of Chla is oriented at 0 -30°with respect to the membrane plane, in agreement with previous work (56).…”

Stoichiometrically Bound β-Carotene in the CytochromebfComplex of Oxygenic Photosynthesis Protects against Oxygen Damage

“…Although this possibility has been proposed (33) in the context of "super-complexes" that could be formed with the PSI reaction center (34), (a) the fractional contribution of the b 6 f-bound pigment to the total antenna function of such a super-complex is small; (b) the lack of inhibition of electron transfer through the b 6 f complex caused by photobleaching of the chlorophyll in the C. reinhardtii b 6 f complex implies that the chlorophyll does not facilitate a ratelimiting step of electron transfer (11); and (c) a light energy transfer function of ␤-carotene to chlorophyll is either small, as shown in the present work (Fig. 1B, inset), or absent (11).…”

“…The bound chlorophyll and ␤-carotene may be "evolutionary relics" that resulted from the appearance of reaction centers prior to that of quinol oxidoreductases (11). However, there is no report of bound bacteriochlorophyll or carotenoid in the cyt bc 1 complex isolated from photosynthetic bacteria.…”

“…This band is absent in a solution mixture of chlorophyll a and ␤-carotene (not shown). Such a band was absent in the excitation spectrum for chlorophyll a fluorescence at low temperature (77 K) for the complex from the green alga C. reinhardtii (11). In the latter work, the issue of a carotenoid excitation band was not specifically addressed because the carotenoid content was found to be less than stoichiometric.…”

“…PCC 6803 (10). A specific binding site of a chlorophyll a in the complex from the green alga, Chlamydomonas reinhardtii, was implied by spectroscopic identification of a specific hydrogen-bonding mode of the chlorophyll and slow rates of exchange of the b 6 f-bound Chl 1 with 3 [H]Chl in detergent micelles (11). The conclusion that the chlorophyll a molecule is bound at a specific site in the b 6 f complex raises the questions of where the two chlorophyll molecules per dimer are located and the functional role of the pigment molecules in the complex.…”