Localization of two phylloquinones, Q(K) and Q(K)', in an improved electron density map of photosystem I at 4-angstrom resolution

An improved electron density map of photosystem I from Synechococcus elonga tus calculated at 4-Angstrom resolution for the first time reveals a second phylloquinone molecule and thereby completes the set of cofactors constitu ting the electron transfer system of this iron-sulfur type photosynthetic r eaction center: six chlorophyll a, two phylloquinones, and three Fe4S4 clus ters. The location of the newly identified phylloquinone pair, the individu al plane orientations of these molecules, and the resulting distances to ot her cofactors of the electron transfer system are discussed and compared wi th those determined by magnetic resonance techniques.