Iron protein structure-function correlation

STRUCTURE-FUNCTION CORRELATIONS IN HIGH-POTENTIAL IRON PROTEINS... 

As is apparent in Fig. 3, considerable similarity exists in the arrangement of the electron transfer cofactors in PS I and PS n. The main differences between the two systems are as follows 1) PS I has three Pe4S4 iron-sulfur clusters. Ex, Ea, and Eb, located on the stromal side of the complex 2) In PS I the primary acceptor is a chlorophyll, not pheophytin and 3) the distance between the primary acceptor (Aqa3 ) and phylloquinone (Aia,b) in PS I is significantly shorter than the corresponding distance between PheoA,B and Qa.b in PS II and Type II reaction centers. These structural differences correlate with functional differences between the two types of reaction centers. In PS II, the mobile electron carrier on the stromal side of the complex is Qb, which is a lipid-soluble, two-electron acceptor. In contrast, the mobile electron carrier in PS I is ferredoxin, which is a water-soluble, one-electron acceptor. The three iron-sulfur clusters in PS I provide a chaimel by which electrons are funneled out of the reaction center to ferredoxin. On the donor side of the complex, plastocyanin, the reductant that replenishes electrons removed from P700, is also a water-soluble protein and is a one-electron donor. Thus, each photon absorbed by the PS I complex leads to the transfer of one electron from plastocyanin to ferredoxin. In Fig. 2, it is apparent that the midpoint potentials of the acceptors in PS I are about 500 to 700 mV more negative than those in PS II, and the... 

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