Bound Acceptors of Photosystem I

Abstract

Photosystem I (PS I) appears to contain five bound electron acceptors (reviewed in (1)):

$$P700\mathop - \limits^l \mathop - \limits^i \mathop - \limits^g \mathop - \limits^h \mathop - \limits^t {A_0} - - - {A_1} - - - X - - - A;B - - - $$

soluble ferredoxin The earliest acceptor (A₀) is a chlorophyll a, the following (A₁) may be a quinone, probably phylloquinone (see below). x appears to be a FeS-centre with somewhat unusual EPR properties. It may be the reductant of the FeS-centres A and B, which - either in series or in parallel - reduce soluble ferredoxin. The protein subunit structure of PS I is fairly well known (2). There are, however, many open questions regarding the function of individual subunits. Here, we report attempts to identify positively the apoproteins of centre x and another FeS-centre, probably B. Localization and some properties of the quinone bound to PS I are described.