Preface Type I photosynthetic reaction centres Peter Heathcote * School of Biological Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK Photosynthesis is the source of energy for virtually all life in the biosphere, and the conversion of light energy into chemical energy is catalysed by mem- brane-bound pigment protein complexes called pho- tosynthetic reaction centres. There are two di¡erent but related types of photosynthetic reaction centre, distinguished by their ability to reduce either qui- nones (type II) or four iron/four sulphur clusters or FeS centres (type I) as terminal electron acceptors. The majority of photosynthetic bacteria possess only one type of photosynthetic reaction centre, whereas cyanobacteria, algae and plants use both types con- nected in series to oxidise water and produce reduc- ing power for photosynthesis. Photosystem I of oxy- genic photosynthesis is a type I reaction centre, and other type I reaction centres are found in the green sulphur photosynthetic bacteria and heliobacteria. The study of type I reaction centres has lagged behind the study of type II reaction centres, partly because type I reaction centres are larger and bind a considerable number of light harvesting antenna pig- ments. The type II reaction centre of purple non- sulphur photosynthetic bacteria was ¢rst isolated as a distinct entity in the late 1960s, and was the ¢rst integral membrane protein to have its structure solved by X-ray crystallography. As a result the re- lationship between the structure of this protein and electron transfer function has been studied in consid- erable detail in the last 17 years, and this protein has been used to test theories of electron transfer. Using photosystem I isolated from the thermo- philic cyanobacterium Synechococcus elongatus the groups of H.T. Witt and W. Saenger in Berlin have been working for the last 10 years to provide a de- tailed X-ray structure of photosystem I. This special issue on type I photosynthetic reaction centres has been timed to coincide with the publication of the structure of photosystem I at 2.5 resolution [1]. The illustration on the cover shows the trimer of photo- system I crystallised, and reveals the complexity of the system. It contains 12 di¡erent polypeptide sub- units, which bind 96 chlorophylls and 31 other small molecules (carotenoids, lipids and electron transfer cofactors). The publication of this structure repre- sents a mammoth achievement, and together with the developing structure of photosystem II from the same groups [2] will allow the ¢eld to develop a complete understanding of the structure and function of these solar energy converters. The issue starts with a review of the structure of photosystem I from three members of the Berlin groups, and then presents reviews from many of the leading research groups in this area. These re- views demonstrate that photosystem I is di¡erent in many fundamental respects from the type II reaction centre of purple photosynthetic bacteria, and the structure reveals many unique and intriguing struc- tural features and protein^cofactor interactions. The type I reaction centres found in the green sulphur photosynthetic bacteria and heliobacteria are also considered in two reviews, and have attracted con- siderable interest following the discovery that the core of these reaction centres consists of two identi- cal polypeptides (homodimeric), unlike the two re- lated but di¡erent polypeptides that form the cata- lytic core of all other heterodimeric photosynthetic 0005-2728 / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII:S0005-2728(01)00193-1 * Fax : +44-208-983-0973. E-mail address : p.heathcote@qmw.ac.uk (P. Heathcote). Biochimica et Biophysica Acta 1507 (2001) 1^2 www.bba-direct.com