Phycobilisomes, the light-harvesting complexes of cyanobacteria and red algae, contain two to four types of chromophores that are attached covalently to seven or more members of a family of homologous proteins, each carrying one to four binding sites. Chromophore binding to apoproteins is catalyzed by lyases, of which only few have been characterized in detail. The situation is complicated by nonenzymatic background binding to some apoproteins. Using a modular multiplasmidic expression-reconstitution assay in Escherichia coli with low background binding, phycobilin:cystein-84 biliprotein lyase (CpeS1) from Anabaena PCC7120, has been characterized as a nearly universal lyase for the cysteine-84-binding site that is conserved in all biliproteins. It catalyzes covalent attachment of phycocyanobilin to all allophycocyanin subunits and to cysteine-84 in the β-subunits of C-phycocyanin and phycoerythrocyanin. Together with the known lyases, it can thereby account for chromophore binding to all binding sites of the phycobiliproteins of Anabaena PCC7120. Moreover, it catalyzes the attachment of phycoerythrobilin to cysteine-84 of both subunits of C-phycoerythrin. The only exceptions not served by CpeS1 among the cysteine-84 sites are the α-subunits from phycocyanin and phycoerythrocyanin, which, by sequence analyses, have been defined as members of a subclass that is served by the more specialized E/F type lyases.

The photostabilities of bacteriochlorophyll a and several of its derivatives, which are of interest as potential sensitizers in photodynamic tumor therapy, were investigated. The pigments were irradiated with light >630 nm in organic solvents (acetone, tetrahydrofuran, pyridine, methanol, ethanol, n‐propanol, 2‐propanol and toluene) and in aqueous detergent solutions (cetyl‐trimethyl‐ammonium bromide [CTAB], lauryldimethyl‐aminoxide [LDAO] or sodium dodecyl‐sulfate [SDS] and Triton X‐100 [TX100]). Their stabilities in these different solvents were determined in the presence and absence of an external sensitizer (pyromethyl‐pheophorbide a), oxygen, sodium ascorbate and inert gas (Ar) or vacuum. The photodegradation products of bacteriochlorophyll a in acetone solution were isolated, purified by HPLC and analyzed by their absorption spectra and mass spectroscopy. Besides the well‐known dehydrogenation products, such as [3‐acetyl]‐chlorophyll a, which were obtained as by‐products, the major products had low absorption in the visible‐near infrared spectral range. The spectral signature of the major component of these products was characteristic of linear open‐chain tetrapyrroles, but they lacked the characteristic protonation‐deprotonation behavior and reactivity of bilins with Zn⁺⁺.

Covalent attachment of phycocyanobilin (PCB) to the α‐subunit of C‐phycocyanin, CpcA, is catalysed by the heterodimeric PCB : CpcA lyase, CpcE/F [Fairchild CD, Zhao J, Zhou J, Colson SE, Bryant DA & Glazer AN (1992) Proc Natl Acad Sci USA89, 7017–7021]. CpcE and CpcF of the cyanobacterium, Mastigocladus laminosus PCC 7603, form a 1 : 1 complex. Lyase‐mutants were constructed to probe functional domains. When in CpcE (276 residues) the N terminus was truncated beyond the R33YYAAWWL motif, or the C terminus beyond amino acid 237, the enzyme became inactive. Activity decreases to 20% when C‐terminal truncations went beyond L275, which is a key residue: the K_m of CpcE(L275D) and (L276D) increased by 61% and 700%, k_cat/K_m decreased 3‐ and 83‐fold, respectively. The enzyme also lost activity when in CpcF (213 residues) the 20 N‐terminal amino acids were truncated; truncation of 53 C‐terminal amino acids inhibited complex formation with CpcE, possibly due to misfolding. According to chemical modifications, one accessible arginine and one accessible tryptophan are essential for CpcE activity, and one carboxylate for CpcF. Both subunits bind PCB, as assayed by Ni²⁺ affinity chromatography, SDS/PAGE and Zn²⁺‐induced fluorescence. The bound PCB could be transferred to CpcA to yield α‐CPC. The PCB transfer capacity correlates with the activity of the lyase, indicating that PCB bound to CpcE/F is an intermediate of the enzymatic reaction. A catalytic mechanism is proposed, in which a CpcE/F complex binds PCB and adjusts via a salt bridge the conformation of PCB, which is then transferred to CpcA.

Chlorophylls provide the basis for photosynthesis and thereby most life on Earth. Besides their involvement in primary charge separation in the reaction center, they serve as light-harvesting and light-sensing pigments, they also have additional functions, e.g., in inter-system electron transfer. Chlorophylls also have a wealth of applications in basic science, medicine, as colorants and, possibly, in optoelectronics. Considering that there has been more than 200 years of chlorophyll research, one would think that all has been said on these pigments. However, the opposite is true: ongoing research evidenced in this Special Issue brings together current work on chlorophylls and on their carotenoid counterparts. These introductory notes give a very brief and in part personal account of the history of chlorophyll research and applications, before concluding with a snapshot of this year’s publications.