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S- and T-type phycobiliprotein (PBP) lyases This family contains the S- and T-type phycobiliprotein (PBP) lyases. PBP lyases are employed by cyanobacteria, red algae, cryptophytes and glaucophytes for light-harvesting. Pigmentation of light-harvesting phycobiliproteins of cyanobacteria and cryptophytes requires covalent attachment of open-chain tetrapyrrole chromophores, the phycobilins, to the apoproteins. PBP lyases mediate this covalent attachment of phycobilin chromophores to apo-PBPs. These lyases are distinguishable in the clades of E/F-, S/U-, and T-type lyases; T-type lyases (which include CpcT) are distantly related to S-type lyases (which include CpcS and CpcU). S- and T-type PBP lyases differ in mechanistic details; the conformation and protonation state in which the chromophore is presented account for their differences in stereochemistry of the chromophore selectivity as well as corresponding binding sites. On the other hand, both lyases carry out the main functions of assisting site selectivity in the apo-PBP, protecting the chromophore and ensuring the regio- and stereoselectivity of the addition. The S- and T-type PBP lyases adopt a beta-barrel structure with a modified lipocalin fold. |
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