The role of Ca2+ and protein scaffolding in the formation in nature's water oxidizing complex

Abstract

The photosystem II (PSII) complex catalyzing the H2O-oxidation reaction of photosynthesis is highly prone to photodamage. Nature has evolved synthesis and repair mechanisms that include the photooxidative self-assembly, termed photoactivation, of the Mn4CaO5 metal cluster responsible for H2O-oxidation. Assembly is a multi-step light-driven process that proceeds with low quantum yield, involves a critical molecular rearrangement between light-activated steps, and is prone to photoinactivation and mis-assembly. A sensitive polarographic technique was used to track the assembly process under flash illumination as a function of the constituent Mn2+ and Ca2+ ions in genetically engineered samples to elucidate the action of Ca2+ and peripheral proteins. We show that the protein scaffolding that organizes this process is modulated allosterically by the assembly protein Psb27, which together with Ca2+, stabilizes the intermediates of photoactivation, a feature especially evident at long intervals between photoactivating flashes. Besides stabilizing intermediates, the Ca2+ ion is also critical to prevent photoinactivation due to inappropriate binding of Mn2+. Overexpression of Psb27, deletion of extrinsic protein PsbO, and excess Ca2+ characteristically modify these processes and retard the dark rearrangement. The results suggest the involvement of three metal binding sites, two Mn and one Ca with occupation of the Ca site by Ca2+ critical for the suppression of inactivation and the long-observed competition between Mn2+ and Ca2+ occurring at the second Mn site necessary for trapping the first stable assembly intermediates. Significance StatementThe oxidation of water by the photosystem II is the foundation of bioproductivity on Earth and represents a blueprint for sustainable, carbon neutral technologies. Water oxidation is catalyzed by a metal cluster containing of 4 Mn and 1 Ca atoms linked via oxo bridges. The initial assembly is a complex sequential reaction harnessing the photochemical reaction center to photooxidatively incorporate Mn2+ and Ca2+ ions into the catalytic unit embedded in the protein matrix. This photoassembly is crucial for both de novo biosynthesis and as part of the self-healing mechanism to cope with incessant photodamage that photosynthetic organisms experience. The results have implications for the natural mechanism as well as the highly desirable biomimetic devices currently envisioned for solar energy production.