Challenges in Enzyme Catalysis - Photosystem II and Orotidine Decarboxylase : A Density Functional Theory Treatment

• Main Author: Lundberg, Marcus
• Format: Doctoral Thesis
• Language: English
• Published: Stockholms universitet, Fysikum 2005
• Subjects: photosystem II; oxyl radical; manganese systems; orotidine decarboxylase; reaction mechanism; density functional theory; Quantum chemistry; Kvantkemi
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-486; http://nbn-resolving.de/urn:isbn:91-7155-057-7

Possibly the most fascinating biochemical mechanism remaining to be solved is the formation of oxygen from water in photosystem II. This is a critical part of the photosynthetic reaction that makes solar energy accessible to living organisms. The present thesis uses quantum chemistry, more specifically the density functional B3LYP, to investigate a mechanism where an oxyl radical bound to manganese is the active species in O-O bond formation. Benchmark calculations on manganese systems confirm that B3LYP can be expected to give accurate results. The effect of the self-interaction error is shown to be limited. Studies of synthetic manganese complexes support the idea of a radical mechanism. A manganese complex with an oxyl radical is active in oxygen formation while manganese-oxo complexes remain inactive. Formation of the O-O bond requires a spin transition but there should be no effect on the rate. Spin transitions are also required in many short-range electron-transfer reactions. Investigations of the superproficient enzyme orotidine decarboxylase support a mechanism that involves an invariant network of charged amino acids, acting together with at least two mobile water molecules.