Sugo Y, Ishikita H. Mechanism of Asparagine-Mediated Proton Transfer in Photosynthetic Reaction Centers.
Biochemistry 2023;
62:1544-1552. [PMID:
37083399 PMCID:
PMC10194076 DOI:
10.1021/acs.biochem.3c00013]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2023] [Revised: 03/29/2023] [Indexed: 04/22/2023]
Abstract
In photosynthetic reaction centers from purple bacteria (PbRCs), light-induced charge separation leads to the reduction of the terminal electron acceptor quinone, QB. The reduction of QB to QB•- is followed by protonation via Asp-L213 and Ser-L223 in PbRC from Rhodobacter sphaeroides. However, Asp-L213 is replaced with nontitratable Asn-L222 and Asn-L213 in PbRCs from Thermochromatium tepidum and Blastochloris viridis, respectively. Here, we investigated the energetics of proton transfer along the asparagine-involved H-bond network using a quantum mechanical/molecular mechanical approach. The potential energy profile for the H-bond between H3O+ and the carbonyl O site of Asn-L222 shows that the proton is predominantly localized at the Asn-L222 moiety in the T. tepidum PbRC protein environment, easily forming the enol species. The release of the proton from the amide -NH2 site toward Ser-L232 via tautomerization suffers from the energy barrier. Upon reorientation of Asn-L222, the enol -OH site forms a short low-barrier H-bond with Ser-L232, facilitating protonation of QB•- in a Grotthuss-like mechanism. This is a basis of how asparagine or glutamine side chains function as acceptors/donors in proton transfer pathways.
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