Mechanism of Asparagine-Mediated Proton Transfer in Photosynthetic Reaction Centers

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.

Elementary Reactions in the Functional Triads of the Blue-Light Photoreceptor BLUF Domain

“…(A) The RNR (PDB 1MXR) starting moiety where PCET and proton rocking occurs. − (B) The PSII (PDB 3ARC) TyrZ is involved in the PCET and proton rocking with H190. , (C) The excited-state proton relay of GFP (PDB 1W7S). (D) The PbRCs (PDB 3I4D) proton relay pathway after the Q B is reduced by one electron. , The proton transfer processes are represented with blue-full arrows, and the electron transfer processes are represented with red-half arrows.…”

“…For example, the proton translocation in cytochrome c oxidase (C c O) transports proton across the cell membrane in a diffusion-controlled-like manner . The other type relays the proton through a compact H-bond network such as that in green fluorescence protein (GFP) , via an H-bond network composed of E222-S205-H 2 O-( p -hydroxybenzilideneimidazolidinone) (Figure C), in photosynthetic reaction centers (PbRCs) , where the proton relay is from a water molecule via a Asp a Ser to the secondary quinone Q B (Figure D), and in PSII , via a number of closely packed H 2 O molecules and amino acids in a H-bond network. As the BLUF domain contains such a compact H-bond system, it is an excellent scaffold to be engineered to include Trp, Tyr, His, Glu, Asp, Asn, Ser, and Thr or water on its proton translocation pathway.…”

Modulation of Electron Transfer Branches by Atrazine and Triazine Herbicides in Photosynthetic Reaction Centers

“…Despite Glu-L212 not participating in this H-bond network, it demonstrates proton uptake (0.3 to 0.8 H + in PbRC from R. sphaeroides − ) upon Q B •– formation. , Protonated Glu-L212 can serve as a proton donor for deprotonated His-L190 after the release of the proton from His-L190, leading to the formation of Q B H 2 . Glu-H177 is situated near Glu-L212.…”

“…In PbRC from B. viridis, an H-bond network involving Ser-L223 and Asn-L213 exists at the distal carbonyl O site of Q B (with respect to the nonheme Fe complex), forming the proton transfer pathway, [Q B ...Ser-L223···Asn-L213] (Figure b). Despite Glu-L212 not participating in this H-bond network, it demonstrates proton uptake (0.3 to 0.8 H + in PbRC from R.…”

Role of Protonation States in the Stability of Molecular Dynamics Simulations of High-Resolution Membrane Protein Structures

“…Besides titratable residues such as glutamic acid and lysine, which can donate or accept protons, other residues that are part of these networks include asparagine and glutamine, and serine, threonine, and tyrosine with hydroxyl groups in their side chains. These polar residues can not only assist in proton transfer reactions by stabilizing charged intermediates but may also undergo protonation/deprotonation reactions in proteinaceous environments. ,− Moreover, titratable acidic and basic residues are well known to act as proton transfer elements and proton loading sites in several enzymes. ,, However, the protonation states in protein structures are usually not explicitly modeled. Such information can be obtained with neutron diffraction techniques, but only for relatively small proteins …”