Exploratory Examination of Photosystem I in Arabidopsis thaliana

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1 Exploratory Examination of Photosystem I in Arabidopsis thaliana A. Role of Photosystem I Photosystem I (PSI) captures the sunlight and transfers the energy through a pigment network to the center of the chlorophyll molecule. The quantum yield of PSI is approximately one in all its forms and is conserved in various organisms for 3.5 billion years. The tremendous chance of possible mutations and the process of evolution itself was not able to hinder the conservation in genetic makeup. The entire process of photocatalysis occurs as a transmembrane electron transfer. Photosystem I was picked as a case study due to its final oxidation that is needed to provide energy for the plant. In addition, the Photosystem I of A. thaliana (PDB: 2WSC) captured our interest. A stromal view of its PSI revealed two distinct but loose units or protein complexes which are separated by a cleft. Plastocyanin (PC) is known to be the universal electron donor in PSI and in plants especially PC is the only mobile electron donor. Additional emphasis was placed on the study of chlorophylls as new Gap chlorophylls were identified in the X-ray structure. Gap chlorophylls are critically important because they are the active molecules that mediate the excitation energy between the two active photocatalytic complexes in P700. 1,2 B. Major Structural Parameters of the Active Site The structure of the protein active site is of a hydrophobic nature as the majority of the P700 protein complex is found in the membrane portion of the chloroplast. This membrane region, the thylakoid membrane is hydrophobic in nature and therefore the P700 complex bound in the membrane region is hydrophobic as well. As evidenced in images A and B in Figure 1, the chlorophyll molecules sit tightly with high affinity in the active site of the protein. The active 1

2 site of the proteins contains both aromatic and increasingly hydrophobic amino acids like, valine, histidine, phenylalanine and tryptophan. 1 A B C Figure 1. Major Structural Parameters. (A) Chlorophyll in the active site of P700 chlorophyll A apoprotein A1. (B) Chlorophyll in the active site of P700 chlorophyll A apoprotein A2. (C) The molecular interactions between chlorophyll and the P700 apoprotein. Some major bond length distances have been identified in C. These include the hydrogen bonding (dashed lines) and distances from the photosystem I P700 chlorophyll A apoprotein A1 to the chlorophyll molecule (not shown) indicated the Van der Waals and weak ionic interactions. 2

3 C. Electron Flow in Photosystem I Photosystem I as shown in Scheme 1 operates in tandem with photosystem II to pass electrons to NADP +, which is eventually used for carbon fixation. It facilitates this process by using its vast pigment array, called the antenna, to create reducing electrons via light absorption. These Scheme 1. Electron Transport Chain Chemical Structure Chlorophyll Ferrodoxin Sulfur Iron Cluster Pheophytin Phylloquinone e 3 3

4 electrons eventually migrate from the antenna to the reaction center, a dimer of chlorophyll molecules called P700, which then initiates a charge transfer cascade ending in NADP + reduction. First the electrons transfer from the reaction center to a metal-less chlorophyll called pheophytin. Next, the electrons move to phylloquinone which transfers the electrons down a triplet of iron-sulfur clusters ending at ferrodoxin. Finally, ferrodoxin reduces NADP + to NADPH, storing the light energy as a chemical bond. 3 Scheme 2. Electron Transport Chain in PS1: From Chlorophyll (Chl) to P700 to Pheophytin (Pheo), to plastoquinone (Q k ), Iron-Sulfur clusters (F x,f a,f b ) and on to Ferrodoxin (blue) D. Molecular Structure of the Photoactive Site The special pair of chlorophylls within the photoactive site as seen in Figure 2, are within the apoprotein meaning that part of the protein outside and inside of the membrane is in presence of an aqueous environment, indicating that the residues are hydrophilic. Therefore, the remaining residues which lay in conjunction with the chloroplast membrane possess a hydrophobic nature. 4

5 There are two chlorophylls in the photoactive sites of P680 and P700 because they act as a heterojunction, thus the need for two chlorophylls. One is ineffective due to its inability to carry the electron transport chain, so the other is needed to function as a complete, sufficient photosystem. The common concept between these biological photosystems and chemical photocatalysts is that they, like photocatalysts, are heterogeneous systems as photosystems I and II work in conjunction to transport electrons. A B C Figure 2. The special pair chlorophylls, chlorophyll (1797) and chlorophyll (1722), from chain A of photosystem I P700 apoprotein are shown as a (A) stick image, (B) space-filling image, and (C) ball and stick image. 5

6 References (1) Amunts, A.; Toporik, H.; Borovikova, A.; Nelson, N. Structure Determination and Improved Model of Plant Photosystem I. J Biol. Chem. 2010, 13, (2) Nelson, N.; Yocum C. F. Structure and function of photosystems I and II. Annu. Rev. Plant Biol. 2006, 57, (3) Nelson, D.; Cox, M. Oxidative Phosphorylation. In Principles of Biochemistry, Sixth; Schultz, L.; Moran, S.; Tontonoz, M.; Michael, A.; McCaffrey, P.; O Neill, J.; Szczepanski, T., Eds.; W. H. Freeman and Company: New York, NY, 2013,