General View of Photosynthesis

Photosynthesis is a physiological process by which green plants, algae and some forms of bacteria utilize energy from sunlight to convert carbon dioxide and water into sugars. For the process of photosynthesis, plants require light energy from the sun, water and carbon dioxide (Johnson, 2016). This process takes place in two steps, one is light reaction and other is dark reaction. In light reaction the harvested light energy will be converted to chemical form i.e., formation of ATP and NADPH+H+ during photosynthetic electron transport. In dark reaction the produced formed energy rich compounds formed in light reaction are utilized to fix CO2 to sugars (Fig. 1).

6 CO2 + 6 H2O? C6H12O6 + 6O2

Organization of thylakoid membrane:

The total process of photosynthesis takes place in the thylakoid membranes (present in the thylakoids) in chloroplast. A series of reactions takes place in the thylakoid membrane. It consists of two main compartments, one is grana where light harvesting takes place and the other one is stroma lamellae, carbon fixation takes place (Dekker and Boekema, 2005). In the thylakoid membranes four membrane bound pigment protein complexes which are embedded to perform the primary process of photosynthesis. They are photosystem (PS) II, cytochrome (Cyt) b6f complex, PS I and ATP synthase complex (Fig. 2). PS I and ATP synthase excluded from the grana membranes and PS II abundantly present in the stacked parts of the thylakoid membranes

Fig.1: Over view of photosynthesis

Fig.2: Organization of pigment proteins in thylakoid membranes.

(Andersson and Andersson, 1980; Barber, 1982), and the Cyt b6f complex is found in both grana and stroma regions (Albertson, 1995). All these components work together to convert light energy into chemical energy.

Structural and functional aspects of photosystem II complex:

Photosystem II (PSII) is a membrane protein supercomplex that executes the initial reaction of photosynthesis in higher plants, algae, and cyanobacteria which can be divided into two building blocks around a P680 molecule, one is PS II core complex and other is light harvesting complex, LHC II (Fig. 3). It drives one of the most oxidizing reactions like the production of O2 which is essential for aerobic life on this planet. It acts as a water- plastoquinone (PQ) oxidoreductase (Shen, 2015; Yamamoto, 2001; Vinyard et al., 2013).

PS II core complex

PS II core complex is a dimer present in the stacked thylakoid membranes. This complex consists of several proteins like D1, D2, CP 47, CP 43, 33kDa extrinsic subunit (Table. 1). In which D1 and D2 proteins bind the primary chlorophyll (Chl) donor P680 as well as two peripheral Chl a and one ?–carotene along with all the cofactors mediating the electron flow, while two others CP 47 and CP 43 bind an array of Chl a and ?–carotene molecules in the form of inner LHCs (Zhou et al., 2009).

[image: ]

Fig. 3: Structure and subunit composition of photsystem II complex

Gene

Protein

Molecular weight (kDa)

Helices

Function

psb A

D1

32

5

Reaction center (RC)

psb B

CP 47

47-51

6

Chl a-binding RC antenna

psb C

CP 43

43-47

6

Chl a-binding RC antenna

psb D

D2

34

5

RC

psb E

Cyt b559?

9

1

Photoprotection

psb F

Cyt b559?

4

1

Photoprotection

psb H

PsbH

10

1

Light dependent phosphorylation

psb I

PsbI

4.8

1

Unknown

psb J

PsbJ

4

0

Stabilization of assembly

psb K

PsbK

3.5

1

Unknown

psb L

PsbL

5

0

Regulation of the P680+ reduction

psb N

PsbN

4.1

1

Unknown

psb O

OEC1

33

0

Regulate O2 evolution

psb P

OEC2

23

0

Regulate O2 evolution

psb Q

OEC3

17

0

Regulate O2 evolution

psb R

PsbR

10

0 or 1

Unknown

psb S

PsbS

22

4

Regulation of lateral location, light harvesting

psb T

PsbTc

3.8

1

Protection of growth

psb W

PsbW

6.1

1

Control of the assembly and accumulation

psb X

PsbX

4.1

1

Unknown

Table 1: Polypeptides of photosystem II complex

psb A -D1protein

D1 is a reaction center core protein which is highly preserved pigment-binding protein. (Ferreira et al., 2004). This protein is a chloroplast genome-encoded protein with molecular weight of 32kDa (Barber et al., 1997). It is an integral thylakoid membrane protein and consists of five transmembrane ?-helices (designated A to E) (Hankamer et al., 2001).

The D1 protein has two important features: It binds the majority of the cofactors involved in PS II mediated electron transport; Tyr 161(YZ), P680 probably via His 198, Pheophytin (Phe) probably via Tyr 126, Tyr 147, Ala 150 and Glu 130, QB via interactions with Tyr 254, Phe 255, Gly 256 and others, Mn cluster possibly via Asp170, Glu 189, Gln 165, Ala 344, His 109, His 332 and His 337 and non-haem iron, probably via His 215 and His 272 (Michel and Deisenhofer, 1988). This is very important protein in the thylakoid membrane which turns over more rapidly than any other protein (Mattoo et al., 1984). Because of this significant feature PS II is susceptible to photoinduced damage (Zavafer et al., 2015).

psb D -D2 protein

D2 protein contains molecular weight of 34kDa which is encoded by a single gene localized in chloroplast genome and an integral membrane protein connected with the thylakoid membrane (Michel and Deisenhofer, 1988). The binding of D1/D2 heterodimer forms of reaction centre II to P680 in addition to two peripheral Chl a and one ?-carotene along with all the cofactors and mediates the electron flow (Aro et al., 2005). The function and stability of PS II depends on the C-terminal domain of D2 and the down regulation of D2 causes the loss in ability to grow photoautotrophically and as well as the loss in the functional RC II in the thylakoid membranes (Caffarri et al.,2009).

psb B - CP 47

PS II core protein is highly preserved protein with molecular weight of 47kDa and have six transmembrane helices (I to V) with N- and C-terminal. This highly conserved PS II core protein consists of about 500 amino acids with a molecular mass of 47 kDa, predicted to have six transmembrane helices (I to V) with the N- and C-terminal covered at the stromal surface (Yeremenko, 2005). This CP 47 protein is an absolute requisite for photoautotrophic growth.

psb C -CP 43

This protein is homologous with CP 47 in having 6 transmembrane helices which contains about 470 amino acids and a molecular weight of 43kDa. It binds to Chl and carotenoids and has a large lumenal loop between helices V and VI (Yeremenko, 2005). The difference between CP 43 and 47 is that it can be easily removed from the isolated core to form a CP 47-RC complex and the irreversible phosphorylation of its N-terminal threonine in case of higher plants (Michel et al. 1988, Dekker et al., 1990).

Water oxidation complex (WOC)

For the function of O2 evolution, three extrinsic water soluble polypeptides i.e. 33, 23 and 17kDa (Hankamer et al., 1997) are essential that are closely associated with the Mn cluster of PS II (Shen, 2015). In all the subunits, 33 kDa subunit helps in the stabilization of Mn cluster and the optimization of the O2 evolution reaction. (Umena et al., 2011; Gururani et al., 2012). WOC undergoes different redox states during ‘S’ cycle from S0-S1, S2-S3 and S3-S4 (Luber, 2011; Wientjes et al., 2013). Two cysteine residues (Cys112 and 135) bind to form a disulfide bridge, which is essential for the function of 33kDa extrinsic polypeptide. The subunit 23 kDa allows PS II to evolve O2 effeciently under severely Cl- limiting conditions (Ifuku et al., 2016). Cl- and Mn+2 are required for optimal O2 evolution (Hankamer et al., 1997). Mn2+ also functions as the substrate binding site of O2 evolution center and water analogues bind to Mn2+ (Yi et al., 2005).

PS II light harvesting complexes

LHC II is dependable for thylakoid membrane adhesion and grana formation (van Amerongen and Croce, 2013). The excitation energy moves from the most blue shifted Chl to the most red shifted one, recommended that Chl a plays the role of the terminal fluorescence emitter within the LHC II monomer (Liu et al., 2004; Standfuss et al., 2005). Lutein neoxanthin molecules of LHC II are occupied in harvesting solar energy, transfer it to Chl a and Chl b respectively (Gradinaru et al., 2000).

The components of LHC II are six peripheral Chl binding apoproteins (Lhc b1-6) with molecular weights of 25, 27,28,28,27 and 23kDa (Zolla et al., 2003). The polypeptide moiety of energetic antenna complex of LHC II are the Lhc b1-3 apoproteins which accounts for about 60% of the total Chl content of thylakoid membranes and Lhc b 4-6 are the polypeptide moiety of minor peripheral LHC II namely CP 29, CP 26 and CP 24 (Lucinski and Jackowski, 2006). According to Hankamer et al. (1997) Lhc b 4-6 are involved in the excitation energy transfer from LHC II to the RC II passing through CP 43 and CP 47. In addition to photosynthetic pigments Lhc b1 binds different lipids, presumably phosphatidylglycerol and digalctosyl (Standfuss et al., 2005).

Cytochrome b6f complex (Q cycle)

In between PS II and PS I, an intersystem electron transport carrier is present called Cyt b6f complex which perform electron transfer in the cyclic process known as Q cycle. Cyt b6f complex helps in the transfer of protons across the membrane, the oxidation of the quinol and the reduction of the plastocyanin (PC) (Fig. 4). It plays a key role in the transfer of electron between two photosystems by considering as a PQ-PC oxidoreductases. PQ is an essential electron carrier between QB and Cyt b6f complex (Kurisu et al., 2003; Carmer et al., 2005).

[image: ]

Fig. 4: Structure and organization of Cytochrome b6f complex in thylakoid membranes.

Structure and functional aspects of photosystem I complex

PS I is a membrane bound protein complex present in the thylakoid membrane, which contains four multisubunit complexes (Brettel and Leibl, 2001; Amunts et al., 2010). It is involved

in the photosynthetic electron transfer from H2O to NADP+ and catalyses the light driven electron transport from PC/Cyt b6 on the luminal side of the membrane to ferridoxin (Fd)/ flavodoxin at the stromal side by a chain of electron carriers(Fig. 5).

PS I complex functions as PC- Fd oxidoreductase (Amunts et al., 2007). The primary electron donor present in PS I is P700+ which donates the electron to A0. The core complex of PS I is surrounded by the LHC I where the charge separation takes place between P700 and A0, from that the electrons then move across intermediate acceptors A1 and FX to final electron acceptors FA and FB (Kou?il et al., 2005b; Qin et al., 2015).

Polypeptides of photosystem I complex

PS I complex consists of 15-20 polypeptides, which are categorized into three groups. are associated with The P700 is associated with 68-70 kDa polypeptides and the Fx is associated with two core polypeptides (Kou?il et al., 2005b). 8-9 kDa polypeptides are associated with the iron sulfer centers FA and FB (Logoutte et al., 1984). In addition to RC, the PSI complex contain photosynthetic pigments and all electron carriers essential to carry out the electron transfer (Nechushtai et al., 1996) (Table 2).

Fig. 5: Structure and subunit composition of photosystem I complex.

Table 2: Components and polypeptides present in the photosystem I.

Acceptor/Donor

Chemical identity

Redox potential (mv)

Polypeptide size/location (kDa)

P700

Chl dimer

+ 490

60-70/CP I

A0

Chl monomer

-

60-70/CP I

A1

Vitamin K1

-

60-70/CP I

FX

(Fe- 4S) cluster

-705

60-70/CP I

FB

(Fe- 4S) cluster

-580

08-08 kDa

FA

(Fe- 4S) cluster

-530

08-09 kDa

P700 the primary electron donor of photosystem I

This is the primary electron donor of PS I complex and regarded as a dimer (Andersson and Vanngard, 1988) . Its midpoint potential value is + 130 mV. In oxidized state it forms an ESR signal at g = 2.0025, ?H = 7.29 gauss and exhibits signal I, typical of organic radical (Warden et al., 1974).

A0 and A1 the primary and intermediate electron acceptors

In PS I, A0 and A1 are the primary and intermediate electron components (Jordan et al., 2001; Kou?il et al., 2014). These electron acceptors function in a sequential order. A0 is a special form of Chl a monomer with absorption maximum at 670 nm attached to the RC I (Malkin, 1996). A1 has been identified as a phylloquinone from analytical studies (Thornber et al., 1997).

FX, FA and FB iron sulfur centers

As well as the above mentioned acceptors, the PS I electron acceptor complex contains a group of bound electron carriers namely FX, FA and FB (Fromme, 2005).

Diffusible electron carriers associated with PS I

The diffusible electron carriers are PC and Fd. PC acts as a diffusible shuttle towards Cyt b6f from P700+. Fd acts as a shuttle from iron sulfer centres towards membrane associated protein Fd-NADP oxidoreductase (Haehnel, 1984; Kou?il et al., 2014). FX contains 2Fe-2S whereas FA and FB contains 4Fe-4S (Lagoutte et al., 1984; Lakshmi et al., 1999).

Ferredoxin

The transfer of electrons between the RC I and NADP+ is mediated by extrinsic iron sulfur protein Fd and the flavin containing proteins ferrodoxin NADP oxidoreductase. This is the final electron acceptors from PS I. Under iron limiting conditions, a special type of flavin containing protein present is flavodoxin (Tollin and Edmonson, 1980). It serves as mobile electron carrier to shuttle the electrons from FA/FB to the site where FNR is bound to the membrane (Forti and Grubas, 1985). RC I and FNR are the two independent sites of Fd on the thylakoid membrane (Merati and Zanetti, 1987). FNR as its sole prosthetic group (Knaff, 1996). This protein is attached to the thylakoid membrane near PS I clusters and ATP synthase complex.

Plastocyanin

PC is a mobile copper protein which serves to mediate electron flow from Cyt b6f to P700+ (Haehnel, 1984), with a molecular mass of 10.5 kDa. The kinetics of the PC donation to P700+ depends on the net charge of membrane surface and the charge of mobile carrier (Sigfridsson, et al., 1997). The primary site of interaction of PC seems to be on a charged region of 17kDa polypeptide which is regulated by ionic strength and pH (Haehnel et al., 1980)

ATP synthase complex

The electrons from PS II to PS I transport and liberate the protons from stroma to the lumen of the thylakoids. The protons are also released into lumen due to oxidation of H2O by PS II (Daum et al., 2010). This electro chemical gradient is responsible for the synthesis of ATP from ADP and Pi (Fig. 6). ATP synthase complex consists of the proton core CF0, which is embedded in the thylakoid membrane ad CF1 the extrinsic enzyme that catalyzes ATP synthesis as well as hydrolysis. The molecular weight of CF1 is 320 kDa. It consists of five subunits of molecular weight 53.4 kDa, 51.6kDa, 36kDa, 21.1kDa and 14.7 kDa respectively (Boekema and Lucken, 1996).

This occurs in a symmetrical ring of six alternating ? and ? subunits with a hole in ? subunits. The CF0 is oligomeric in nature and comprises four different intrinsic protein subunits in both green algae and higher plants (Hahn et al., 2018). It is self-assembled in the membrane bilayer to form a proton conducting pore as well as the site to which CF0 binds (Richter and Mills, 1996).

Thylakoid membrane lipids

Thylakoid membranes are having higher concentrations of glyceroglycolipids and phospholipids (PL). The glycerolipids are monogalactosyldiacyl glycerol (MGDG), digalactodiacyl glycerol (DGDG) and sulphoquinovasyl diacyl glycerol (SQDG) and their proportion for 40-50 %, 20-30 % and 5-10 % respectively. Thylakoid membranes consist of 10-20 % phosphotidyl glycerol (PG) of the total lipids (Boudière et al., 2014). The physical properties and asymmetric distribution of these glycolipids and PL in the thylakoid membranes play a key role in the molecular organization and thylakoid photofunctions (Murphy, 1986).

Fig. 6: Structure of ATP synthase complex.

Photosynthetic pigments in plant system

Chlorophylls

Chl a and Chl b are the two types of chlorophyll that are found in plants and green algae. Both types are associated with integral membrane proteins in the thylakoid membrane of chloroplast. Both Chls absorb light strongly in the red and blue parts of the spectrum. There are a few different forms of Chls that occur naturally, which differ in some small changes in the ring structure, as well as in different side chains (Goedheer, 1968) (Table 3).

Carotenoids and Xanthophylls

Carotenoids and xanthophyll are also having absorption maxima at 480 nm of the visible spectrum like Chl. These pigments have to transfer excitation energy to Chl molecules before it can be used for photosynthesis (Young et al., 2017). Chemically carotenoids (?-carotene, lutein, neoxanthin and violoxanthin) are long poly isoprenoid molecules having conjugated double bonds and each end of the molecule contains an unsaturated cyclohexene ring. Xanthophylls are very similar to carotenoids in structure but contain oxygen atoms in their terminal but contain oxygen atoms in their terminal rings (Gruszecki et al., 2016).

Table 3. Characteristics of natural chlorophylls.

Chl a

Chl b

Chl c 1

Chl c 2

Chl d

Molecular formula

C55H72O5N4Mg

C55H70O6N4 Mg

C35H30O5N4Mg

C35H28O5N4Mg

C54H70 O6N4Mg

C3 group

-CH=CH2

-CH=CH2

-CH=CH2

-CH=CH2

-CHO

C7 group

-CH3

-CHO

-CH3

-CH3

-CH3

C8 group

-CH2CH3

-CH2CH3

-CH2CH3

-CH=CH2

-CH2CH3

C17 group

-CH2CH2COO-Phytyl

-CH2CH2COO-Phytyl

-CH=CHCOOH

-CH=CHCOOH

-CH2CH2COO-Phytyl

C17-C18 bond

Single

Single

Double

Double

Single

Occurrence

Universal

Mostly plants

Various algae

Various algae

cyanobacteria

Spectral characteristics

The spectral characteristics of thylakoid membrane denote the interaction of chromophore with its microenvironment occupied by proteins and lipids and /or H2O or other chromophores (Thornber, 1975; Ladygin, 2015).

Absorption spectral characteristics

In higher plants, at room temperature the absorption spectra of thylakoid membranes exhibit two peaks and two shoulders (Barber, 2014). The peaks at 680 nm, 440 nm derive from Chl a, while the shoulder at 650 nm from Chl b while the 480 nm(s) peak is due to the absorption of carotenoids (Table 4). Absorption of light by antenna pigments and the efficient transfer of absorbed excitation energy to photochemical reaction centers are one of the key processes in photosynthesis.

Chlorophyll Fluorescence

The excition of Chl takes place and loses some of the absorbed energy as fluorescence and thus fluorescence is a competitive process with photochemistry (Adams and Demmig-Adams, 2004). This Chl a fluorescence measurements have been extensively used as an intrinsic probe to study the photosynthetic reactions (Baker and Rosenqvist, 2004). After the absorption of light by antenna complex present in the PS, it will be trapped by the RC (P680 and P700) for PS II and PS I respectively. Once the charge separation occurs at the RC the following events proceed.

Table 4: Absorption and Chl fluorescence emission characteristics of oxygenic thylakoid membranes.

Absorption

Peak position

Possible region

Comments

680

PS II Chl a

Peak

650

Chl b

Hump

440

Soret band of Chl a

Hump

480

Carotenoids

Peak

Fluorescence emission at room temperature

Emission range

Possible region

Comments

683- 687 (F 685)

Chl a PS II core: CP43, CP 47

Major band

693- 698 (F 695)

Chl a PS II core: CP47

Minor shoulder: with high (QA-)

705-712 (F 710)

PS I core or antenna

Minor: F0/Fm spectrum

720- 760 (F 740)

PS II + PS I

Broad vibrational satellite bands

DPA ? DP+A ? DP+A- ? D+ PA- ? D+ P+A ? Fluorescence.

D is an electron donor to the RC, P and A are electron acceptors. In the first state of the above scheme, DPA, the RC is called an open center which is ready for photochemistry but not for fluorescence. The RC in all other states considered to be closed because of its inability to carryout charge separation. The P+ state is a quencher of fluorescence can be seen. When both P and A are reduced (D+PA-), the fluorescenec occurs. The fluorescene intensity is weak in PS I irrespective of opening or closing of the RC. The D+PA- state after re-excitation mostly emits light as a variable fluorescence (Fv) (Adams and Demmig-Adams, 2004). At room temperature the Chl a fluorescence emission spectrum of chloroplast is characterized by a major peak at 683 n and a shoulder ranging from 700 to 750 nm depending on the organism (Govindjee, 1995) (Table 4).

The emission of fluorescecne light in actively photosynthesizing organisms oscillates with time and is known as ‘Kautsky effect’ (Krause and Weiss, 1984). Most of the fluorescence emission at room temperature is mainly from PS II and LHC II and the contribution of PS I is not significant. The fluorescence emitted by PS II is associated with photochemical activities (Duysens and Sweers, 1963). When the photosynthetic electron transport chain between QA and QB is blocked by a herbicide such as DCMU, Chl a fluorescence rises fast to high maximal fluorescence (Fm).

Electron transport

The most widely accepted model for the interaction of PS is the Z-scheme of Hill and Bendall (1960) which represents the transfer of electrons in a series from PS II to PS I by an electron transport chain (Fig. 7). A great number of Chl a and b and carotenoid molecules associated with light harvesting antenna proteins capture the light energy that is used to drive the PS II reaction. The formation of radical pair, P680+ Pheo- (Shikanai and Yamamoto, 2017) takes place by the transfer of electron the special pair Chl (P680) to the primary acceptor (Phe). P680+ is the most oxidizing redox component of PS II and it accepts an electron from a specific amino acid residue (D1-Tyr161), and for that reason it is reduced to P680 (Lucinski and Jackowski, 2006). The Mn cluster passes through a series of oxidation states referred to as the S0-S4 cycle (Kok et al., 1970). Four turnover of primary charge separation are needed to build the oxidizing equivalents required for the conversion of two molecules of H2O to O2 (Renger, 1997).

The electrons, which are accepted by P680+, are passed down the electron transport chain. In this way, Phe accepts electrons from P680 and passes them on to a plastoquinone molecule (QA), tightly bind to the D2 protein (Lancaster et al., 1996). Then this electron from QA- passes on to a second plastoquinone molecule, associated with the QB site on the D1 protein (Mc Pherson et al., 1994). This electron transfer is promoted by the presence of a non-heme iron located between QA and QB (Diner et al., 1991). Each PQ associated with the QB site can accept two electrons derived from H2O and two protons from the stroma before being released into the lipid matrix in the form of reduced plastoquinone (PQH2). The PQH2 is oxidized by Cyt b6f

Fig. 7: Schematic representation of electron transport (Z scheme).

Component is involved in the generation of proton motive force across the thylakoid membrane (Ort and Yocum, 1996).

Introduction

The electrons from Cyt b6f complex are then transferred to PC. As the P700 is excited upon absorption of light, an electron is transferred to the primary acceptor; A0 and electrons reduce the oxidized P700 from PC. Electrons from A0 are lastly transferred to Fd, which reduces NADP+ via A1 (Phylloquinone), FX and FA/FB. Four photons are required to reduce one molecule of NADP+ by this model. The path of electron transfer through FA/FB remains an unsettled area in the electron transfer pathway in PS I (Jung et al., 1995; Mannan et al., 1996).

Partial photochemical reactions

In general partial photochemical reactions are measured by the help of artificial exogenously added electron donors and acceptors and by the use of electron transport inhibitors. Their sites of electron donation, reception and inhibition are shown in the (Fig. 8). These exogenous donors and acceptors are used to appraise the photochemical activities of thylakoid membrane as well as photochemical reactions catalyzed by PS II and PS I separately or together (Izawa, 1980; Kleczkowski, 1994). These partial photochemical reactions are valuable tools in estimating the photochemical potential of chloroplast.

Acceptors: a1: Silicomolybdic acid; a2: Phenylenediamine, p-Benzoquinone; 2,5-dimethyl p-benzoquinone, and 2,5-dichloro-p-benzoquinone; a3 Methyl viologen, Anthroquinone, Ferricyanide

Donors: d1: Catechol; Ascorbate; H2O, Diphenylcarbazide, NH2OH; d2: Duroquinol; d3: Diaminodurene; Dichlorophenol indophenol; Tetramethyl phenyl durene. All are reduced by ascorbate.

Inhibitors: in1: NH2OH; in2: diuron; in3: Dibromothymoquinone; in4: KCN and HgCl2; in5: DSPD

Fig. 8: Commonly used artificial electron acceptors, donors and inhibitors of electron transport chain.

Cite this page