In this article we will discuss about Photosynthetic Antenna Pigments. After reading this article you will learn about: 1. Composition of Photosynthetic Antenna Pigments 2. Role of the Antenna in Photosynthesis.
Composition of Photosynthetic Antenna Pigments:
All functional pigments in photosynthetic membranes are bound in a variety of pigment-protein complexes. In general, photosynthetic pigments and pigment-protein complexes serve two primary functions. They function either as antennae or as reaction center. The antennae of plants consist of large number of protein-bound pigment molecules which absorb photons and transfer their energy to the reaction center.
These light-harvesting complexes are formed by polypeptides, which bind chlorophyll a, chlorophyll b, carotene and xanthophyll’s.
When the pigments are very close to each other, it is possible that the quantum energy of an irradiated photon is transferred from one pigment to the next. One quantum of light energy is called a photon, whereas one quantum of excitation energy transferred from one molecule to the next is termed exciton.
Antennae of plants consist of an inner and outer part. The outer or the peripheral part which is formed by the light-harvesting complex (LHC) collects the light. The inner part of the antenna, which consists of the core complex, is generally an integral component of the reaction center, and transfers the excitons collected in the outer part of the antenna into the photosynthetic reaction centers.
The mechanism by which the excitation energy is conveyed from the light-harvesting chlorophyll to the reaction center is resonance transfer. This process is also known as Forster transfer, after the name of the scientist who first described the phenomenon.
In this process of resonance energy transfer in antenna complexes, photons are not simply emitted by one molecule and absorbed by another, on the contrary, it is a non-radioactive process by means of which the excitation energy is transferred from one molecule to another.
There is great diversity in the antenna pigments, particularly among the algae. In contrast, there is evidence that PSI and PSII reaction center complexes are highly conserved in nature, composition and function among all oxygenic photosynthetic organisms. In higher plants and algae, PSI and PSII have unique antenna systems that differ in both pigment and protein composition.
The antennae of both photosystems can be divided into two groups:
The core and the peripheral antenna complexes. In PSI, the core antenna consists of about 100-120 Chl a and 15 β-carotene molecules bound in the same complex that contains P700 reaction center.
The PSII core antenna is composed of two pigment-protein complexes, CP 43 and CP 47, which are separate from PSII reaction center complex. Each binds 20-25 Chl a and several β-carotene molecules. The pigment and protein components of the core antenna are uniform among the photosynthetic organisms.
There is diversity in the outer peripheral antenna complexes. There is evidence that the size and composition of peripheral antenna can be adjusted in response to environmental conditions. In higher plants and green algae, the outer peripheral antennae of PSI and PSII are composed of two classes of Chl a/b binding proteins, which are light harvesting complexes termed LHCI and LHCII respectively which collect the light.
The unique properties of the individual peripheral antenna proteins can lead to heterogenous function, especially in PSII. The antennae of PSII reaction center have been shown to contain four light harvesting complexes, viz., LHCII a – d. The principal component is, however, LHC IIb, which occurs in the membrane as a trimmer.
The monomer is represented by a polypeptide to which two molecules of lutein are bound. The main LHC II complexes fall into two groups – those that are tightly associated with PSII and that can reversibly dissociate from PSII upon phosphorylation.
In fact, the activity of LHC II can be regulated by phosphorylation by ATP through a protein kinase, and one threonine residue on the polypeptide is the site of phosphorylation. In addition, PSII contains three minor peripheral antenna complexes, CP29, CP26 and CP24 which provide connections between the main LHC antenna and PSII core.
Role of the Antenna in Photosynthesis:
A. Light-harvesting Function:
All photosynthetic pigments either belonging to the reaction center or the antenna are capable of direct absorption of sunlight. Under average daily light intensities, the rate of light absorption by a reaction center pigment alone is far below the capacity for photosynthetic electron transport and would not provide sufficient energy to drive the process.
Therefore, efficient photosynthesis is only possible when energy of photons of various wavelengths is captured over a certain surface comprising hundred of antenna pigments.
Since the accessory pigments such as Chl b and carotenoids belong to the antenna and absorb wavelengths of light which are only weakly absorbed by Chl a, the range of wavelengths over which the light absorption can occur becomes wider, thus improving the efficiency of light absorption.
B. Protection against Active Oxygen Species:
The excited singlet state of chl a which forms by light absorption is unstable and will decay back to the lowest energy (ground state) by means of three intrinsic processes.
These are intersystem crossing to the triplet state, radiation decay (fluorescence) and thermal emission (heat). The yield of triplet Chl in the antenna far exceeds that produced in the reaction center. The triplet state of Chl a can sensitize the formation of an excited singlet state of oxygen (1O2) via triplet energy transfer.
3Chl* + O2 → Chl + 1O2
The singlet oxygen produced is highly reactive and can oxidize many important biological molecules, particularly lipids. In all photosynthetic organisms, this problem is overcome by the presence, in the antenna complexes, of carotenoids which rapidly quench Chl triplet states. The quenching reaction involves the triplet energy transfer from Chl a to the carotenoid followed by thermal dissipation of the triplet energy on the carotenoid.
3Chl* + carotenoid → Chl + 3carotenoid* → Chl + carotenoid + heat
In addition, carotenoids can quench singlet oxygen directly through a similar triplet transfer reaction.
1O2 + carotenoid → O2 + 3carotenoid* → O2 + carotenoid + heat
Thus, the carotenoid in photosynthetic antennae play the essential role of photo-protection by preventing the formation and accumulation of singlet O2. This is the reason why plants without carotenoids cannot grow and survive in strong light conditions.
C. Regulation of Light Utilization:
At limiting light intensities, the rate of photosynthesis is linear with the incident light intensity. As light intensity increases, the rate of photosynthesis becomes saturated and ultimately becomes independent of light intensity. It is interesting to note that saturation of photosynthesis is thought to result from a limitation in the capacity of the dark reactions and not in photosynthetic electron transport.
On the other hand, the rate of photon absorption remains linear with increasing light intensity. As a result, plants growing under excess light intensities always absorb light energy in excess of the photosynthetic capacity to utilize the energy for CO2 fixation. Thus excess light absorption is a problem, which is frequently faced by the plants in the field.
When we look at the coupling between light and dark reactions of photosynthesis, it becomes apparent that a limitation of photosynthetic dark reactions is caused by slower regeneration of NADP+, ADP and inorganic phosphate which eventually leads to an inhibition of electron transport.
The subsequent limitation of PSII reaction centers results in increased fluorescence and triplet formation, leading to the production of singlet O2 with concomitant oxidative damage.
At the same time, limited availability of NADP+ allows molecular O2 to compete with ferredoxin as an acceptor from PSI, resulting in the production of superoxide and other free radicals which also cause oxidative damage. The antenna pigment-protein complexes play a significant role in regulating the utilization of absorbed light energy.
This is achieved by excited state energy transfer among the antenna pigments, while the damage caused by excess light absorption is minimized either by energy transfer to functional reaction centers (photochemical quenching) or by non-destructive dissipation of excess light energy as heat (non-photochemical quenching).
Thus, an optimum light-harvesting efficiency under low light conditions is determined by the structure and composition of the antenna.
A mechanism has been proposed to give an account of the components of the quenching reaction in the antenna and the site at which the quenching occurs.
A correlation has been observed between the capacity for quenching and the accumulation of the carotenoid zeaxanthin in thylakoid membrane (Demming-Adams, 1990). Zeaxanthin is formed in thylakoids by reversible de-epoxidation of violaxanthin (a diepoxide) via the monoepoxide antheraxanthin in a process called the “xanthophyll cycle”.
It has been further noticed that the de-epoxidation reaction forming zeaxanthin is favoured by low pH of the thylakoid lumen. The enzyme epoxidase converting zeaxanthin to violaxanthin has a pH optimum of 5.1 and its activity is favoured under excessive light.
D. Regulation of Energy Distribution from PSII to PSI:
The relative light harvesting capacity of the two photosystems must be controlled in order to achieve maximum rates of electron transport at a particular light intensity (Allen, 1992). This is another mechanism in antenna complex of regulating the utilization of light energy in photosynthesis.
When NADP+ becomes limiting due to high light or due to limitations in the dark reactions, the rate of PQH2 oxidation by PSI decreases because of slower electron transport through PSI.
Reduction of the PQ pool activates a protein kinase that phosphorylates a pool of LHC II component of antenna, and includes the lateral movement of the phospho-LHC II from PSII in the appressed regions to the stroma-exposed regions of the thylakoid membrane.
Phospho-LHC II is then able to interact with and deliver excitation energy to PSI at the expense of PSII. When the PQ pool becomes oxidized, the kinase is deactivated and the process is reversed by a phosphatase enzyme via de-phosphorylation of phospho-LHC II causing the resulting LHC II to migrate back into the appressed membrane regions of antenna.