Lecture 20

Introduction to photosynthesis

Introduction to photosynthesis


Carbon cycle and Global Change

Light Harvesting

Overview of light harvesting

At ambient intensities, an antenna consisting of 1 chlorophyll molecule would intercept a photon once every few seconds. The decay rates of some of the energy storage mechanisms (charge separation in the photochemical reactions, the proton gradient) have half-times in this range, so photosynthesis with an antenna this size would be inefficient. In order to achieve an efficient use of light energy, the photosynthetic organisms have evolved light harvesting antenna, which allow many pigments to cooperate in the collection of light for a single reaction center. In many photosynthetic systems, the size of the antenna can be adjusted to suit the intensity of light. An effective antenna is particularly important for growth at low light intensity.

Click here for a brief review of antenna function by Bob Blankenship.

Transfer of excitation from the antenna to the reaction center occurs with high efficiency. Evolution has designed structures which meet the requirements of physics and chemistry, as determined by the absorption in the atmosphere, the potential for photochemical damage, and the requirements for efficient transfer as described by Forster theory for energy exchange between neighboring pigments (see below). The requirements are:

Fluorescence and light harvesting

Fluorescence from photosynthetic systems varies with the physiological state, and measurement of fluorescence is much used as a tool for exploring mechanism. General principles are covered here, and use of fluorescence to explore mechanism in photosynthesis in Lecture 21.

The Forster mechanism of excitation transfer

A brief description of the main parameters of Forster mechanism for inductive resonance transfer is given here.

Bacterial light harvesting

The structure of the light harvesting complex LH2 from a photosynthetic bacterium has recently been solved by X-ray crystallography by RRichard Cogdell and colleagues. Click here for a nice introduction to the structure of the bacterial light harvesting complex from Rhodopseudomonas acidophila, pictures from the Nature article, and a Chime tutorial on the structure from Richard Cogdell's group.

Click here for a review of bacterial light harvesting complexes from the Schulten lab, which gives a nice overview of their recent structural and theoretical work.

Click here for an overview of work on the light harvesting complex from Rs. molischianum, from Schulten's group.

The Rhodopseudomonas acidophila complex is typical of the light harvesting complexes from the purple bacteria, and contains two small protein subunits, a and b, three bacteriochlorophylls (BChl), and two carotenoids. The spectrum of the complex shows two BChl peaks in the near IR, at 800nm and at 850 nm. These correspond to 1 B800 BChl, and two B850 BChls.

Stereo-pair view of a single a, b-subunit pair from the light harvesting complex from Rhodopseudomonas acidophila. The BChl of B800 is shown as dark green, the B850 pair is shown as cyan and purple, carotenoids are orange and yellow, water oxygens are red; a-subunit is blue, b-subunit is pale green. For a Chime tutorial on the structure, click here. Coordinates kindly provided by Dr. Cogdell.

In the membrane (and also in the crystals), the complete protein is made of 9 such a, b-subunit pairs, arrange in a cylinder, as seen in the links above, and this image from the Cogdell lab.

and this one, which has 8 a, b-subunit pairs, from Schulten's lab:

These cylindrical assemblies allow the B850 BChls to form a closely coupled ring in which excitation transfer by an exciton mechanism is very rapid. Excitation transfer in the B800 BChl ring, and between B800 and B850 molecules, is less rapid, but still on the fast end of Forster theory.

A structure for the LH1 complex from Rhodospirillum rubrum at ~10 Å resolution has been solved by electron diffraction crystallography by Robin Ghosh and colleagues. On the basis of this structure and the sequence similarity, it is clear that the LH1 forms a similar structure with a, b-subunit pairs as in LH2, but with only the two BChls (called B875) corresponding to B850. Ghosh has pointed out that the LH1 supercomplex has 16 a, b-subunit pairs in its cylinder. This is sufficient to contain 1 reaction center within the pallisade. Cogdell and colleagues, in collaboration with Werner Kühlbrandt, have recently solved a low resolution electron diffraction structure for the LH1-reaction center complex, which confirms Ghosh's suggestion.

In the bacterium, the LH1 complexes are synthesized in a fixed, and the LH2 complexes in a variable stoichiometry, with respect to the reaction center. The LH2 proteins are coded by the puc-operon, and the LH1 subunits, and the M and L subunits of the reaction center, are coded by the puf-operon. The two operons are under different control, so that the LH2 complex is expressed in greater amounts at lower light intensities.

In the complete photosynthetic apparatus, the LH2 structures lie outside the LH1 pallisade, but the B850 ring at the same level in the membrane as the B870, which line up with the ancillary BChls of the reaction center, to provide a pathway for rapid excitation transfer from the peripheral antenna to the BChl-dimer (P865) of the reaction center. This picture, adapted from an image from Richard Cogdell's lab, shows a cross section through a model structure containing all three complexes.

A similar model based on the Rs. molischianum structure, from Schulten's group:

Light harvesting in green plants

The light harvesting antenna of photosystem II consists of three main groups of proteins.