May 9, 2007 | David F. Coppedge

Details of Photosynthesis Coming to Light

New tools of science are unveiling the secrets of what was long a “black box” in biology: photosynthesis.  A paper in Nature last week1 described the structure of the plant PhotoSystem I complex (PSI) in near-atomic resolution.  Next day, a paper in Science2 described some of the protein interactions that occur when plants turn light into energy for work.  Both papers praised the exceptional efficiency of “the most efficient nano-photochemical machine in nature.
    As is common in the scientific literature, the paper in Nature used engineering language when discussing photosynthesis.  It referred to the “reaction centre” as a “light-harvesting complex” and to certain parts as “antennas.”  The authors used the root efficient eight times in the paper: for example, “This highly efficient nano-photoelectric machine is expected to interact with other proteins in a regulated and efficient manner” – there are two instances in the same sentence.  The paper ended:

The complexity of PSI belies its efficiency: almost every photon absorbed by the PSI complex is used to drive electron transport.  It is remarkable that PSI exhibits a quantum yield of nearly 1 (refs 47, 48), and every captured photon is eventually trapped and results in electron translocation.  The structural information on the proteins, the cofactors and their interactions that is described in this work provides a step towards understanding how the unprecedented high quantum-yield of PSI in light capturing and electron transfer is achieved.

The authors only referred to evolution once: “The two principal subunits of the reaction centre, PsaA and PsaB, share similarities in their amino acid sequences and constitute a pseudosymmetric structure that evolved from an ancient homodimeric assembly.”  Yet this was stated dogmatically without any explanation of how that could have occurred.
    The paper in Science explored photosynthesis from the protein’s perspective.  The authors of this paper also spoke of the “efficient transfer of electrons across biomembranes” and the “high efficiency of the reaction (an electron is transferred for each photon absorbed)” – i.e., there is no loss or waste of input.
    The authors discussed how certain protein parts physically move in response to their inputs.  These movements among the chlorophylls and other parts modulate the speed of the downstream reactions.  Rather than quote their jargon about biomechanics and biomolecular dynamics, let’s attempt an analogy that suggested itself from one of the illustrations: it’s like catching eggs dropping out of the sky into a soft, gentle net, where they can be safely transported to the kitchen.  Those who prefer the original jargon can see the footnote.4

1Amuntz, Drory and Nelson, “The structure of a plant photosystem I supercomplex at 3.4-angstrom resolution,” Nature 447, 58-63 (3 May 2007) | doi:10.1038/nature05687.
2Skourtis and Beratan, “Photosynthesis from the Protein’s Perspective,” Science, 4 May 2007: Vol. 316. no. 5825, pp. 703-704, doi: 10.1126/science.1142330.
3The second paper also spoke of the efficient use of quantum mechanical properties of light: “The experimental data reported by Wang et al. also encourage renewed theoretical attention to the early events in photosynthesis.  Models that include quantized nuclear dynamics seem particularly important, because high-frequency quantum modes influence fast electron transfer, producing nonexponential kinetics and unusual temperature dependence.”
4“Wang et al. suggest that the slow protein dynamics discussed above may help to overcome reaction barriers produced by membrane potentials or by environmental factors that perturb the photosynthetic reaction center and potentially slow down the electron-transfer rate.  Thus, protein motion could overcome reaction barriers produced by cellular factors that might otherwise perturb the electron-transfer kinetics.”

Those who studied high school biology decades ago can revel in these facts about photosynthesis that are now coming to light (pardon the pun).  At the time, our teachers and professors saw light going in, and sugars coming out, but were nearly clueless about what magic was going on inside.  The black box is now opening, and we’re finding out that highly efficient molecular machines were there all along.  So that’s how it’s done!

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