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Tuesday, September 08, 2009

Monday's Molecule #135: Winner!

Yesterday's molecule was the type II reaction center from the photosynthetic purple bacterium Rhodobacter spaeroides.

When light shines on the special pair of chlorophyll molecules known as P870, a single electron is boosted to a high energy level by absorbing a photon. This electron is transferred to an adjacent bacteriochlorophyll a heme group in a typical oxidation reaction. The single electron then travels down the electron transport pathway to a bacteriopheophytin heme and then to a quinone molecule that's part of the pathway.

In the final step, the electron reduces another quinone bound to the "mobile" site. This reduction is mediated by an iron atom (red ball). Quinol (QH2) is released after two electrons have been transferred sequentially. The quinol molecule carrying two electrons then diffuses to a cytochrome bc complex that pumps protons across the membrane. The creation of a proton gradient drives the synthesis of ATP.

The electron deficient P870 chlorophylls are re-supplied with electrons from cytochrome c, which gets them from the chytorchrome bc complex in a cyclic reaction [see A Simple Version of Photosynthesis].

Purple bacteria are strict anaerobes—oxygen is poisonous to them so their The purple bacteria version of photosynthesis does not involve the splitting of water and generation of O2. This is an important point since many students think that water (H2O) is the only possible electron donor in photosynthesis. In fact, the ability to oxidize water evolved much later.

It's much better to think of photosynthesis as a light-activated oxidation-reduction system where there are several possible electron donors and acceptors.

The type II reaction center molecules are embedded in a membrane-spanning protein complex whose structure has been solved [see Nobel Laureates: Deisenhofer, Huber, and Michel]. In the version shown here, the electron donor is cytochrome c, which binds to the top part of the molecule on the exterior surface of the membrane.

Photosynthesis is a complex example of an electron transfer reaction. Rudolph A. Marcus was awarded a Nobel Prize for his work on understanding this type of chemical reaction.

This week's winner is Philip Johnson of the University of Toronto. He blogs at Biocurious. While Philip was the first to get the right answer, an honorable mention has to go to Wibowo Arindrarto from Jakata, Indonesia for the best answer.

Your task for today is to identify the molecules with the question marks and explain (briefly) what's going on.

There's a Nobel Prize associated with the type of reaction that you're seeing here. Focus on the red arrows. The prize wasn't for this particular reaction although it is depicted in the Nobel Lecture as an example of the type of reaction that was being described. Name the Nobel Laureate.

The first person to describe the reaction and name the Nobel Laureate wins a free lunch. Previous winners are ineligible for six weeks from the time they first won the prize.

There are only three ineligible candidates for this week's reward: Alex Ling of the University of Toronto, and Markus-Frederik Bohn of the Lehrstuhl für Biotechnik in Erlangen, Germany, and Maria Altshuler of the University of Toronto

I have an extra free lunch for a deserving undergraduate so I'm going to continue to award an additional prize to the first undergraduate student who can accept it. Please indicate in your email message whether you are an undergraduate and whether you can make it for lunch.


Nobel Laureates
Send your guess to Sandwalk (sandwalk (at) and I'll pick the first email message that correctly identifies the molecule(s) and names the Nobel Laureate(s). Note that I'm not going to repeat Nobel Prizes so you might want to check the list of previous Sandwalk postings by clicking on the link in the theme box.

Correct responses will be posted tomorrow.

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