Probing photosynthesis is professor's career focus

Updated 10:00 AM March 20, 2006

Probing photosynthesis is professor's career focus

By Nancy Ross-Flanigan
News Service

From rainforests to rosebushes, from farm products to fossil fuels—virtually all of life as we know it depends directly or indirectly on a single, ancient process.

(Photo by Martin Vloet,
U-M Photo Services)

That process is photosynthesis—the chain of reactions through which green plants, algae and some bacteria use the sun's energy to convert water and carbon dioxide into oxygen and energy-rich organic compounds. Although there's evidence that photosynthesis has been going on for nearly four billion years, and scientists have been studying it for a couple of centuries, much remains to be learned about the details, says Charles Yocum, the Alfred S. Sussman Distinguished University Professor (DUP) of Molecular, Cellular and Developmental Biology.

Yocum has spent his career teasing apart the workings of a key enzyme in the first step of photosynthesis: photosystem II. He will discuss the latest phase of that work in his DUP lecture at 4 p.m. March 28 in the Rackham Amphitheatre.

When Yocum began his research in the 1970s, the oxygen-producing step in which photosystem II participates was "a black box," he recalls. "Water went in one side, and oxygen and electrons came out the other side, but what went on in between was largely a matter of conjecture. Now, we've taken the system completely apart—we've extracted the proteins, we know what they are and we know the genes that encode them.

"Other researchers have determined the crystal structure of the enzyme, so we know what it looks like. That's one of the rewarding things about science—especially basic biological science—to be able to start on something at the beginning and see it go to completion."

One of Yocum's early and pivotal contributions was being the first person to isolate photosystem II in its active form. "That was neat," he says, "but then we had to do something with it, and doing something with it is what I've been up to for most of my career."

Recently, one branch of his research has focused on figuring out which parts of the photosystem II enzyme—which is made up of 30 different proteins—are essential to its function.

"I'm a biochemist, so I like to manipulate enzyme systems and take them apart," he says. "If you begin to take this enzyme apart, you very quickly find out that it won't work without certain key proteins." In earlier research, he learned that two of those proteins are necessary for keeping the metal ions that the enzyme contains in their proper places. Lately, he's been studying a third protein, without which the enzyme is useless.

"You can put the whole complex together and it won't work unless it has this one key protein attached to it, so we've been doing experiments to try and learn how this protein works and why you have to have it for the whole system to function," he says.

By introducing mutations into the protein to make defective versions of it, then checking to see what the malfunctioning protein can and can't do, Yocum learned that its role is to trap chloride ions and hold them in place in the enzyme. If the protein doesn't do its job, the enzyme can't retain the chloride ions, and if the enzyme lacks chloride, it can't convert water to oxygen.

Understanding the finer points of a process as essential as photosynthesis is intellectually satisfying, but there also are practical reasons for probing the reactions, Yocum says.

"When we understand the chemistry of these reactions, it probably will be possible to create synthetic models of the whole system which could be used—along with solar energy—to generate oxygen, electrons and protons. You can then take those electrons and protons and convert them into hydrogen," an important step toward a hydrogen-power economy.

Probing photosynthesis is professor's career focus

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