Sunrise in the Garden of Dreams

Gerhard Schrader had been feeling sick for a while, but he didn’t realize how bad it was until he tried to drive home. There was something wrong with his eyes. He felt like he was going blind; he could hardly see the road, and it was becoming difficult and painful to breathe. It was a struggle for him to get home. When he finally reached the house, he peered into the mirror through his glasses. He discovered the pupils of his eyes had shrunk to tiny pinpoints.

He quickly guessed what had happened. Schrader worked at IG Farben, at that time in 1936 the world’s largest chemical company. He’d recently completed the initial synthesis of a new compound — a clear apple-scented liquid. Its sweet scent belied its real nature, however, for while he now knew the chemical’s vapor must be quite toxic, he didn’t yet realize how dangerous it was, or how close he’d come to death.

After a couple weeks in the hospital, Schrader returned to the lab just before Christmas, where he resumed work on his new compound — this time being just a little more careful. He soon realized it was incredibly toxic. Animals exposed to low vapor concentrations went into convulsions and died within minutes.

Schrader was disappointed. He was trying to develop new pesticides; he wanted chemicals that killed insects but not mammals. Word of his unusual find got round to the Wehrmacht, however, and unlike Schrader’s managers at IG Farben, the Wehrmacht was deeply interested. As it turned out, in trying to find the perfect pesticide, Schrader had stumbled across history’s most potent class of chemical warfare agents.

Tabun: When death smells like fruit.

Luckily, neither tabun nor sarin (another similar compound synthesized by Schrader) saw use in the Second World War. Despite the sinister nature of some of the compounds he made, however, Schrader’s work ultimately proved beneficial, because his research gave rise to the modern world’s most popular family of insecticides. Organophosphate pesticides (OPPs) are similar to tabun in many respects, and the way they kill insects is very similar to the way tabun nearly killed Schrader.

The Master of All Poisoners

So how do OPPs work their lethal magic? Let’s start by thinking about an insect as a biochemical machine we want to break. I know that sounds horrible — I like most insects, with the exception of cockroaches (especially when one turns up in a glass of orange juice I’m about to drink). But when it comes to pests that eat our crops, killing is exactly what we want to do. And just as you’d expect, it turns out some ways to put them out of action are more effective than others.

Take a look at this chart from [1]. (I realize this may look like just a bunch of names, but hold on — explanation forthcoming).

 The original paper[1] includes similar data for herbicides and fungicides and a chart with a more complete breakdown. I have to admit I really like this kind of data, geek that I am. If you also are so inclined, check out [1] for more.

The chart displays insecticide targets as a percentage of world sales for 2003 — in other words, out of total world insecticide sales in 2003, what percentage was composed of chemicals that hit a particular target. As the chart illustrates, the bulk of the ammo we fire at pests is aimed at their nervous systems. That’s hardly surprising; if you want to kill a bug, knocking out its nervous system is a fast and foolproof way to do it. Many of Nature’s most lethal poisons act on the nervous system as well. Out of the targets in the nervous system, however, the most popular is by far and away acetylcholinesterase (AChE). This too is unsurprising, and it has a lot to do with the way nervous systems work.

Every time you move your fingers or take a deep breath, your motor neurons convey the instructions from your central nervous system to your muscle fibers. At the junction between the motor neuron and the muscle fiber (the NMJ), the motor neuron floods the gap between the two cells (the synaptic cleft) with a neurotransmitter called acetylcholine.

Acetylcholine: Messenger boy.

On the other side of the cleft, acetylcholine binds to receptors on the muscle fiber, triggering a chain of events that causes the muscle fiber to contract. You don’t want the acetylcholine to linger around, however, because that would keep the muscle in a contracted state. It’s essential to get rid of the acetylcholine quickly. Which is why you have the enzyme acetylcholinesterase (AChE).

Ribbon diagram of acetylcholinesterase with bound ligand — see PDB 1ACJ, of which more later.

Just thinking about Nature’s catalytic toolkit is enough to turn chemists green with envy, and when you look at AChE, it’s easy to see why. AChE boasts a turnover number of 1.4 x 10^4 (see [3]), meaning that when fully saturated a single AChE molecule can break down 14,000 molecules of acetylcholine per second. When acetylcholine enters the synaptic cleft, AChE immediately starts chopping it up, so as soon as the motor neuron stops dumping acetylcholine, the concentrations in the synaptic cleft drop quickly and the signal dies away.

This system generally works quite well, but from a strategic point of view, it’s a weak point vulnerable to enemy attack. If you disable this system, you deprive an animal of the ability to control its own muscles — you paralyze it. And Nature, the queen* and master of all poisoners, has found ways to do precisely that. Botulinum toxin or Botox, to take one famous example, paralyzes you by preventing your motor neurons from releasing acetylcholine. FAS2, a component of green mamba venom, relies on a different but no less vicious approach — it locks onto AChE itself.

We humans can hardly hope to surpass Nature’s cruel cunning as a poisoner, but at least we can imitate her ways. Like FAS2, OPPs and nerve agents bind to AChE to knock it out of action. Let’s zoom in and take a quick look to see how this works.

From the Protein Data Bank, 1ACJ.

This is a drawing of three amino acids (“residues”) in the active site of the AChE enzyme from the Pacific electric ray. The translucent gray ribbons and loops are part of the cartoon drawing of the protein’s secondary structure elements — alpha helices, beta sheets and so forth. Everything else is hidden, except for the side chains of three particular amino acids. I’m not going to draw the full mechanism because some parts of it are still unclear, primarily the role of glutamate (see [4] among others). We do know, however, that these three residues are critical to the enzyme’s function. They serve as a kind of “catalytic triad” or trio, if you will. They’re the cutting blades on the meat grinder, the teeth on the diamond-blade saw.

Acetylcholine fits right into the active site of AChE like a hand slipping into a pocket, bringing it into close proximity with the three residues shown above. The interactions between acetylcholine and AChE subtly alter the shape of the enzyme and trigger a series of events that split acetylcholine into acetate and choline. Acetate is just acetic acid (vinegar) minus a hydrogen ion, while choline is a nutrient you get from your food.

Party of Four: Acetylcholine (top left) plus water (top right) to give choline (bottom left) and acetic acid (bottom right).

Just like acetylcholine, nerve agents and OPPs also bind to the active site of AChE. Initially, the first few steps of the same reaction mechanism take place. But about midway through the reaction, something goes wrong.

Ordinarily, the oxygen on serine forms a bond with a carbon in acetylcholine, and this bond breaks during a later step in the reaction. When a nerve agent like sarin or an OPP is bound to the enzyme, however, a bond forms between the oxygen in serine and the phosphorus atom in the OPP, altering the normal sequence of events in a way that leaves the enzyme stuck. The end result looks like this:

 From PDB 1CFJ. Again, courtesy of the Pacific electric ray.

Notice the serine has a new group attached to it (a methyl phosphonate group). The result is the molecular equivalent of throwing a steel wrench into an intricate machine. With the methyl phosphonate stuck on the serine in the active site, the enzyme is disabled; it can no longer break up acetylcholine. The next time the motor neuron fires, acetylcholine will accumulate in the synaptic cleft with nothing to break it down. Even modest doses of OPPs condemn insects to paralysis and death. Some pests, of course, have evolved varying levels of resistance to OPPs, usually through mutations that alter the structure of the AChE or alter other biochemical pathways that detoxify the OPP and render it harmless.

Another major class of pesticides called the carbamates also latches onto AChE, although the carbamates work in a different and less irreversible way. Nonetheless, they too are murderous to insects. Some carbamate pesticides like aldicarb are quite toxic to humans/other mammals, although aldicarb remains invaluable thanks to its potency against certain nematodes.

Aldicarb: the potato grower’s friend.

But wait just a minute here. If OPPs and carbamates are so toxic, how can we use them as pesticides? As it turns out, however, not all are equally toxic to all alike. Many of our OPPs like malathion aren’t deadly until they’re converted into an active form by enzymes inside the body, and inside a human these OPPs get broken down much faster than they get converted into the active or deadly form. Also human and insect AChEs have different structures so some OPPs are better at binding to an insect AChE than a human AChE. In other words, when compared with nerve agents like tabun, most of these chemicals are more toxic to insects and less so to us. That doesn’t mean they are harmless, of course, and in fact some of them can be very deadly, but they are at least more dangerous to insects than to humans. OPPs and carbamate pesticides are also thought to break down relatively quickly in the environment.

Chemical companies have occasionally erred in this regard. VX — the most deadly nerve agent of them all — was briefly marketed as an insecticide. The manufacturer soon discovered its startling toxicity and yanked the product off the market. Moreover, many of the OPPs and carbamates have long aroused bitter controversy, partly on account of evidence they may affect human/animal biochemistry through other mechanisms besides AChE inhibition. Many OPPs and carbamates can pose serious risks to farm workers and potentially poison non-target organisms like birds or fish. In 2009, for example, the EPA banned carbofuran, a carbamate pesticide infamous for its lethality to birds.

Consumers often seem worried about pesticides in their food. It’s worth noting that the USDA tests produce samples as part of their Pesticide Data Program (you can find the methodology they use here, and the data from recent years here). The levels of pesticides present are extremely low and well below the tolerance levels set by the EPA, which suggests this problem is far less significant than generally believed.

These kinds of controversies are difficult to resolve, however, not least because they reflect two different philosophies about how to regulate industrial chemicals. One of these is the precautionary principle, which holds that if there is any plausible risk associated with its use, a chemical should be treated as guilty until proven innocent in order to protect the public. Critics of the precautionary principle see this kind of approach as excessive. They prefer to consider these agents innocent until proven guilty.

A Poison is A Poison

But wait! Do we have to use pesticides? Instead of arguing about which of them pose a health or environmental hazard, why can’t we farm the “natural way” instead? That would be nice, of course, if only a “natural way to farm” actually existed. Agriculture is unnatural by definition, a human invention that enables us to feed a much larger population than what uncultivated land would ordinarily support. And when we plant vast tracts of land with crops bred to maximize yield, we are in essence making a giant Bug Magnet. We either have to control pests or accept significant losses. If we can find a different way to control pests, of course, then that’s another story.

Critics of modern agriculture often cite “organic” farming as an alternative. The phrase “organic farming” secretly bothers me, because whenever somebody refers to “organic”, the first thing that comes to mind is organic solvent. Nonetheless, misnomer though it may be, the name seems to have stuck, so we’ll go with it. In any event, there are a lot of problems with the “organic alternative”. For one, it’s very dubious that humankind could live by organic farming alone. For another, organic farmers use pesticides too. Sure, they’re “natural” pesticides, but that doesn’t always mean very much, because nicotine sulfate and rotenone aren’t necessarily environmentally-friendly agents either. There is a good post by a blogger at SciAm about this, so I won’t belabor the point. At the end of the day, a poison is a poison is a poison, as Gertrude Stein might say.

Despite these drawbacks, there IS a way we might be able to make organic farming viable in the long-term. Unfortunately, it’s the same approach the organic farming movement has already rejected. That’s right: GMOs.

How Green Was My Garden

There’s a widespread misconception out there that natural = wholesome, good, healthy while synthetic = dark, evil, designed by corporate gnomes to poison you. And there’s some truth to this, in that recent history provides innumerable examples of cases where companies adopted practices later found to damage the environment. Think about tetraethyl lead or Silent Spring and you’ll see why environmentalists are right to be on their guard. But at the end of the day, it’s clearly a massive exaggeration to believe that everything large companies do is evil, or that natural > synthetic by default.

Unfortunately, I think this mindset is still pervasive in the environmental and organic farming communities, and it explains much of the hostility towards GMOs. To my mind, GMOs and organic farming could very well make an excellent partnership. I know there are a lot of reasons why this looks like a rocky marriage — Monsanto and organic farmers love each other about as much as monks loved Viking raiders. But I think we need to get past this stumbling block somehow, because given further development, GMOs could ultimately prove to be the best thing that ever happened to organic farming.

I’m a little biased, though, it’s true. I always find GMOs very exciting, because the potential is so vast. We’re still in the early days; we’re still just gearing up for action. There’s a lot we have yet to learn about plant biology. But even the little we’ve accomplished so far gives you a hint of the promise to come. Nature’s molecular toolkit is vast. If we can take greater advantage of that toolkit — use modern molecular biology to turn it to our own advantage — perhaps we can at last marry environmental stewardship to increased production. It’s a beautiful dream, but today we might actually have the means to make it come true.

[1] John E. Casida. “Pest Toxicology: The Primary Mechanisms of Pesticide Action.” Chemical Research in Toxicology, 22(4): 609-619, March 2009.

[2] Jonathan Tucker. War of Nerves: Chemical Warfare from WWI to Al-Qaeda. New York, NY: Anchor Books, 2007.

[3] David Nelson & Michael Cox. Lehninger Principles of Biochemistry. New York, NY: W. H. Freeman and Company, 2008.

[4] Yanzi Zhou et al. “Catalytic Reaction Mechanism of Acetylcholinesterase Determined by Born-Oppenheimer ab initio QM/MM Molecular Dynamics Simulations”. Journal of Physical Chemistry 114(26): 8817-8825, July 2010.

[5] USGS National Wildlife Health Center, Field Manual of wildlife Disease, Chapter 39.

Casida, J. (2009). Pest Toxicology: The Primary Mechanisms of Pesticide Action Chemical Research in Toxicology, 22 (4), 609-619 DOI: 10.1021/tx8004949