The enzymatic target

I've been doing some net-casting for some novel compounds to try in a lovely new model I'm building. I've had several interesting directions suggested to me, and as a result I've done a bit of reading slightly outside my usual-suspects list. Always a good thing.

One of the candidates I considered was an enzyme inhibitor. In theory, it modified the rate of a key enzyme-catalyzed reaction in the synthesis of a particular neurotransmitter. Now whether I actually believed the data... that's another story. (Hint: I didn't bother looking into how I would go about acquiring some of the compound.) But I thought it was interesting enough to talk about the mechanism on the blog.

Enzymes! What are they good for?!

To start off, I'd like to say that I've always admired the enzyme. I started out my science career as a biochemist playing with them in the lab, and have always thought those little workhorse machines are fucking awesome. They do all kinds of things. Mostly this involves taking substrates (reactants) and positioning them the right way to kick-start a reaction, and they facilitate the making and breaking of bonds that are not otherwise particularly favored by physics. They make reactions possible that would totally not happen in a reasonable amount of time otherwise- reactions that would be so slow that a cell might use up its stores of a given molecule before random physics of molecules running into each other supplied more. In short, they're necessary for life.

Figure 1. Enzymes! They're frequently presented using a pac-man analogy, and I just can't help myself. In this case, our pac-man enzyme binds to a little blue triangle, and makes it into a little blue diamond. Fancy stuff here.

What about enzymes for the BRAIN?!

Of particular interest to me now that I'm a little more specialized and have joined up with the neuropharmacologists' club- we have a series of enzymes that work to change precursor amino acids and other building materials into the neurotransmitters that make our brains functional. Enzymes also break down neurotransmitters, to make sure they're not just floating around all uncontrolled in the brain. And just so you can see some real substrates and products, and a real example of step-by-step synthesis (and simultaneously breakdown) pathways...

Figure 2. Catecholamine biosynthesis is one of my favorite examples. The enzymes responsible for each step are named in light blue. Note that these enzymes both create and destroy neurotransmitters. A dopamine molecule is used up to make norepinephrine.

How do we target enzymes with pharmacology?

The idea of interrupting an enzyme pathway and reducing/increasing the number of target molecules hanging out in the cell (or wherever else) is nothing new- we've been implementing it for some time. One very prominent example of this is statins, a class of drugs that inhibit the enzyme HMG co-A reductase. Inhibiting this enzyme inhibits cholesterol synthesis, a good thing for people who have too much cholesterol. And since we have a lot of different enzymes changing a lot of different molecules, the principle has lent itself nicely to many applications. Many of those possible applications are very messy and full of potential side effects- so like much of pharmacology, we're limited to some of the more clear-cut and simple ways to use enzymes as targets.

Figure 3. The idealized version. Inhibit enzyme 2, less molecule C gets produced. Easy!

Figure 4. A slightly more real version, though probably still oversimplified. Suddenly inhibiting enzyme 2 causes much more than a drop in molecule C. Now levels of D, E, and Z are all affected. Possible side effects! What else are we disrupting by screwing with all these other molecules?!

Pharmacology Tip Numero Uno: Really. It's *never* that simple.

As if this didn't seem like a sufficient wrench-in-the-gears, there are different enzyme inhibition mechanisms with different implications. There are competitive inhibitors, that work by reducing the likelihood that a substrate will enter the active site of the enzyme. There are irreversible inhibitors, that permanently bond to the active site of the enzyme, after which it's out for the count. There are noncompetitive inhibitors, that affect the enzyme's action by binding somewhere that is not the active site (and this may or may not be reversible). Finally, there are uncompetitive inhibitors- distinct from noncompetitive inhibitors- that only bind to the enzyme when there is a substrate molecule in the active site.

Figure 5. Enzyme inhibitor sites and mechanisms, illustrated.

Competitive inhibitors can be useful little tools- the great thing (or double-edged sword, depending on your view) about them is that they do wear off with time. This is not the case for the irreversible types- a dose of those and you're stuck waiting for some new enzymes to be made. Many nerve agents are irreversible or semi-irreversible inhibitors of the acetylcholinesterase enzyme- there is potential to do a lot of damage with nerve agents. Noncompetitive inhibitors don't block the active site of an enzyme, so your substrate will still bind, but the enzyme won't kick-start any reactions. The uncompetitive inhibitors are similar in that way to the noncompetitives. These last two mechanisms are useful when you don't want to use a drug that resembles your substrate enough to fit into the same active site. Say your enzyme binds to any triphosphate molecule (there's ATP and all your nucleotides to start)- you don't want to chance widespread side effects by using something very nonspecific that looks like ATP.

In summary

Enzymes can be good targets for future therapeutics. We have a number of widely-used pharmacological tools that target enzymes, both in CNS pharmacology as well as other areas (like statins in cardiovascular pharmacology). However, there are a lot of fine details, and like many other potential targets of interest, there are a lot of ways to create side effects.

Doesn't mean I think it's any less cool as a concept, though.