Enzymes are the chemical engines and machinery of life. They are the catalysts that move things around inside our cells, that break apart proteins and other molecules to either recycle them or get rid of them out of the body. They are the machines that read, translate, replicate and repair our DNA. They are proteins themselves: big long amino acid polymers that glob up on themselves to give three-dimensional shapes with velcro-like molecular surfaces and little sticky pockets for other molecules. In an enzyme, those sticky pockets can do more than just stick (i.e. bind) to another molecule--they can perform chemical reactions on those other molecules. They do this by bringing those molecules so close in space either to other molecules they also bind into their sticky pockets, or to their own amino acid chemical groups, that the atoms in the molecules just can't help but interact and form (or break) bonds with each other. Think of static-y balloons: rub them on your hair, then hold them juuuuust close enough to the wall and they POP on over to stick. Or of how even though you didn't like your college roommate very much at first, you spent enough time in close proximity and eventually bonded and became friends. These kinds of forces happen at the atomic scale, bringing molecules together that otherwise either would not ever get close enough together to react, or who don't really WANT to react but can't help it when they get stuck together in a sticky pocket and stay there together for a long enough time. For molecules, even a few milliseconds can be long enough.
The molecules that enzymes are bringing into sticky pockets and forcing to become friends with themselves or others are called their "substrates." The stuff that gets formed in the sticky pocket is called the "product." How much a given substrate "likes" an enzyme (and vice versa) is governed by the chemical properties of the molecule and how well they are compatible with the sticky pocket. A key aspect of this is that in order to be a good enzyme, a good catalyst, there has to be just the right balance between how much the substrate and product stick to the sticky pocket: if either one of them sticks too much, the enzyme has trouble kicking them out to move on to the next molecule of substrate and therefore can only do the reaction once. How fast the enzyme can go through this process (the "rate" of the reaction) is thus also governed by the stickiness, but additionally by the amount of time it takes for molecules of the substrate to randomly bump into the enzyme when they are both floating around in a solution where at any given time, they may be very, very far away from each other (think trying to find another human in a vacant building vs. a crowded airport). This seems pretty straightforward as a conceptual illustration, but how do we actually quantify this? How do we figure out how fast an enzyme goes with a given substrate, how well a substrate sticks to an enzyme, how well its product sticks to the enzyme, and how to relate any of this to what we want to know about the world and how it works? A fascinating new translation (from the original German) of the paper by Michaelis and Menten that is considered to be the first quantitative description of all of this, along with some commentary and historical contextualization, was published this week in the journal Biochemistry.
In the early 1900s, a Canadian woman named Maud Leonora Menten went to Germany to work with Leonor Michaelis at the University of Berlin. By this time in scientific history, Michaelis and Menten knew that enzymes could perform chemical reactions. They knew approximately that this stickiness of pocket for substrate was a factor in determining the fundamental properties of the reaction (things like its rate and the mechanism by which it occurred). Catalyzed themselves by work from Victor Henri, they hypothesized that there would be a quantitative relationship between the amount of enzyme around compared to the concentration (or density of molecules floating in the solution--trying to find each other in the vacant building or the crowded airport)--but nobody had ever analyzed this properly before. Henri had the basic idea right, but had forgotten to take some key factors into account (things like changes to enzyme products that just happen on their own without the enzyme being involved, and how much an enzyme depends on having the right pH balance in order to work properly) that made it too hard for him to figure it out. In just that one year, Menten worked with Michaelis to set up experiments to test this and to properly control those other aspects that Henri didn't, and collected the data to write the paper describing analyses and equations that transformed the way scientists thought about enzyme function and provided the foundation on which modern biochemistry is built.
All of this was done without even knowing how much enzyme they were working with, just diluting some kind of preparation of it (which, Johnson and Goody point out, Michaelis and Menten didn't even describe in the paper) in different proportions to substrate (which was sucrose) and without having modern molecular analysis tools available. The "readout" (i.e. detection method) of the enzyme activity that Menten used was the optical rotation of the solution she was working with: how much the solution "twisted" some polarized light that was passed through it. The enzyme, invertase, was breaking apart sucrose into fructose and glucose, and removing the bond between them had an effect on the optical rotation. She had to handle the solutions just right and put them in conditions that would minimize the unrelated conversion of the glucose to make sure that those effects wouldn't complicate the analysis. With that scrappy, deeply thought-out experiment, she and Michaelis are a reminder about how much you can figure out with very few resources. This piece of history also illustrates how much discovery of the un'seeable' can come from the basic function of human exploration, even without fancy machines and lots of pre-determined knowledge about a system.
One thing their models don't account for is how enzymes probably ACTUALLY function inside cells, where concentrations and densities of proteins and other molecules are extremely high and dynamic, relative to when they are free floating in solutions. The interior of a cell is like an obstacle course, with strings of stuff and big protein chunks in glommed up 'complexes' everywhere you turn. Molecular distances are measured in Angstroms, where one Angstrom is 1/10th of a nanometer. A typical enzyme protein is, on average, about 4-5 nanometers in diameter (40-50 Angstroms) when its amino acid polymer chain is all velcroed up on itself (i.e. the protein is 'folded'). An enzyme can be sequestered over in one region of a cell (which is about a picoliter, or a billionth of a liter, for a human white blood cell), thousands of nanometers away from its substrates and with protein after protein in between them. Having an obstacle course of proteins between you and your substrate is a lot different from only having a bunch of tiny water molecules filling that distance. Proteins do not get out of your way easily the way water molecules do--however, they do sometimes actively facilitate your travels to bring you to your substrate (through their own enzymatic machinery functioning as kinetic motors). Also, you might just end up next door to some other substrate that you don't like that much, that doesn't stick very well in your sticky pocket, but hey, you're hanging around in the same locale and what the heck. You might crawl from there to some other molecule of another substrate in a complex next door, without ever letting completely go and floating away (the way the traditional understanding of enzyme reaction rates assumes is happening).
These factors and differences play a major role in actual biological enzyme catalysis, and add so much complexity to the system that these simple Michaelis-Menten models break down. It's analogous to the difference between Newton's Law of Gravity and General Relativity. The rates could be either faster or slower than you would expect from how sticky the enzyme pocket is for the substrate (faster because of proximity increases for the substrate, slower because of more chances of sticking to something else nearby including inhibitory, grabby product, and not being released to find another molecule of substrate), and we don't have good ways of measuring these effects to any accuracy yet. This will be the next challenge for biochemistry, to update this model and incorporate all of these complex molecular interactions into a fuller, integrated picture of how these machines work inside the cell. This is, to put it simply, a loooooong way off. But we need to remember as modern scientists how much we can do when we make the best possible assumptions we can with all of the information we have available, are rigorous about defining and remembering those assumptions, and not be afraid to ask these kinds of questions just because they are hard.
Menten didn't stay in Germany--she went to the University of Chicago to complete a PhD degree (because Canada didn't allow women to get PhD degrees at the time--way to go, dudes). She went on to publish other work and be very successful as a faculty member at the University of Pittsburgh--where, surprise surprise, she wasn't promoted to full professor until she was 70 years old and had been there for 26 years. From all accounts available, she was a fascinating person, researcher, doctor and painter, and was multifaceted and accomplished in all aspects of her life. Also interestingly, I don't see any discussion of her family or marital status in the sources I've trolled through (admittedly briefly)--it's nice to know that a woman's accomplishments can be discussed independently of whether or not she had a husband and children. I wonder why I never knew Menten was a woman before now. I'll make sure to tell my students about her.
Johnson, K., & Goody, R. (2011). The Original Michaelis Constant: Translation of the 1913 Michaelis–Menten Paper Biochemistry, 50 (39), 8264-8269 DOI: 10.1021/bi201284u