Allosteric Regulation
Some enzymes do not follow a simple Michaelis-Menten curve. Their velocity-vs-substrate plot is not a hyperbola but an S-shape (sigmoid). These enzymes are allosteric - they have multiple subunits and multiple binding sites that talk to each other. The conversation between subunits produces cooperativity, which is how your cells do precise, switch-like regulation of critical metabolic steps.
Four ways to change enzyme activity, fastest first
Scroll sideways to see the whole map.
Why zymogens exist at allSome enzymes are too dangerous to build in working form. A protease made active inside the cell that produced it would digest that cell, so it is made as an inactive zymogen and only cut open where it is meant to work. Acute pancreatitis is what happens when that cutting starts in the wrong place.
Feedback inhibitionThe classic allosteric arrangement: the end product of a pathway inhibits the enzyme that catalyzed its first committed step. It is the cheapest possible control, because the cell measures the thing it actually cares about and acts on the one step that commits resources.
Cooperativity is allostery tooIn a multi-subunit enzyme the substrate itself is the allosteric regulator: binding at one site changes the shape of the others. That converts the hyperbolic Michaelis-Menten curve into a sigmoid, which makes the enzyme behave like a switch rather than a dial.
What “Allosteric” Means
Allos (other) + steric (place) = “binds at another place.” An allosteric site is any site on an enzyme other than the active site where a regulator can bind. Binding there triggers a conformational change that ripples through the protein, changing activity at the distant active site.
- Allosteric activators stabilize the high-activity shape (R state, “relaxed”).
- Allosteric inhibitors stabilize the low-activity shape (T state, “tense”).
Cooperativity and Sigmoidal Kinetics
Most allosteric enzymes have multiple substrate-binding sites. When one subunit binds substrate, it changes shape - and nudges the other subunits into a higher-affinity shape. The second substrate binds more easily, the third even more easily, and so on. That is positive cooperativity.
Plotting velocity vs. [S] for a cooperative enzyme gives a sigmoidal curve instead of a hyperbola. At low [S], the enzyme is sluggish. Once a critical [S] is reached, activity ramps up sharply. Past that threshold, the enzyme is essentially maxed out. Sigmoidal kinetics turn an enzyme into a switch.
- Positive cooperativity: binding one substrate makes subsequent binding easier. Sigmoidal curve. Classic example: hemoglobin binding oxygen.
- Negative cooperativity: binding one substrate makes subsequent binding harder. Less common but real (insulin receptor kinase).
Quantifying Cooperativity: The Hill Coefficient
Cooperativity is not just qualitative. You can put a number on it. The Hill coefficient (n or nH) is a measure of how strongly the subunits “talk” to each other.
You do not need to calculate Hill coefficients on the MCAT, but you should recognize a number like “n = 2.8” as “this protein shows strong positive cooperativity.” A passage that gives you a Hill plot or Hill value is testing whether you know which direction the cooperativity runs.
T State vs. R State
Two conformations interconvert:
- T state (tense) - low affinity, low activity. Promoted by allosteric inhibitors.
- R state (relaxed) - high affinity, high activity. Promoted by allosteric activators and by substrate itself.
Substrate binding pulls the equilibrium toward R. Inhibitors lock the enzyme in T. Activators lock it in R. The sigmoidal curve emerges from this T-to-R transition, which happens in a concerted all-or-nothing fashion in some models (MWC / concerted model) or one subunit at a time (KNF / sequential model). The MCAT does not require naming these models, but the concept of shifting between two states is essential.
Feedback Inhibition
Most metabolic pathways end in a product that allosterically inhibits the first committed step of its own pathway. This is feedback inhibition, and it is the main way cells avoid overproducing things they already have plenty of.
Canonical MCAT examples:
- ATP is an allosteric inhibitor of phosphofructokinase-1 (PFK-1), the rate-limiting enzyme of glycolysis. When the cell has enough energy (high ATP), glycolysis slows.
- AMP is an allosteric activator of PFK-1. Low energy (high AMP) accelerates glycolysis.
- Citrate is an allosteric inhibitor of PFK-1. A full TCA cycle signals no need for more fuel.
- CTP feedback-inhibits aspartate transcarbamoylase (ATCase), the classic textbook allosteric enzyme of pyrimidine synthesis.
Hemoglobin Is the MCAT’s Favorite Allosteric Protein
Hemoglobin is not an enzyme, but it is the textbook cooperative protein. Four subunits, each binding one O2. Binding the first O2 is hard (T state). Once bound, the subunit shifts to R, pulling the others with it. The second, third, and fourth O2 molecules bind much more easily. The result is a sigmoidal O2 saturation curve that allows hemoglobin to load O2 efficiently in the lungs (high O2) and unload it in tissues (low O2).
Negative regulators that stabilize T (2,3-BPG, H+, CO2) cause hemoglobin to release more O2 in tissues - the Bohr effect. Know this example cold; it shows up constantly. Full coverage of hemoglobin is in Chapter 3.