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As biological catalysts, enzymes transform their target substrates into products. Enzyme kinetics is the rate of that transformation. By understanding how an enzyme’s behavior is affected, you can figure out how it functions in physiology or fails to function in disease.
Now it gets complicated…
In the first place, most enzymes are tightly regulated: they won’t be active or synthesized until needed. Furthermore, many enzymes need small molecule cofactors to do part of their reactions. These cofactors can be metals, like zinc or iron, or organic coenzymes (e.g. NADPH).
The reaction rate also depends on the environment. Enzymes function at an optimal pH; when the environmental pH is different, the enzyme loses structural stability and its reaction rate decreases. Enzymes also have optimal temperatures, and their rates accelerate as temperatures increase to 50°C, but decrease abruptly as the enzyme denatures around 55-60°C. The exceptions to this are enzymes found within thermophilic bacteria, which operate maximally at a higher temperature.
An enzyme’s kinetics can also be controlled by inhibitors. Inhibitors reduce activity by either blocking the interaction between the enzyme and substrate or altering the substrate’s transformation rate. Many pesticides (e.g. chlorpyrifos) and drugs (e.g. ACE inhibitors) are specific enzyme inhibitors.
To determine kinetics, we analyze an enzyme’s activity – the number of reactions it manages over time. After setting the length of time and amount of enzyme and substrate, it’s possible to determine the velocity – the reaction rate – by measuring product appearance (or substrate disappearance) at different times. By plotting the amount of product versus time, we can determine velocity from the linear portion of the curve.
Is it really that simple? Perhaps – if you are fortunate enough to study a classic Michaelis-Menten enzyme! The Michaelis-Menten theory describes a reaction involving one substrate (S), enzyme (E), an intermediate enzyme-substrate complex (ES), and a product (P) and regenerated enzyme. This assumes that when the enzyme complexes with the substrate, it either dissociates into unchanged substrate and enzyme or proceeds irreversibly forward to product.
The second step’s rate (k2) represents the enzyme’s turnover for enzyme-substrate converting to product. We get the Michaelis constant, Km, from the ratio of the reactions affecting the enzyme-substrate complex: its formation (k1), dissociation (k-1), and progress towards product (k2). A lower Km value indicates that an enzyme has a higher affinity for a substrate.
Ultimately, a Michaelis-Menten enzyme’s velocity depends on its Km, substrate concentration, and maximum possible velocity (Vm), which occurs when there is much more substrate than enzyme. This means that when the substrate concentration equals Km, the enzyme’s velocity is half of its Vm.
If an inhibitor acts irreversibly, it destroys an enzyme’s ability to transform its substrate, usually by covalently modifying the enzyme. The effects of an irreversible inhibitor will be observed in a decreased reaction velocity over time. If an inhibitor is reversible, we can measure its effects on Vm and Km. These effects depend on whether the inhibitor competitively blocks the enzyme’s active site (Vm stays the same, Km increases), noncompetitively binds outside the active site (Vm decreases, Km stays the same), or noncompetitively binds to the enzyme-substrate complex (Vm and Km both decrease).
What if the enzyme doesn’t follow Michaelis-Menten kinetics? What if the product can transform into substrate, inhibit its own formation, or the enzyme’s behavior depends on substrate concentration?
Not all enzymes follow Michaelis-Menten kinetics. For example, allosteric enzymes – which have at least two substrate sites – do not follow Michaelis-Menten; when the first substrate binds to the enzyme it alters the enzyme’s affinity for the second enzyme, often via a conformational change. To learn more about allosteric kinetics, I would recommend looking into the resources below. However, bear this in mind: kinetics is fascinating, but not for the faint of heart!
Atkins WM. Michaelis-Menten Kinetics and Briggs-Haldane Kinetics. http://depts.washington.edu/wmatkins/kinetics/michaelis-menten.html.
Costa LG. Toxic Effects of Pesticides. In: Klaassen CD, ed. Casarett and Doull’s Toxicology: The Basic Science of Poisons. 7th ed. New York, NY: The McGraw-Hill Companies; 2006:883-930.
Robyt JF, White BJ. Biochemical Techniques Theory and Practice. Prospect Heights, IL: Waveland Press, Inc.; 1990:291-320.
Voet D, Voet J. Biochemistry. 3rd ed. Hoboken, NJ: John Wiley & Sons; 2004:459-546.