Most Commonly Missed Concept #1: Michaelis-Menten Kinetics
Overview: After Amino Acids and the Metabolic Pathways, Michaelis-Menten Kinetics is easily the next most commonly tested topic on the MCAT. In fact, you are likely to see about as many Michaelis-Mentin Kinetics questions on test day as you are to see Organic Chemistry questions. Yet, Kaplan devotes an entire book to Organic Chemistry and only part of a chapter to Michaelis-Menten Kinetics. That is the primary purpose of this lesson as well as the primary purpose of this course. My goal is to cover some of the most commonly tested (yet missed) concepts, and Michaelis-Menten Kinetics easily ranks at the top of the list. You’ll want to know this lesson inside and out.
For starters, what is the purpose of Michaelis-Menten Kinetics? Simply put, the goal is to characterize the speed at which an enzyme is able to convert a substrate into its product. Remember that enzymes will bind to their substrate and then convert it into its product(s) as seen in the following simplified diagram:
One reason that Michaelis-Menten Kinetics can be so hard for students to grasp is that professors, books, and videos make the topic overly complicated by trying to teach you the derivations of the various equations and charts, etc. Let me fill you in on a little secret — The MCAT could care less about these useless details! 🙂 We are going to maintain our focus on the essential features of this topic, the stuff that the MCAT will actually test you on.
With our basic understanding of this enzymatic process, let’s take a look at the standard Michaelis-Menten graph (see image to the right). On the x-axis, you have the substrate concentration ([S]), and on the y-axis, you have the initial velocity of the reaction (Vo). The purpose of this graph is to determine how much substrate we need to add before this reaction will no longer proceed any faster. At what point will the enzymes not be able to convert the substrate into product any faster? It is at this point that all the enzymes will be “saturated” (filled) with substrate. This is what we call Vmax, the maximum speed of the reaction. On the graph, Vmax is where the hyperbolic curve flattens out.
Keep in mind that at Vmax, adding more substrate will do nothing because all the enzymes are saturated. Typically the only way to increase Vmax is to increase the amount of enzyme. This is a well-tested idea, so don’t forget it — Increasing the substrate concentration will NOT increase Vmax.
Something that can be hard for students to grasp is the idea of Km, the Michaelis-Menten constant. Let’s lay it out nice and simple. Km is the…
Associated Questions from the AAMC:
- Question 7 of the Biology/Biochemistry Section of the Official Guide Questions.
- Questions 14 and 30 of the Chemistry/Physics Section of the Official Guide Questions.
- Questions 33, 54, 55, 73, 75, and 90 of the Biology/Biochemistry Section of the Section Bank.
- Questions 30, 33, 39, 48, 49, 50, 55, 56, 75, 77, 95, and 99 of the Chemistry/Physics Section of the Section Bank.
- Question 20 of the Chemistry/Physics Section of the Sample Test.
- Questions 23 through 25 of the Biology/Biochemistry Section of the Sample Test.
- Question 8 of the Chemistry/Physics Section of Practice Exam 2.