Enzyme Biochemistry Practice Problems

You are a research scientist studying a novel enzyme X, and you want to characterize this new enzyme. You measure the velocity of the reaction with different substrate concentrations and get the following data:
```
    [substrate] (mM)                Initial Velocity (mmol/min)
---------------------------------------------------------------------
             3.0                            10.4
             5.0                            14.5
            10.0                            22.5
            30.0                            33.8
            90.0                            40.5
```
- a) Graph the above data. From the graph, estimate KM.
- b) Calculate Vmax. Show any equations and calculations.
- c) Is X an allosteric enzyme? Explain.
- d) You decide to do this experiment again, but this time with only one third of the enzyme X concentration used in the first experiment. Draw a new graph on the same graph that you did the first graph on. Estimate Km and Vmax from the new graph.
- e) You wish to find the amino acid sequence of the enzyme X. What methods might you use to determine this? Name at least three.
Five reaction mixtures containing 10 x 10-9 M of an enzyme are made up with five different substrate concentrations and the initial rates of the reactions were measured:
- a) Determine Km and Vmax, using a Lineweaver-Burke plot.
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- b) Suppose the experiment were repeated with five times the amount of enzyme. How would Km and Vmax be affected?
- c) A poison is added to the reaction mixture that is known to bind the enzyme at a site different from where the substrate binds. The poison and the substrate can bind the enzyme simultaneously but the poison prevents the formation of product. Qualitatively, how does the poison affect Km and Vmax?
In certain bacteria, the biosynthesis of aromatic amino acids (phenylalanine, tyrosine, and tryptophan) proceeds by a branched pathway where some intermediates are used to make more than one amino acid. The pathway and some key intermediates are shown below.

This pathway is regulated by feedback inhibition at enzymes a through e. For parts a through e, list the intermediates and products shown which you might expect to be inhibitors of each enzyme. Explain your answer briefly.
Lactose is a disaccharide found in milk. Although in the United States we are told that milk, it does a body good, many adults throughout the world get sick from drinking milk because they cannot digest lactose. Lactose intolerance varies markedly among various human populations. (For example, only about 3% of people of Danish descent are lactose intolerant, compared with 97% of people of Thai descent.) When someone who is lactose intolerant ingests milk, the lactose accumulates in the lumen of the small intestine because there is no mechanism for uptake of the disaccharide. This causes abdominal distension, cramping, and watery diarrhea.
- a) Why can't lactose diffuse across the membranes of the intestinal epithelial cells in the absence of a carrier-mediat