• Write your names or group number on each page of the exam you turn in.
• You may refer to your notes, course reader, handouts, or graded homework assignments. (Wireless laptop computers and textbooks cannot be used.)
• Please read the question carefully and make sure that you have thought it through with everyone’s input before converging on a solution.
• If you do not agree with your group, you may submit the examination under your own name for separate grading.
1. Hemoglobin was one of the first proteins crystallized and one of the first proteins to have its three-dimensional structure determined by X-ray crystallography. Appropriately for Valentine’s Day, the cover of this year’s February 14th issue of Science displays a bright red crystal of Hemoglobin C, a human hemoglobin variant most common in West Africa. In maximum dimension it is about 0.6 mm, much smaller than Zinoffsky’s crystals of horse hemoglobin and produced in a much more controlled way using vapor diffusion. The procedure is as follows: a small drop (microliters) of a purified and concentrated, but not supersaturated, protein sample dissolved in some aqueous solution (e.g. polyethylene glycol in a buffer), is placed on see-through lid (“hanging drop”) of a sealed chamber containing a much larger volume of a more concentrated, the same solution without protein. The sample is kept in a refrigerator and examined periodically through the lid with a microscope. If the conditions are right, crystals will form in a few days as the system come to equilibrium. Based on the information provided. [Polyethylene glycol is a water-soluble, non-volatile, high molecular weight polymer used to precipitate proteins.]
A. Draw a picture of the crystallization set up.
B. Using words and illustrations as needed, describe conceptually what happens in the hanging drop over time and why this is a gentle and controlled way to obtain protein crystals.
From Science 299, 1023 (2003)