Lec35n - Lecture 37. Part I: Protein Structure. 6th Ed;...

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1 Lecture 37. Part I: Protein Structure. 6 th Ed; 313, 366 + Fig 19.10 7 th Ed.; 319, 380 8 th Ed. 336,441 Part II: Cancer. Campbell 6 th Ed.; pp. 368-371 7 th Ed.; 370-373 8 th Ed.; 373-376 Part I: Structural biology is the branch of biology concerned with figuring out the 3-dimensional structures of macromolecules. These days, most work is being done on proteins. Techniques like X-ray crystallography (used in the 1950’s to determine the structure of DNA) are very valuable for determining the structures of proteins even today. This is because each protein has a different structure, and it’s not possible to predict the structure of an entire protein by looking at its amino acid sequence. Proteins often (but not always!) consist of separate domains that fold independently. In some cases, these domains are encoded by separate exons. Each domain can contain combinations of different secondary structure elements: α -helices and β -sheets. These elements are arranged in a characteristic shape in each type of domain. That is, the hallmark of each domain is a characteristic 3-D shape. Different domains in one protein can have independent functions. For instance, one domain might have enzymatic activity (such as kinase activity), while another domain functions to bind another protein. Furthermore, the same domain is often found in different proteins. That is, proteins can be mix-and-match combinations of different domains. For instance, “Protein #1” may consist of 3 domains; A, B, and C. “Protein #2” may consist of 4 domains; 2 copies of A, B, and D. In this case, domains A and B are present in both proteins, while domains C and D are only in one of them. The same domain will have the same overall shape in the different proteins. In our example, we’ll be able to recognize a “Domain A” in both Protein #1 and Protein #2. In addition, the amino acid sequences of Domain A in the two proteins will probably be very similar. Importantly, however, they won’t be identical. As we’ll see, small changes in the same domain found in two different proteins can have important consequences for function. Individual exons can encode discrete domains. Removal of intervening introns by splicing joins sequences coding for the different domains, so they’re linked in the final protein. Over the course of evolutionary time, large-scale chromosomal rearrangements (discussed in the lecture on mutations) can shuffle different exons in the genome. As a result, exons that were originally parts of different genes can be brought together into the same gene, where they might code for different domains of the same protein. One way this could occur is by reciprocal translocation (breakage and rejoining of 2 non-homologous chromosomes to swap the ends), if both chromosomes broke at points that happened to be in the middle of introns. Such an event would combine the exons next to the breakpoints into the same gene, which would now contain exons from both genes. We’ll look at two examples of common domains, that are both found in many different
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This note was uploaded on 09/23/2010 for the course BIO 202 taught by Professor Dean during the Fall '08 term at SUNY Stony Brook.

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Lec35n - Lecture 37. Part I: Protein Structure. 6th Ed;...

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