5PEP:
reference: X-ray analyses of aspartic proteinases. II. Three-dimensional structure of the hexagonal crystal form of porcine pepsin at 2.3 A resolution., Cooper JB, Khan G, Taylor G, Tickle IJ, Blundell TL, J Mol Biol 1990 Jul 5;214(1):199-222. PMID: 2115088

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