bonding". In a way this continued work on the nature of the chemical bond by his doctoral advisor at the California Institute of Technology, Linus Pauling, who was awarded the 1954 Nobel Prize in Chemistry "for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances." The source for about half of this section is Lipscomb's Nobel Lecture. Large biological molecule structure and function Lipscomb's later research focused on the atomic structure of proteins, particularly how enzymes work. His group used x-ray diffraction to solve the three-dimensional structure of these proteins to atomic resolution, and then to analyze the atomic detail of how the molecules work. The images below are of Lipscomb's structures from the Protein Data Bank displayed in simplified form with atomic detail suppressed. Proteins are chains of amino acids, and the continuous ribbon shows the trace of the chain with, for example, several amino acids for each turn of a helix. Carboxypeptidase A (left) was the first protein structure from Lipscomb's group. Carboxypeptidase A is a digestive enzyme, a protein that digests other proteins. It is made in the pancreas and transported in inactive form to the intestines where it is activated. Carboxypeptidase A digests by chopping off certain amino acids one-by-one from one end of a protein. The size of this structure was ambitious. Carboxypeptidase A was a much larger molecule than anything solved previously. Aspartate carbamoyltransferase. (right) was the second protein structure from Lipscomb's group. For a copy of DNA to be made, a duplicate set of its nucleotides is required. Aspartate carbamoyltransferase performs a step in building the pyrimidine nucleotides (cytosine and thymidine). Aspartate carbamoyltransferase also ensures that just the right amount of pyrimidine nucleotides is available, as activator and inhibitor molecules attach to aspartate carbamoyltransferase to speed it up and to slow it down. Aspartate carbamoyltransferase is a complex of twelve molecules. Six large catalytic molecules in the interior do the work, and six small regulatory molecules on the outside control how fast the catalytic units work. The size of this structure was ambitious. Aspartate carbamoyltransferase was a much larger molecule than anything solved previously. Leucine aminopeptidase, (left) a little like carboxypeptidase A, chops off certain amino acids one-by-one from one end of a protein or peptide. HaeIII methyltransferase (right) binds to DNA where it methylates (adds a methy group to) it.