1965

Protein Crystallography

John Desmond Bernal (1901–1971), Dorothy Crowfoot Hodgkin (1910–1994), David Chilton Phillips (1924–1999)

The enzyme lysozyme—which takes its name from the Greek word lysis, meaning “loosening”—had been known about for some time when British biophysicist David Chilton Phillips tried to solve its structure by X-ray crystallography. Alexander Fleming (of penicillin fame) named it in 1923, and it was known as the reason egg whites and tears showed antibacterial properties. That’s the role it has in humans as well. As part of the innate immune system, it attacks compounds called peptidoglycans, which are found in the cell walls of bacteria. It’s also found in saliva where it degrades starch, which is why a piece of popcorn disappears so rapidly when it touches your tongue.

Applying X-ray crystallography to proteins was a big step, since they were much larger and more complex than the usual molecules examined by this technique. British X-ray crystallographers John Desmond Bernal and Dorothy Crowfoot Hodgkin showed that protein diffraction was possible in 1934, but the first structure solved was myoglobin in 1958 by Phillips. His next target was an enzyme. It was thought that a three-dimensional structure might provide insights into how enzymes were able to speed up chemical reactions so dramatically.

Lysozyme had many things in its favor as a target—it’s easily available, and it crystallizes well, which is a step that even today can be something of a black art in protein work. Solving the diffraction pattern was a huge effort with the computational resources available, but lysozyme’s structure was determined, as were somewhat fuzzier structures of lysozyme with inhibitor molecules inside its active site. Crystallizing proteins in this way now provides countless insights into biochemistry and drug discovery (since most pharmaceuticals work by binding to specific protein sites). There are pitfalls, naturally. A crystal structure is a static shot of a mobile protein, a picture that may or may not reflect what goes on out in the real world where proteins can move and shift. But there’s always NMR to fall back on, which can sometimes be used to solve protein structures in solution.

Lysozyme crystals as seen through a polarized-light microscope. These well-formed blocklike crystals are ideal for X-ray crystallographic studies, but few proteins form them as well as lysozyme does.