The big reveal: Beta galactosidase and cryo-EM

My final Structure of Christmas may look like an unremarkable enzyme, but it heralded the arrival of a game-changing method in structural biology.

My ninth (and final) Structure of Christmas is beta-galactosidase: a pretty run-of-the-mill enzyme that turns compound sugars into monosaccharides. When you put a special dye on it, it turns the dye blue (whee!). It’s a mainstay of molecular biology and millions of students have used it in countless experiments, both fascinating and mundane. It doesn’t have much of a ‘wow’ factor – it’s a solid member of a respectable family of sugar-cleaving enzymes.

What is so special about it is the way its structure was determined.

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Tropomyosin and actin: Move!

My penultimate structure of Christmas is actually two molecular partners, which work together to make muscle move.

Most of my Christmas structures have been separable units – some large, some small – that float around in cells or cell membranes. But to move physically, organisms need to have more at their disposal than some things floating in solution. For most life forms, movement is managed by proteins working together. A perfect example of this is the beautiful partnership between actin and tropomyosin.

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Antibodies: Defend!

Once a parasite makes it past our outer defences, it encounters some seriously sophisticated weaponry. One of these is the ever-shifting antibody, my seventh structure of Christmas.

Every large organism – you included – is just a feast laid out for any parasite (bacteria, virus or beastie) clever enough to break in and access its carefully amassed energy. Throughout the Billion-Years’ Evolutionary War between hosts and parasites, the host has always been on the defensive, endlessly innovating to fend off invaders.

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The twilight world between chemistry and life

Viruses live in a twilight zone, somewhere between life and its ingredients. My fifth structure of Christmas emerges from that zone to wreak havoc on cattle: the foot-and-mouth-disease virus.

Consider the virus: a beautifully crafted set of molecules perfectly arranged to do one thing, and one thing only: subvert life forms to make more of itself. But what is it? Is it ‘alive’, in the conventional sense?

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