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Your body burns up a lot of food every day. However, cells don’t burn food like a fireplace. Instead, food molecules are combined with oxygen molecules one-by-one, in many carefully controlled steps. In this way, the energy that is released can be captured in convenient forms, like ATP or NADH, which are then used elsewhere to power essential cellular functions. Our cells get most of their energy from a long series of reactions that combine oxygen and glucose, forming carbon dioxide and water, and creating lots of ATP and NADH in the process.
Citrate synthase is a central enzyme in this process of sugar oxidation. It is the first step of the citric acid cycle, also known as the Krebs cycle. Glucose has previously been broken into several pieces by glycolysis, releasing two carbon atoms as carbon dioxide and leaving the rest as two molecules of acetate, carried in an activated form on special cofactor molecules. In the citric acid cycle, these remaining carbon atoms are fully oxidized to form carbon dioxide. Citrate synthase starts this process by taking the molecules of acetate and attaching them to oxaloacetate, which acts as a convenient handle as the carbon atoms are passed from enzyme to enzyme in the citric acid cycle.
More info by David Goodsell at the PDB, here.
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