Structure
Citrate synthase's 437 amino acid residues are organized into two main subunits, each consisting of 20 alpha-helices. These alpha helices compose approximately 75% of citrate synthase's tertiary structure, while the remaining residues mainly compose irregular extensions of the structure, save a single beta-sheet of 13 residues. Between these two subunits, a single cleft exists containing the active site. Two binding sites can be found therein: one reserved for citrate or oxaloacetate and the other for Coenzyme A. The active site contains three key residues: His274, His320, and Asp375 that are highly selective in their interactions with substrates. The image to the right highlights the three key amino acids of citrate synthase's active site in its open state (the substrate is absent). The specific atoms involved in interactions are designated by color, and both a drawing and video of their mechanism can be found in the section labeled "Mechanism" below. The images to the left display the tertiary structure of citrate synthase in its opened and closed form. The enzyme changes from opened to closed with the addition of one of its substrates (such as oxaloacetate).
Read more about this topic: Citrate Synthase
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