Catabolite activator protein

Catabolite Activator Protein or CAP is a transcriptional activator that exists as a homodimer in solution, with each subunit comprising a ligand-binding domain at the N-terminus (CAPN, residues 1-138), which is also responsible for the dimerization of the protein, and a DNA-binding domain at the C-terminus (DBD, residues 139-209). Two cAMP (cyclic AMP) molecules bind dimeric CAP with negative cooperativity and function as allosteric effectors by increasing the protein's affinity for DNA. Cytosolic cAMP levels rise when the amount of glucose transported into the cell is low, but lactose is readily available.

CAP has a characteristic helix-turn-helix structure which allows it to bind to successive major grooves on DNA. The two helices are reinforcing each causing a 43° turn in the structure so overall causing a 94° degree turn in the DNA.

This opens the DNA molecule up, allowing RNA polymerase to bind and transcribe the genes involved in lactose catabolism. CAP thus enhances the expression of the lac operon when lactose is present but not glucose.

This requirement reflects the greater simplicity with which glucose may be metabolized in comparison to lactose. The cell "prefers" glucose, and if it is available, the lac operon is not activated, even when lactose is present.