On the surface of cells, there are many proteins. Some proteins are involved in recognition, attachment, or transportation. The citrate-malate shuttle system consists of citrate shuttle and malate shuttle, which are carrier proteins. Carrier proteins are present on the cell surface. They transport different molecules across the mitochondria. In this system, the substances being transported are malate and citrate.
The starting material is acetyl-CoA. It is a molecule that is involved in ATP synthesis, protein metabolism, and lipid metabolism. As the inner membrane is not permeable to this molecule, acetyl-CoA needs to be converted into other products for effective transport. It is also the first step of the reaction.