If the protein includes more than one amino acid chain, then tertiary structure is the three-dimensional structure of the component chain. The picture below illustrates one model of the tertiary structure of a protein called hexokinase. In the model, each round ball stands for one atom, and adjacent atoms are attached to each other. The four units are stuck together by amino acid interactions in a distribution which leaves room for heme groups, which bind oxygen and to carry to cells. The shapes and interactions of proteins allow them to perform the tasks which keep the cell alive.
Proteins can also act as enzymes , which transform certain chemical compounds into other compounds.