 The specific sequence of amino acids results in a unique three @-@ dimensional structure for that protein , and the three @-@ dimensional structures of proteins are related to their functions . Some are simple structural molecules , like the fibers formed by the protein collagen . Proteins can bind to other proteins and simple molecules , sometimes acting as enzymes by facilitating chemical reactions within the bound molecules ( without changing the structure of the protein itself ) . Protein structure is dynamic ; the protein hemoglobin bends into slightly different forms as it facilitates the capture , transport , and release of oxygen molecules within mammalian blood . 
