Proteins are the workhorses of the cell (Figure 4.1): They provide structural support in the cytoskeleton, facilitate communication with other cells by acting as receptors, neutralize foreign pathogens, generate contraction forces in muscle, and most ubiquitously catalyze chemical reactions. Proteins are abundant in biological systems, such as eggs (Figure 4.2). Proteins are one of the major macronutrients in the human diet (Figure 4.3).
Some recombinant proteins now serve as therapeutic drugs for treatment or prevention of disease. Biomedical engineers also use recombinant proteins, such as growth factors, to promote growth and differentiation of cells in engineered tissues. Some biomedical engineers have been using techniques of protein engineering to design new biomaterials for use in tissue engineering, drug-delivery systems, or other medical applications. Biomedical engineers working in the area of systems biology are developing models for the function of signaling networks in cells: these signaling networks are interconnected sets of biochemical reactions, which happen inside cells and which are controlled largely by intracellular proteins.
This chapter describes the structure and function of proteins and also includes a brief introduction to some of the techniques used to determine protein structure, chiefly nuclear magnetic resonance (NMR) and x-ray crystallography. Researchers in the pharmaceutical industry use these protein structures in structure-guided drug design. Chemicals that interfere with protein function have long been used as drugs, but traditionally these chemical-protein interactions have been discovered empirically (or by accident). Now, chemical agents can be designed rationally, based on a detailed knowledge of the protein's structure, to interact with enzymes or receptors and enhance or inhibit their function. Because the structure of a protein determines its function, and because chemicals that interact with specific structural units within proteins can be useful as drugs, biomedical engineers and computer scientists are developing computer programs that will help predict the three-dimensional structure of a protein based on its primary amino acid sequence.