Probando tutory



3.5 WHAT ARE PROTEINS?



Proteins are composed of one or more chains of . Proteins perform many functions; this diversity of function is made possible by the diversity of protein structures. Most cells contain hundreds of different enzymes, which are proteins that guide almost all the chemical occurring inside cells, as you will learn in Chapter 6. Other types of proteins are used for structural purposes. These include elastin, which gives skin its elasticity; keratin, the principal protein of hair, horns, nails, scales, and feathers; and the silk of spider webs and silk moth cocoons (Fig. 3-17). Still other proteins provide a source of amino acids for developing young animals, such as albumin protein in egg white and casein protein in milk. The protein hemoglobin transports oxygen in the blood, while contractile proteins, such as those in muscle, allow both individual cells and entire animal bodies to move. Some hormones, including insulin and growth hormone, are proteins, as are antibodies (which help fight disease and infection) and many poisons (such as rattlesnake venom) produced by animals.