After studying this chapter, you should be able to:
Appreciate the importance of the extracellular matrix (ECM) and its components in health and disease.
Describe the structural and functional properties of collagen and elastin, the major proteins of the ECM.
Indicate the major features of fibrillin, fibronectin, and laminin, other important proteins of the ECM.
Describe the properties and general features of the synthesis and degradation of glycosaminoglycans and proteoglycans, and their contributions to the ECM.
Give a brief account of the major biochemical features of bone and cartilage.
Most mammalian cells are located in tissues where they are surrounded by a complex ECM often referred to as “connective tissue,” which protects the organs and also provides elasticity where required (eg, in blood vessels, lungs, and skin). The ECM contains three major classes of biomolecules: structural proteins (1), for example, collagen, elastin, and fibrillin, certain specialized proteins (2) such as fibronectin and laminin, which form a mesh of fibers that are embedded in proteoglycans (3). The ECM has been found to be involved in many normal and pathologic processes—for example, it plays important roles in development, in inflammatory states, and in the spread of cancer cells. Involvement of certain components of the ECM has been documented in both rheumatoid arthritis and osteoarthritis. Several diseases (eg, osteogenesis imperfecta and a number of types of the Ehlers-Danlos syndrome) are due to genetic disturbances of the synthesis of collagen. Specific components of proteoglycans (the glycosaminoglycans; GAGs) are affected in the group of genetic disorders known as the mucopolysaccharidoses. Changes occur in the ECM during the aging process. This chapter describes the basic biochemistry of the three major classes of biomolecules found in the ECM and illustrates their biomedical significance. Major biochemical features of two specialized forms of ECM—bone and cartilage—and of a number of diseases involving them are also briefly considered.
COLLAGEN IS THE MOST ABUNDANT PROTEIN IN THE ANIMAL WORLD
Collagen, the major component of most connective tissues, constitutes approximately 25% of the protein of mammals. It provides an extracellular framework for all metazoan animals and exists in virtually every animal tissue. At least 28 distinct types of collagen made up of over 30 distinct polypeptide chains (each encoded by a separate gene) have been identified in human tissues (Table 50–1). Although several of these are present only in small proportions, they may play important roles in determining the physical properties of specific tissues. In addition, a number of proteins (eg, the C1q component of the complement system, pulmonary surfactant proteins SPA and SPD) that are not classified as collagens have collagen-like domains in their structures; these proteins are sometimes referred to as “noncollagen collagens.”
TABLE 50–1Types of Collagen and Their Tissue Distribution