English to Portuguese Medical Medical (general)

PROTEOGLYCAN
Many types of PGs are found in cartilage. Fundamentally, it is a large protein-polysaccharide molecule composed of a protein core to which one or more glycosaminoglycans (GAGs) are attached (Fosang and Hardingham, 1996; Muir, 1983; Ratcliffe and Mow, 1996). Even the smallest of these molecules, biglycan and decorin, are quite large (approximately 1 × 104 mw), but they comprise less than 10% of all PGs present in the tissue. Aggrecans are much larger (1–4 × l06 mw), and they have the remarkable capability to attach to a hyaluronan molecule (HA: 5 × 105 mw) via a specific HA-binding region (HABR). This binding is stabilized by a link protein (LP) (40–48 × 103 mw). Stabilization is crucial to the function of normal cartilage; without it, the components of the PG molecule would rapidly escape from the tissue (Hardingham and Muir, 1974; Hascall, 1977; Muir, 1983).
Two types of GAGs comprise aggrecan: chondroitin sulfate (CS) and keratan sulfate (KS). Each CS chain con tains 25 to 30 disaccharide units, whereas the shorter KS chain contains 13 disaccharide units (Muir, 1983). Aggrecans (previously referred to as subunits in the American literature or as monomers in the UK and European litera ture) consist of an approximately 200-nanometer-long protein core to which approximately 150 GAG chains, and both O-linked and N-linked oligosaccharides, are covalently attached (Fosang and Hardingham, 1996; Muir, 1983). Furthermore, the distribution of GAGs along the protein core is heterogeneous; there is a region rich in KS and O-linked oligosaccharides and a region rich in CS (Fig. 3-6A). Figure 3-6A depicts the famous “bottle-brush” model for an aggrecan (Muir, 1983). Also shown in Figure 3-6A is the heterogeneity of the protein core that con tains three globular regions: Gl, the HABR located at the N-terminus that contains a small amount of KS (Poole, 1986) and a few N-linked oligosaccharides, G2, located between the HABRand the KS-rich region (Hardingham et al., 1987), and G3, the core protein C-terminus. A 1:1 stoichiometry exists between the LP and the G1 binding region in cartilage. More recently, the other two globular regions have been extensively studied (Fosang and Hardingham, 1996), but their functional significance has not yet been elucidated. Figure 3-6B is the accepted molecular conformation of a PG aggregate; Rosenberg et al. (1975) were the first to obtain an electron micrograph of this molecule (Fig. 3-6C). Atomic force microscopy (AFM) techniques have been used to image (Seog et al., 2005) as well as to measure the net intermolecular interaction forces (Ng et al., 2003) of proteoglycans.