Globular proteins immune system

Some polypeptide chains fold into two or more compact regions that may be connected by a flexible segment of polypeptide chain, rather like pearls on a string. These compact globular units, called domains, range in size from about 30 to 400 amino acid residues. For example, the extracellular part of CD4, the cell-surface protein on certain cells of the immune system to which the human immunodeficiency virus (HIV) attaches itself, comprises four similar domains of approximately 100 amino acids each (Figure 3.47). Often, proteins are found to have domains in common even if their overall tertiary structures are different. 

In general, there are two types of protein structures (1) fibrous, elongated proteins which are not soluble in water and provide structural support and (2) globular spherical proteins which are water soluble and have specific functions in the immune system and metabolism. 

Proteins are relatively complicated molecules made up from the 20 naturally occurring a-L-amino acids, which are linked to each other via peptide bonds. Fibrous proteins give mechanical strength to bones and muscles. Globular proteins such as antibodies and enzymes have specific functions in the immune system and in metabolism. The macromolecular chain of a protein is folded in a very specific way. This folding is essential for the protein s function and activity. The sequence of the amino acids is referred to as the primary structure of the protein. Parts of the amino acid chain form domains of regular structures such as a-helices and /3-pleated sheets. These make up the secondary structure of the protein. The whole three-dimensional shape of the amino acid chain including interactions between different secondary domains is referred as the tertiary structure. Some proteins consist of... 

Collagen-like triple helices also occur within other proteins. One of these is protein Clq, a component of the complement system of blood (Chapter 31). This protein interacts with antibodies to trigger a major aspect of the immune response. Clq has six subunits, each made up of three different polypeptide chains of about 200 residues apiece. Beginning a few residues from the N termini, there are over 80 residues in each chain with collagen-like sequences. The three chains apparently form a triple helix within each subunit. However, the C-terminal portions are globular in nature.200 Collagen-like tails also are present on some forms of the enzyme acetylcholinesterase (see Chapter 12C,10). Tire extensins of plant cell walls contain 4-hydroxyproline and evidently have a structure... 

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