 Serum (or plasma) albumin is the major protein component in the serum of vertebrates. It constitutes approximately 50% of the total protein in blood plasma. Serum albumin is a single polypeptide chain with a molecular weight of approximately 66,000 D, and is highly soluble in water. Serum albumin plays a large role in the maintenance of osmotic pressure in the blood and also serves to transport free fatty acids. We will use serum albumin as an example to discuss a number of the principles of protein structure and folding, and we will also be interactively learning how to use the graphics program Chime to explore the three-dimensional organization of proteins. Most of what we will explore in this segment is derived from X-ray crystallography and a knowledge of the amino acid sequence of serum albumin. In particular, we will be utilizing information about human serum albumin.  Units of protein structure are amino acids. There are 20 types of amino acids commonly found in proteins. All amino acids have the basic structure H2NCHRCOOH, with the R representing a side chain group. Each of the amino acids has a different side chain. It is the side chain that gives each amino acid its characteristic properties, such as polar, hydrophobic, charged, aromatic, acidic, basic, etc.
 The amino acids are linked covalently through the alpha-carboxyl and alpha-amino groups to form an unbranched polypeptide. The sequence of amino acids in this polypeptide is termed the primary structure or sequence, and is genetically determined.
The way a single polypeptide chain folds up in three-dimensional space is governed by the amino acid sequence of the protein. 
We will begin our examination of human serum albumin by looking at the overall shape of the molecule:
 This view of human serum albumin (HSA) depicts the protein in a "space-filling" mode. Such a view is useful in providing an overall feel for the molecule. We can see that it is a knobby structure, a bit like an asteroid, and semi-spherical in nature. It is classed as a "globular" protein.
Clicking on the button brings up the same view of the molecule in the interactive 3D program "Chime." 
  
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