What are the different classes of amino acids and how do they relate to physiology?
The most common way of grouping the 20 major amino acids is based on the chemical properties of the side chain (or "R group").
The first group is the non-polar amino acids-Glycine Alanine, Valine, Cysteine, Proline, Leucine, Isoleucine, Methionine, Tryptophan, and Pheylalanine. These amino acids are generally high in hydrocarbon content, which makes them hydrophobic, meaning they are repelled from water. These amino acids will most commonly be found in the core of a protein away from surfaces and typically are a critical component of a protein's overall structure and stability.
The second group, is the basic (or + charge) group, which includes Lysine, Arginine, and Histidine. As suggested by the name, these groups are capable of acting as Bronsted bases, which means they are most stable when they take on a hydrogen and gain a positive charge. These side chains are hydrophillic, which means they will be more likly found on the surface. Due to their ability to take on charge, they are very common components of a protein's active site (to help catalyze a proton transfer for example).
Next, the acidic group (or - charge group) is aspartic acid and glutamic acid. Again, as the name would suggest, they act as bronsted bases and give up their proton to develop a negative charge. They will be found in similar places as the basic group members, though they will typically be involved in different reactions (or may require a combination of acid and base activity).
While the basic and acidic groups are technically also considered polar, "the polar group" includes the amino acids which are polar but do not have acid or base activity. (Note: some people may consider the acid and base groups as a subcategory of the larger polar group.) These amino acids are Serine, Threonin, Tyrosine, Asparagine, and Glutamine. Like the acid and base amino acids, the polar amino acids will most commonly be found at the surfaces of proteins, and may be important components of the active site.
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