I have also begun to look at the functions of and relationships between amino acids, not because I need to know it for the quiz, but because I am curious. On Wikipedia I ran across the Protein Digestibility Corrected Amino Acid Score (PDCAAS) which was adopted by the FDA in 1993 as the latest greatest measure of amino acid digestion from various food sources.
The four food sources that scored the highest in this scale are: whey, egg white, casein and milk. Mercola promotes whey as his favorite protein supplement, and sells powders made of it for your smoothies. We've always known that eggs are good for you--best when raw, it turns out. My mother made me drink my milk. But I had no idea what casein is, so I had to look it up.
Casein is used by weight lifters because it is a slowly digested source of amino acids, relative to the whey which is quickly absorbed. Casein is the predominant phosphoprotein found in milk and cheese, and it is fairly hydrophobic. It is a salt of calcium when in milk, or more specifically it forms a suspension of casien micelles.
Casein contains lots of proline peptides. Proline is an amino acid with a non-aromatic cyclical structure that does not interact with neighboring peptides. So normally proline causes a bend in the primary structure of a protein. When the primary structure bends, the segments of usually amino acids bond to each other with disulphide bridges, yielding secondary and tertiary protein structures. It is the secondary and tertiary structures that give proteins their functionalities. But casien has too much proline (which doesn't interact chemically) and doesn't form disulphide bridges (I'm guessing it lacks the sulfur-containing aa's: cysteine and methionine) so it has little secondary or tertiary structure. Since it already doesn't have higher levels of structural organization, it cannot denature and does not coagulate when heated. It does precipitate in acid (and rennic enzymes from cow stomachs), and trypsin can cleave off phosphate containing bits.
Trypsin is an intestinal enzyme that breaks down proteins. Enzymes are proteins with secondary and tertiary structures such that they have functions, such as that of an enzyme--to catalyze a chemical reaction. An aspartate residue (one aa) in the catalytic "pocket" of trypsin attracts positively charged lysine and/or arginine (aa's) and cleaves the polypeptide strand on the carboxyl side of them. Lysine and arginine are two aa's that when out of balance cause outbreaks of herpes simplex virus. Supplementation of lysine tends to prevent or reduce outbreaks in carriers of the virus.
Is aspartate negatively charged? YES. It has two carboxyl groups to one amine.
But back to casein--A casein-free diet is used by 26.8% of parents as a therapy for autistic children (Green 2006). Autistic children placed on a casein-free diet for eight weeks showed significant behavior improvements (Lucarelli 1995).
I have learned this one thing about learning. When I learn something, its relations in the universe begin to appear. The more I learn, the more obvious it is that everything is interconnected.