ANSC 221v - SAG 106

PROTEINS and Amino Acids

Definitions:

• Proteins are very large, complex molecules made up of amino acids
• Proteins are composed of the elements C, H, O, N and usually S and P
• Proteins are characterized by the amount of Nitrogen (N) they contain. Since it would be complicated and expensive to determine protein per se, instead we just analyze for the amount of N and because proteins on the average contain 16% N, we multiply amount of N X 6.25 (1/16=6.25)
• Proteins are very complicated, with the exact stereochemistry of the molecule being important. There is
  • primary structure
    • number and exact order of the amino acids contained (of which there are about 20, so the complexity is roughly that of the alphabet)
  • Secondary structure
  • the polypeptides align themselves along one dimension in the form of a right-turning alpha helix, stabilized by hydrogen bonding between CO and NH groups, which then intertwine with each other to form a supercoil, much like a rope.
  • Tertiary structure
  • the polypeptides are folded and tightly coiled into a globular form.
  • Quaternary structure
  • several tertiary structures align into one protein.

  Proteins are complex substances of high molecular weight. Altering just one amino acid in a whole string of thousands, or changing the spot of a fold, can make the whole difference between a protein being active or inactive.

  The individual characteristics of proteins are what distinguish one individual from another. Your genetic makeup dictates your proteins, which in some cases are unique to you, so that your immune system can recognize you from anyone else.

Amino Acids

About 20 amino acids are found in proteins. Of these, the body is able to manufacture some, but not others.

The amino acids the body cannot manufacture, or cannot make fast enough for normal function, are called essential amino acids or indispensible amino acids. For the rat (and pig) there are 10 essential amino acids.

The essential amino acids are:

• phenylalanine
• valine
• threonine
• tryptophan
• isoleucine
• methionine
• histidine
• arginine
• lysine
• leucine

The first letter of those amino acids, in that order, spell out the name PVT TIM HALL, which can be used as an aid to remember them. MEMORIZE THIS LIST and SPELL them CORRECTLY.

For the baby chick, PROLINE and GLYCINE are also essential.

The amino acid LYSINE is the most limiting for most animals in our common feeds, like corn, wheat, or grain sorghum. For the pig, the second limiting amino acid is tryptophan. For the chicken, because of the connective tissue in feathers, the second limiting amino acid is methionine which contains sulfur.

Structure of amino Acids

All amino acids contain amino groups, NH₂. They are connected to the alpha carbon, which is the one next to the COOH, or carboxyl group (which is what makes it an acid).

Hence, the structure for all amino acids can be represented as (where "R" stands for "something - ANYTHING - else" is:

     NH2-CH-COOH
         |
         R

When amino acids are hooked together to form a chain, they are connected through a peptide bond, with the OH from the COOH in one amino acid and an H connected to N in the other being removed as HOH, or water and the linkage connecting N to C.

         -------
   HCH-NH|H  HO|OC-CH2-NH2
         -------