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What are Proteins?

Polymers (large molecules) are made up of individual units known as monomers (monomeric units) that are joined together. For proteins, the monomers are amino acids, composed of the elements carbon, hydrogen, oxygen, nitrogen and usually with sulphur. Occasionally there is iron, copper, and phosphorus, etc. About 16% of a protein is nitrogen by weight. 100 amino acids or more are a protein. Less than 100 amino acids are a peptide. Two amino acids joined together is a dipeptide. A tri-3, tetra-4 peptide. Polypeptide is more than ten amino acids.

Amino Acids

The building blocks or basic units of proteins are amino acids. There are 20 different amino acids needed to make our body’s proteins:  [essential to humans – threonine, isoleucine, leucine, valine, phenylalanine, tryptophan, methionine, lysine, histidine (essential for infants)], [non-essential  for humans – glycine, alanine, serine, tyrosine, proline, cysteine, aspartic acid, asparagine, glutamic acid, glutamine, arginine, histidine].

What does this mean for you? You have a need for eight essential amino acids in your diet each day. That is why so many people eat meat– it has all the essential amino acids in one serving. Peas are vegetable protein. Rice is a grain protein. Protein is found in almost all foods, but in association with carbohydrate and fat. Google “essential amino acids” or any one of them individually and see what happens. 

Amino acids are joined together by peptide bonds. The carboxyl group of one amino acid forms a covalent chemical bond with the amino group of an adjacent amino acid. In the process a molecule of water is released as the peptide bond is formed. The cells in the body assemble proteins by adding amino acids end to end via peptide bonds to make long chains. When the protein of the body is being broken down inside cells or when food protein is digested in the gastrointestinal tract, peptide bonds are broken by the addition of water molecules.

  • To make a protein you make peptide bonds between adjacent amino acids and release one molecule of water of each peptide bond.
  • To break peptide bonds, introduce one molecule of water for each peptide bond that is broken.

This is the give and take, the dance of life, with the come and go of positive and negative charges.

Non-essential amono acids glycine and alanine join to make the peptide bond glycylalanine. If water is released by the formation of a peptide bond, then water is required to be added when a peptide bond is broken.

Structure and Types of Proteins


Primary Structure: the sequence of amino acids in a protein. Linear arrangement, must be exact. Each protein is coded for in a gene. When one amino acid is substituted for another the resulting protein would likely be functionless.

Secondary and Tertiary Structure: the 3-dimensional arrangement of peptide chain or conformation of the protein molecule. This must be exact for biological function; easily disrupted– denatured by heat, acid, mechanical agitation. Examples: boiling egg or beating egg white.

Types of Proteins

Proteins can be classified according to function in the body:

  1. enzymes – catalytic
  2. hormones- regulation
  3. antibodies- protection
  4. structural- bone, cartilage, tendon, nails, hair, etc.
  5. contractile- muscle
  6. transport- hemoglobin, etc or proteins in cell membranes and substances transferred from one side to the other of the cell

The proteins had a function in the animal or plant from which they were derived.

Digestion and Absorption

Digestion of Protein:

Beginning in the stomach, an acidic medium, the low stomach pH helps denature the protein molecules, making them more susceptable to protein breakdown. There are 3 kinds of protein molecules that the digestive enzymes might attack: food protein, protein in intestinal cell, digestive enzymes.

Pepsin (stomach enzyme) breaks down peptide bonds in protein molecules adjacent to specific amino acids to produce peptides. In the small intestine, the pH slightly alkaline, protein digesting enzymes from the pancreas and intestinal cells attack other peptide bonds in the middle and at the ends of peptides. Net result is free amino acids. Proteins that are digested also come from sloughed-off intestinal cells and also the digestive enzymes themselves.

Digestability of proteins from a mixed diet averages 92%; meat and egg protein averages 97% while vegetable, fruit, cereal and nut protein 75-85% digested.

Absorption of Amino Acids:

Free amino acids active transport across intestinal cell wall. Carriers in cell membrane recognize specific groups of like amino acids. Absorbed amino acids enter capilliaries to larger veins to portal vein to liver.

Janet Wiebe