Nutrition Science and Everyday Application - beta v 0.1

PROTEIN STRUCTURE 309
Essential
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Nonessential
Alanine
Arginine*
Asparagine
Aspartic Acid
Cysteine*
Glutamic Acid
Glutamine
Glycine*
Proline*
Serine
Tyrosine*
*Conditionally essential
Table 6.1. Essential and nonessential amino acids
Sometimes during infancy, growth, and in diseased states, the body cannot synthesize
enough of some of the nonessential amino acids and more of them are required in the diet.
These types of amino acids are called conditionally essential amino acids.
The nutritional value of a protein is dependent on what amino acids it contains and in
what quantities. As we’ll discuss later, a food that contains all of the essential amino acids in
adequate amounts is called a complete protein source, whereas one that does not is called an
incomplete protein source.
THE MANY DIFFERENT TYPES OF PROTEINS
There are over 100,000 different proteins in the human body. Proteins are similar to
carbohydrates and lipids in that they are polymers (simple repeating units); however,
proteins are much more structurally complex. In contrast to carbohydrates, which have
identical repeating units, proteins are made up of amino acids that are different from one
another. Different proteins are produced because there are 20 types of naturally occurring
amino acids that are combined in unique sequences.
Additionally, proteins come in many different sizes. The hormone insulin, which regulates
blood glucose, is composed of only 51 amino acids. On the other hand, collagen, a protein
that acts like glue between cells, consists of more than 1,000 amino acids. Titin is the largest
known protein. It accounts for the elasticity of muscles and consists of more than 25,000
amino acids!
The huge diversity of proteins is also due to the unending number of amino acid
sequences that can be formed. To understand how so many different proteins can be made
from only 20 amino acids, think about music. All of the music that exists in the world has
been derived from a basic set of seven notes C, D, E, F, G, A, B (with the addition of sharps
and flats), and there is a vast array of music all composed of specific sequences from these