People commonly associate protein with meat. Consequently people eat steak and chicken to build muscle but their thinking is only partially correct. Protein is a vital structural and working substances in all cells, not just muscle cells. Meat is a good source of protein but so are milk, eggs, legumes, grains and some vegetables. People who overvalue protein may overemphasize meat in their diets as the expense of other equally important nutrients and foods.

Chemically, proteins contain the same atoms as carbohydrates and lipids; carbon, hydrogen and oxygen. But proteins also contain nitrogen atoms which gives the name amino (nitrogen containing) to amino acids.

Amino acids have the same basic structure, a central carbon atom with a hydrogen, an amino group and an acid group attached to it. Carbon atoms need to form four bonds though so a fourth attachment is necessary. It is this fourth site that distinguishes each amino acids from the others. At this attachment a distinct atom or group of atoms is known as the side group or side chain. Even though all amino acids share a common structure they differ in size, shape, electrical charge and other characteristics because of the differences in these side groups.

More than half of the amino acids are nonessential, meaning that the body can synthesize them for itself when needed. Proteins in foods usually deliver these amino’s but it is not essential that they do so. Given nitrogen to form the amino group and fragments form carbohydrates and fat to form the rest of the structure, the body can make any nonessential amino acids. Some examples are:

Alanine
Arginine
Asparagine
Aspartic Acid
Cysteine
Glutamic Acid
Glutamine
Glycine
Proline
Serine
Tryosine

There are nine amino acids that the human body either cannot make at all or cannot make in sufficient quantity to meet its needs. These nine amino acids must be supplied by the diet and they are the essential amino acids. THey are:

Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine


There is also another group which has been recently been identified and that is the Conditionally Essential Amino Acids. Sometimes a nonessential amino becomes essential under special circumstances. For example, the body usually uses amino acid phenylalanine to make tyrosine but if the diet fails to supply enough phenylalanine, or if the body cannot make the conversion for some reason (in some diseases) then tyrosine becomes conditionally essential.

Cells link amino acids end-to-end in a virtually infinite variety of sequences to form thousands of different proteins. Each amino is connected to the next by a peptide bond. Two amino acids bonded together form a dipeptide. A third amino can be added to form a tripeptide. As additional amino acids join the chain, a polypeptide if formed. Most proteins are a few dozen to several hundred amino acids long.

The extraordinary and unique shapes of proteins enable them to perform their various tasks in the body. Some form hollow balls that carry and store materials within them and some such as those of tendons are more then ten times as long as they are wide to form strong rod like structures. Some polypeptides are functioning proteins as they are, while others need to associate with other polypeptides to form larger working complexes. Some proteins require minerals to activate them. One molecule of hemoglobin (large globular protein molecule that packs red blood cells and carries oxygen) is made of four associated polypeptide chains each holding the mineral iron.

When proteins are subjected to heat, acid, or other conditions that disturb their stability they undergo denaturation. That is, they uncoil and lose their shapes and consequently their ability to function. Past a certain point this is irreversible. Such examples are the curdling of milk and the stiffening of egg whites when they are whipped. Proteins in food do not become proteins directly. Instead they supply the amino acids form which the body makes its own proteins. When a person east foods containing proteins, enzymes break the long polypeptide strands into shorter strands, the shorter strands into tripepetides and dipeptides and finally into amino acids.


Part 2:

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