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Proteins

The word Protein comes from the Greek word 'proteios' meaning "of first importance." The world of proteins is vast, as protein molecules are involved in so many areas of life. Here are just a few of the many examples of proteins in action:

• Protein molecules are structural materials in muscles, skin, nails, hair and even bone.

• Insulin, for example, is a hormone made from protein that controls the uptake of glucose into cells. Most hormones are made of protein.
  

• Protein molecules called enzymes serve as catalysts in biochemical reactions. The function of a catalyst is to speed up or slow down a chemical reaction.
  

• Hemoglobin and myoglobin are examples of transport proteins, in this instance, transporting oxygen (O₂) gas molecules to cells. Antibodies, which are produced by our immune system and are the first line of defense against viruses and bacteria, are constructed from protein molecules. These are just a few of the many examples of proteins in action.

Amino Acids

Amino acids are the building blocks of protein molecules. Humans use about 20 different amino acids in constructing the various proteins we need. The following table gives the names of these amino acids:


Amino Acids Humans Use


Alanine (Ala) ________Glutamic acid (Glu) ___Leucine (Leu) _______Serine (Ser)

Arginine (Arg)_______ Glutamine (Gln)_______Lysine (Lys) ________Threonine (Thr)

Asparagine (Asn)_____Glycine (Gly) _________Methionine (Met) ____Tryptophan (Trp)

Aspartic acid (Asp)____Histidine (His) ________Phenylalanine (Phe)__ Tyrosine (Tyr)

Cysteine (Cys) _______Isoleucine (Ile)_________Proline (Pro) ________Valine (Val)



Amino acids all have a hydrogen atom (–H), an amino group (-NH₂) and a carboxyl group (–COOH) bonded to the same central carbon atom.


_____Representation of an Amino Acid
aminoacid_1

Also bonded to this carbon atom is either a hydrogen atom (as in the case of glycine) or side chain symbolized by the letter “R.” In the case of the amino acid called alanine, for example, R = CH₃. If the amino acid is serine, then R = CH₂OH and so on for the other amino acids.

Amino acids and therefore, proteins are largely composed of the following elements:

• Carbon (C)

• Oxygen (O)

• Hydrogen (H)

• Nitrogen (N)
  

A number of other elements are found in the structure of proteins as well.

• Most proteins, for example, contain Sulfur (S).

• Iron (Fe) is an element found in the O₂ transport proteins hemoglobin and myoglobin.

The simplest amino acid is Glycine:


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Below are the structures of the essential Amino Acids. The black spheres represent carbon atoms, red is oxygen, blue is nitrogen, yellow is sulfur and the white spheres are hydrogen atoms. Amino acids are the building blocks of proteins. Proteins are in turn, long chains of amino acids. The human body requires 20 different amino acids in order to function properly. Nine of the 20 amino acids are called "essential," which means that they must be supplied by our diet, since we cannot make them in the body. The remaining 11 amino acids are called "nonessential" because our body can make or "synthesize" them.

The 9 Essential Amino Acids

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__Histidine_________Isoleucine_________Leucine______________Lysine



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_Methionine_____Phenylalanine______Threonine______Tryptophan________Valine



You can find molecules like these and others at the following website:

http://www.iconbazaar.com/molecules/index.html





Making Proteins

Amino acids add one to another (known as peptide bonding) like the links of a chain. We call this the primary structure of the protein. It is the unique amino acid sequence for the protein being made. Some proteins or "polypeptides" as they are called can be a few amino acids long or hundreds to thousands of amino acids long.


__Making Proteins from Proteins

Once the primary structure of the protein is established the protein may find itself twisting into a tightly coiled spiral structure (alpha-helix) or forming sideways bonds with adjoining proteins (called a beta-pleated sheet) in a sort of zig-zag pattern. Fibrous proteins, such as collagen, keratin and elastin have this type of zig-zag formation. The alpha-helix and the beta-pleated sheet are examples of a protein's secondary structure.

Some proteins undergo an extensive folding of the polypeptide chain into a complex three-dimensional geometry. This is what scientists call the protein's tertiary (third) structure. Globular proteins are an example of this type of behavior.

When two or more polypeptide chains fold together in such a way that the new protein behaves like a single entity, then the protein is said to exhibit a quaternary structure. For example, hemoglobin has a quaternary structure.

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