Chemistry of Life » Proteins

Proteins consist of a number of sub units called AMINO ACIDS. In fact, if we want to be really accurate and pedantic alpha amino acid residues. Although the number of possible amino acids could run into hundreds of thousands life has only evolved to use 20 different ones. They are 19 coded for (and also listed below) and cysteine which is made by joining two cystine units together to form S-S sulphur-sulphur bridges. In their turn amino acids are made up of different arrangements of carbon, oxygen, nitrogen and hydrogen atoms (and in three cases methionine, cystine and cysteine sulphur is also present. The amino acids coded for by DNA are shown below.

  1. Alanine - Ala,
  2. Arginine - Arg,
  3. Aspartic acid - Asp,
  4. Asparagine - Asn,
  5. Cystin - Cys,
  6. Glutamic acid - Glu,
  7. Glutamine - Gln,
  8. Glycine - Gly,
  9. Hisidine - His,
  10. Isoleucine - Ile,
  11. Leucine - Leu,
  12. Lycine - Lys,
  13. Methionine - Met,
  14. Phenylalanine - Phe,
  15. Proline - Pro,
  16. Serine - Ser,
  17. Threonine - Thr,
  18. Tryptophane - Try and
  19. Tyrosine - Tyr.

The two simplest amino acids, glycine and alanine, can be shown as

where H=Hydrogen C=carbon N=Nitrogen and O=oxygen.

Glycine and alanine can combine together with the elimination of a molecule of water to produce a dipeptide. It is possible for this to happen in one of two different ways - so you can get two different dipeptides.


In each case, the linkage shown in blue in the structure of the dipeptide is known as a peptide link.
If you joined three amino acids together, you would get a tripeptide.
If you joined several together you get a polypeptide.
If you join very many together you get a protein.

Now proteins are found in all cellular organisms from primitive archae and bacteria right up to complex multicellular organisms like plants and animals. They are also found in viruses. DNA and RNA give the orders to the cell as to what to do and proteins carry out these orders. Proteins vary in structure as well as function. They are constructed from a set of 20 amino acids and have distinctive individual three-dimensional shapes.

The human body is about 50% dry weight protein. Proteins have a large number of important functions in the human body—and in fact they are involved in virtually all cell functions. Each protein within the body has a specific function. Below is a list of several types of proteins and their functions.

Antibodies - are specialized proteins involved in defending the body from foreign invaders called antigens.
Contractile Proteins – make up a large proportion of the muscles and are involved in contraction and movement.
Enzymes - are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Examples include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.
Hormonal Proteins - are messenger proteins which help to coordinate certain bodily activities. An example is insulin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration.
Structural Proteins - are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as hair, quills, feathers, horns, and beaks. They are fond in connective tissues such as elastins and collagens which are used in tendons and ligaments.
Storage Proteins - store amino acids. Examples include ovalbumin and casein. Ovalbumin is found in egg whites and casein is a milk-based protein.
Transport Proteins - are carrier proteins which move molecules from one place to another around the body. An example is haemoglobin. Haemoglobin transports oxygen through the blood.

One well known protein is the hormone insulin. The insulin molecule has served as a model for multitudes of studies on the fundamental structure and properties of proteins. It was the first protein to have its amino acid sequence sequenced, in 1955, by Fred Sanger (Sanger 1988), earning him a Nobel prize in 1958. The three-dimensional structure of insulin was ultimately solved by Dorothy Crowfoot Hodgkin and her colleagues in 1969, using X-ray crystallographic methods (Adams et al. 1969).

It is composed of two polypeptide chains: In humans chain A has 21 amino acids and chain B has 30.

The theoretical number of ways of arranging the amino acids in a simple protein like insulin runs into billions. However there will only be a few ways of arranging the amino acid sub groups to give a biologically active protein. Evolution has led to small differences occurring in different species. Pig insulin is very similar to human.

Ray Goodwin

Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know

Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see

Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands

Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are

One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face

The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.

Readers of this web site are invited to e-mail the author ( and discuss their opinions of the topics dealt with and suggest any changes which they think may be helpful.

Life in the Cosmos Website
Version 01.00 - April 20, 2015.