Aside from water, Proteins are the most abundant kind of molecules in the body.

http://en.wikipedia.org/wiki/Protein_(nutrient)

Proteins are essential nutrients for the human body.They are one of the building blocks of body tissue, and can also serve as a fuel source. As a fuel, proteins contain 4 kcal per gram, just like carbohydrates and unlike lipids, which contain 9 kcal per gram.

Proteins are polymer chains made of amino acids linked together by peptide bonds.

Amino acids can be divided into three categories: essential amino acids, non-essential amino acids, and conditional amino acids. Essential amino acids cannot be made by the body, and must be supplied by food.

 Non-essential amino acids are made by the body from essential amino acids or in the normal breakdown of proteins. Conditional amino acids are usually not essential, except in times of illness, stress, or for someone challenged with a lifelong medical condition.