Protein molecules are large, complex molecules with a huge variety of structures and functions within cells. Most chemical reactions in cells are catalyzed by proteins called enzymes. Proteins form the basis of the cytoskeleton of cells, providing structure and motility. Proteins are also essential for the communication between cells and within cells. In plants, the largest concentration of proteins can be found in some seeds.
Proteins are polymers of nitrogen containing molecules called amino acids. The amino acids are much simpler molecules than the nitrogenous bases found in nucleic acids. The same twenty amino acids is are used in the manufacture of proteins in the cells of all living organisms. An amino acid is built around a single carbon atom called the alpha carbon. Bonded to the alpha carbon are a hydrogen atom (H), a carboxyl group (COOH), and an amino group that contains nitrogen (NH2). A specialized "R" group is attached at the last site. The R-groups are different for each of the twenty amino acids, and their chemical properties, such as charge, hydrophilic or hydrophobic nature, and size, dictate protein function and shape.
The order and number of amino acids that are linked together to form a protein are determined by the order of the codons in the DNA that encode that protein. The order and number of the amino acids in a protein is called the primary structure, and it ultimately determines the shape of the protein. Proteins can have secondary structures formed by hydrogen bonding between the peptide bonds that link the amino acids together. The two common secondary structures in proteins are the alpha helix and the beta pleated sheet. The amino acid chain (also called a peptide chain) can fold up on itself to form globular structures. This is known as tertiary structure. Tertiary structure is determined by the number and order of amino acids in the protein and is formed when molecules in the R-groups of the amino acids interact with one another. When two or more peptide chains interact to form a single functional molecule, the protein is said to have quaternary structure.
See also: Nucleic Acids, Lipids
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