Proteins are molecular machines that perform the majority of the biochemical processes within cells. They also serve as the building blocks for many structural components. Their ubiquitous use as builders and building materials alike makes it easy to see why proteins are indeed the material that makes life happen.
Proteins are assembled in a complex series of biochemical reactions known as gene expression. Genes are basically the instructions or blueprints that tell the cell how to make proteins, and the information contained within many gene is usually required to produce a single functional protein.
Proteins are composed of amino acids that are linked together by peptide bonding. Amino acids have many structures, but the basic formula is NH2-C-COOH. Each contains an amine group (NH2) and a carboxylic acid (COOH), thus their name - amino acid. Amino Acids are first linked together to form long chains called polypeptides, which after further modification are called proteins.
Proteins are composed of twenty different types of amino acids. Most proteins form globular structure and others are chain-like. Proteins have 2 distinctive types of functions: structural and biological activity. However, some proteins serve both functions. Important structural proteins in animals include collagen, which is the component of the bones, muscles and skin, and keratin, which is the component of the hair, feather, and finger nails.
- Main article: Protein synthesis
Protein synthesis is the process during which the chemical instructions on genes are used by a living cell to make proteins. It is a highly complex procedure that involves a great number of chemical reactions, both inside and outside the nucleus, which are catalyzed by a host of enzymes and cofactors.
In brief, protein synthesis begins in the nucleus, where the protein-coding instructions from the genes are transmitted indirectly through messenger ribonucleic acid (mRNA), a transient intermediary molecule similar to a single strand of DNA. This mRNA is moved from the nucleus to the cellular cytoplasm, where it serves as the template for protein synthesis. The cells protein-synthesizing machinery (ribosomes) then translates the codons into a string of amino acids that will eventually constitute the protein molecule.
|Examples of nucleotide codons|
and their corresponding amino acid:
Codon - Amino Acid TGC = Cysteine CTG = Leucine AGT = Serine GCA = Alanine
The product of a single gene is a string of amino acids known as a peptide or polypeptide. This is known as the primary structure of a protein. This peptide is folded into a particular subunit, and then joined together with other peptides to make a functional protein. Protein structures are known by 4 levels of organization: primary, secondary, tertiary, and quaternary protein structures.
The first structure of protein is primary protein structure or primary protein stage. The primary structure consists the repeating sequence of three atoms: N-C-C. N is alpha helix, and C is carboxyl group. The structure of proteins is bonded by the covalent bonds. From the primary stage, the translation is occurring; the amino acids form to bond into the special order of them. (Polypeptide, group of amino acids, is the sequence of amino acids in the primary stage.) The sequence of amino acids determines how to protein to twist or form. Proteins can be distinguished by the stable structures. 
Secondary structure is occurring after the primary structure. It consists the regular repeating patterns in the polypeptide chains. Hydrogen bonding plays an important role in stabilizing these folding patterns. In the secondary structures, there are two different types of the structures: alpha helix and beta pleated sheet. Each of the structures work differently, even though they are occurring in the secondary structure. 
- Alpha helix: Alpha is a right handed coil. The alpha helical secondary structure is common in keratins, which is the fibrous structural protein: hair, feather, and toe nails.
- Beta pleated sheet: Beta pleated sheet is formed from two or more polypeptide chains. It is stabilized by the hydrogen bonds between N-H and C-O groups.
Of course, tertiary structure occurs after the secondary structure. The tertiary structure is the polypeptide and proteins are composing it as the secondary structure of protein. Polypeptides in tertiary structure can be bent and be folded back and forth. Many of the interactions between atoms are occurred in tertiary structure.
- Covalent disulfide bridges: They can hold the folded polypeptides
- Hydrophobic: Side chains are gathered together in protein to fold the polypeptides that the process occurs away from water
- Van der Waals force: It stabilize the interactions between the hydrophobic residues.
- Ionic bonds: They form between positive and negative charges.
The quaternary structure describes the interaction of proteinaceous subunits with other subunits or different molecules. The quaternary stricture is specifically the result of binding of the subunits together and their interactions. The protein subunits are the short tertiary polypeptide chains. The tetrameric metalloprotein hemoglobin is a good illustration of the quaternary structure. Hydrogen bonds, ionic bonds, Van der Waals forces, and hydrophobic bonds hold four different subunits together (2 α subunits and 2β subunits) along with an Fe(II) ion. The presence of the iron allows oxygen binding and transportation in the form of oxyhemoglobin during cellular respiration. When oxyhemoglobin releases O2 in the cells of the body, the quaternary structure undergoes a change. 
- Primer on Molecular Genetics: Introduction by the U.S. Department of Energy, Accessed January 22, 2011.
- Protein and Polypeptide Structure: Four Conformation Levels of Protein Structure By Anne Marie Helmenstine, Ph.D., About.com Guide, Accessed January 22, 2011.
- Protein Structure Advancing the Chemical Science
- Some Protein History Associated Content
- Protein & Polypeptide Structure About Chemistry