The functions of proteins are the essence of life itself. They make up more than 50% of the dry mass of animals. Many of an organism’s proteins are enzymes, special proteins that speed up the rate of chemical reactions in the cell. Enzymes are tiny molecular tools that temporarily combine with the substrates (ingredients) for a specific reaction and hold them at the correct angle for a reaction to occur. The enzyme may also pull on bonds and loosen them. This lowers the amount of energy needed for the reaction to proceed so it can occur at a much lower temperature than would be necessary without the enzyme (meaning that your cells don’t have to heat up every time a reaction occurs). Each of the thousands of enzymes in the human body is specific to one (or, in some cases, more) particular reaction. In addition, proteins play a myriad of other roles in cells. A few examples are shown in the table below.
It is the order of the amino acids in a protein that determines its shape and it is the shape of the protein that determines its function.
It is the function of DNA to encode, utilize, and pass on with amazing accuracy the code for the order for the order of the amino acids in every protein in every living cell.
| Some Functions of Proteins |
| Type of Protein | Example | Function |
| Enzymes | Amylase | Promotes the break down of starch to a simple sugar |
| Structural | keratin, collagen | Hair, wool, nails, horns, hoofs, tendons, cartilage |
| Contractile | actin, myosin | Contracting fibers in the muscles |
| Storage | ferritin | Stores iron in spleen |
| Transport | hemoglobin | Carries oxygen in blood |
| Immunological | antibodies | Rid the body of foreign proteins |
| Toxins | neurotoxin | Cobra venom, blocker of nerve functions |
| Hormones | insulin, glucagon | Regulate use of blood sugar |
Proteins are organic compounds that contain nitrogen in addition to carbon, hydrogen and oxygen. The subunits of proteins are called amino acids. There are twenty different kinds of amino acids found in living cells. All amino acids have a central carbon atom which makes four attachments. Three of the attachments are the same for all kinds of amino acids.
1) a single hydrogen atom
The Four Parts of an Amino Acid
2) a COOH group on one side
3) an NH2 group on the other.
4) R differs for each amino acid
The Four Parts of an Amino Acid
1) a single hydrogen atom
2) a COOH group on one side
3) an NH2 group on the other.
4) R differs for each amino acid
Proteins are long, unbranched chains of amino acids called polypeptides. Polypeptide chains fold up into complex shapes because of attractive and repulsive forces between the R groups of different kinds of amino acids. Some proteins are composed of more than one polypeptide chain that fit together to form a complex three-dimensional functional protein.
Scientists describe the structure of a protein on four levels of complexity: primary, secondary, tertiary, and quaternary levels.
The primary structure of a protein is the sequence of amino acids.
The secondary structure of a protein occurs when the sequence of amino acids are linked by hydrogen bonds. This level of structure takes the form of either a pleated sheet or a helix.
The tertiary structure describes the folding and other contortions of a polypeptide chain that result from the molecular interactions among the R groups of the different amino acids.
The arrangement of two or more polypeptide chains in a protein make up its quaternary structure
The primary structure of a protein is the sequence of amino acids.
The secondary structure of a protein occurs when the sequence of amino acids are linked by hydrogen bonds. This level of structure takes the form of either a pleated sheet or a helix.
The tertiary structure describes the folding and other contortions of a polypeptide chain that result from the molecular interactions among the R groups of the different amino acids.
The arrangement of two or more polypeptide chains in a protein make up its quaternary structure
The figure above was adapted in 2002 from the website of the
Office of Science Education and Outreach of the National Human Genome Research Institute: http://www.nhgri.nih.gov/DIR/VIP/Glossary/Illustration/protein.html
Having the correct amino acids in the correct positions is crucial to the protein’s overall shape and consequently to its function. For example, the change of just one amino acid alters the shape of hemoglobin enough to create the condition of sickle cell anemia. Though proteins themselves do not mutate, a mutation in the genetic material of an organism is expressed as a change in the order of amino acids of a protein.
It is the order of the amino acids in a protein that determines its shape and it is the shape that determines the function.
Clearly, it is critical for every cell to have a process that guarantees accurate ordering of amino acids in every protein that it needs to carry out its life activities.
Work your way through the pages of DNA Basics to find out how that happens.
