Proteins are an important class of molecules that carry out most of the work inside cells. The building blocks of proteins are smaller organic molecules called amino acids. Most organisms, including humans, use only 20 different amino acids to assemble the vast multitude of proteins needed to build and run a cell.
To build proteins, cells use a complex assembly of molecules called a ribosome. The ribosome assembles amino acids into the proper order and links them together via peptide bonds. This process, known as translation, creates a long string of amino acids called a polypeptide chain.
After the polypeptide chain is synthesized, it will sometimes undergo additional processing. For example, some proteins will have certain amino acids removed. Or extra molecules such as sugars or phosphates may be attached to some of the amino acids in a protein.
What Do Proteins Do?
Proteins are responsible for a large set of cellular functions. Many proteins, like microtubules, provide structure to cells. Others help in the transport or storage of other molecules. A good example is hemoglobin in red blood cells, which take oxygen to and carbon dioxide away from cells.
Still other proteins, known as antibodies, allow the body’s immune system to recognize and target potentially harmful microbes. Signaling proteins, such as peptide hormones, serve as messengers that carry information between different cells or organs.
Enzymes are another particularly important type of protein. Cells perform thousands of different chemical reactions, each of which requires a certain amount of energy. Enzymes help to reduce the energy needed for a chemical reaction, allowing cells to function more efficiently.
When individual proteins combine into larger structures, they can form molecular assemblies that carry out more complex tasks. These multi-subunit proteins include DNA polymerase, which replicates DNA; myosin, a motor protein that promotes muscle contraction; and RNA polymerase which copies segments of DNA into RNA.
How Is DNA Used to Make Proteins?
The instructions for proteins are encoded in sequences of DNA. The process of “reading” DNA sequences and using them to build a protein requires two phases: transcription and translation. During transcription, the instructions in DNA, which are found in short sequences of DNA called codons, are copied into RNA.
After being transcribed, the finished RNA, called messenger RNA (mRNA), binds to the ribosome where it undergoes translation. Translation is the process where a long string of codons is converted into a long string of amino acids. Each string of amino acids arranged in a particular order codes for a specific protein.
What Is the Structure of Proteins?
Proteins contain up to four different orders of structure. The string of amino acids is the protein’s primary structure. Interactions between the different amino acids cause certain regions of the polypeptide chain to fold into stable patterns called a secondary structure. Examples of secondary structure include alpha helices or beta sheets. These secondary structures can in turn interact with each other, giving rise to the tertiary structure.
And finally, there are times when more than one copy of a protein has to work together to get its job done. In these cases, each of the proteins is referred to as a subunit. The quaternary structure of a protein is the final structure of all the subunits together.