Proteins differ largely in their amino acid sequence, which is determined by the nucleotide sequence of their genes and generally results in the protein folding into a certain 3D structure that dictates its function. At least one lengthy polypeptide is found in a protein. Proteins are composed of amino acids which contain nitrogen and carbon atoms together with hydrogen as the most common chemical element. The basic building block of all proteins is the peptide bond, which is formed when an amino group on one molecule attaches to a carboxyl group on another. Amino acids can be classified according to their chemical properties: acidic, basic, or neutral. Each protein contains a unique sequence of these amino acids.
In general, proteins are either enzymes or hormones. Enzymes speed up reactions without being consumed themselves. Proteins play important roles in many aspects of our lives including health, nutrition, and technology. For example, enzymes help digest our food, release nutrients during digestion, and allow us to absorb vitamins and minerals from the foods we eat. Other examples include blood clotting factors, antibodies, and DNA receptors. Proteins are also responsible for some diseases: insulin becomes toxic when it binds to itself instead of reacting with sugar molecules, and this self-binding causes diabetes. Cancer cells produce large amounts of proteases, which break down tissue matrix components.
A lengthy polypeptide is generated when a specific protein is created. The polypeptide has a certain number of amino acids. The reactive groups of the amino acids are exposed. Distinct genes contain different nucleotide sequences, and so create different proteins. This fact was first discovered by Joseph Lister in 1872. He showed that bacteria could produce different enzymes with distinct chemical properties. Thus, proteins can be used as markers for different genes in any organism including humans.
Polypeptides are made up of long sequences of amino acids. The main structure of proteins is formed by the sequence of amino acids in polypeptides. Proteins have a higher amount of structure as well. The capacity of proteins to interact selectively and firmly with other molecules is the primary feature that enables their various activities. These activities include muscle contraction, DNA replication, enzyme catalysis, immunity, and odor detection among others.
Proteins can be classified into four general types based on their sequence and structure: globular proteins, membrane-associated proteins, enzymes, and RNA binding proteins. Globular proteins contain a fixed three-dimensional structure composed of several helices wound around each other like a spring. They include antibodies, most hormones, structural proteins such as collagen and actin, and some transcription factors. Membrane-associated proteins include receptors for extracellular signals. These proteins have an alpha-helical transmembrane domain and an extracellular domain that interacts with ligands or other proteins. Enzymes are responsible for most chemical reactions in cells and include proteins from various enzyme families such as oxidases, transferases, and phosphatases. Some ribosomal proteins are also enzymes that play a key role in protein synthesis inside cells. Finally, some proteins bind RNA molecules non-specifically e.g. chaperones, while others bind specific RNAs to perform their function e.g. tRNA synthetases.
The Basic Structure Peptides are amino acid sequences of fewer than 50 amino acids, whereas protein and polypeptide are employed for longer sequences. One or more polypeptide molecules can be combined to form a protein. Proteins perform many functions inside cells. They provide structure to cells, regulate how quickly cells divide, enable cells to recognize other proteins produced by themselves or other cells, and act as enzymes to catalyze chemical reactions.
The three major classes of proteins are structural proteins, regulatory proteins, and catalytic proteins. Structural proteins include collagen, elastin, and fibronectin. These proteins provide shape to cells and support tissues without changing their function. They are essential for growing tumors because they help them grow and spread into other parts of the body. Collagen is the most abundant protein in the human body. It provides strength to bones, muscles, and connective tissue. Elastin aids in the elasticity of blood vessels, while fibronectin helps cells attach to each other and to surfaces within the body.
Regulatory proteins include hormones, cytokines, and growth factors. These proteins control the activity of cells, organs, and entire bodies. Hormones are chemical messengers that cells produce that influence the activities of other cells or groups of cells. Examples include insulin, cortisol, and erythropoietin.