Proteins are biological macromolecules. They are composed of 20 so-called proteinogenic amino acids. Humans must ingest proteins with food, since eight amino acids are essential, therefore cannot be produced by the body.
The number of different proteins in a cell depends strongly on the type of cell and which organism it comes from. In a yeast cell there are about 5900 different proteins.
Amino acids are divided into different groups based on their properties. These are acidic, basic, hydrophilic and hydrophobic amino acids. These determine the properties of the protein.
The formation of proteins is protein biosynthesis. During translation, the code on the DNA is translated into amino acids. The individual amino acids are then linked by ribosomes (protein-RNA complex) via peptide bonds. This creates chains of several hundred or thousand amino acids. The longest human protein is titin, with 27,000 amino acids in one chain. It is found in muscle cells and is used for muscle elasticity and stability.
The structure of proteins is characterized by a hierarchy. The first level is the primary structure, it describes the sequence of amino acids linked by peptide bonds.
The secondary structure describes structural elements that occur due to interactions of amino acids. These structures include the α-helix and the ß-sheet structure, which are stabilized by hydrogen bonds. The next stage is the tertiary structure. This represents the spatial structure of the protein and brings together the individual secondary structures into higher-level "folds." In this structure, the protein is biologically active. The quaternary structure describes the assembly of two or more polypeptide chain in tertiary structure into a higher-level structure to form a functional unit.
The function of proteins is determined by their structure. Proteins are the basic building blocks of the cell and perform many different functions. Because of their structure, they are very specific. These include stability, elasticity, the catalysis of reactions by enzymes, they act as messengers or hormones for communication within the cell or between different cells, they serve to protect the cell as toxins or antibodies and are important transporters for other small molecules.