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Amino Acids
Amino acids are the building blocks of proteins. Amino acids consist of a central carbon bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable R group. There are 20 types of amino acids, each with a unique R group. The R group dictates the properties of the amino acid, such as being hydrophilic, hydrophobic, or ionic.
Structure
To form proteins, amino acids are linearly linked together using peptide bonds which are a type of covalent bond. These bonds form between the amino group of one amino acid and the carboxyl group of another amino acid. This creates a chain that at one end has an amino group, called the amino terminus, and at the other end has a carboxyl group, called the carboxyl terminus. This creates a molecule that has directionality which is critical in how the protein folds and functions.
The specific order of amino acids in a polypeptide or protein is the primary structure. Since amino acids have variable R groups, the order determines the overall shape of the protein as the R groups interact with other amino acids. The shape of the protein is categorized into two levels – secondary and tertiary structure. The secondary structure refers to the formation of alpha helixes and beta-pleated sheets. The tertiary structure refers to the 3D folding that occurs. These shapes are ultimately what determine the function of the protein. Hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bonds and other bonds can form between amino acids and work together to stabilize the shape. Quaternary structure occurs when multiple polypeptide units come together to form a larger protein complex. Examples of proteins include enzymes, antibodies, and ribosomes.
Function
Proteins serve a variety of roles within organisms. Amongst many other things, they provide structural support, facilitate the transport of other molecules, catalyze reactions, transmit signals, and are key in immune defense. All these functions are dictated by the four levels of protein structure. In addition, the location of the amino terminus and the carboxyl terminus play a role in orienting the protein in a way that enables proper interaction and activity. Recognition of the amino terminus or carboxyl terminus plays a role in protein-protein interaction, directional binding to other molecules such as DNA, and cell signalling.
