Peptide bonds are a type of covalent bond that is only found within protein molecules. These bonds join amino acids together to create peptide chains, which are then joined together to form proteins. Covalent bonds form when one atom within a molecule shares one, two or three electrons with an atom from another molecule. These types of bonds are strong and can be difficult to break.
All proteins are made from chains of amino acids that bond together in a very specific way. Most amino acids have a single carboxyl group (-COOH) on one side and an amino group (-NH2) on the other. Adjacent amino acids can form a peptide bond when one acid's carboxyl group joins with the other's amino group.
When peptide bonds are formed between amino acids, a water molecule is lost. This type of reaction is called a condensation reaction. The water molecule (H2O) is created by the loss of a hydroxyl (-OH) from the carboxyl group and a hydrogen atom (H) from the amino group. The fact that all amino acids bond together in this way is one of the factors that determines the shape of the protein being made.
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Single peptide bonds occur between each amino acid pairing. Proteins are also called polypeptides as they are often made up of tens and even hundreds of amino acids that have been joined together into peptide chains. This means that proteins contain many peptide bonds.
To break a peptide bond, a hydrolysis reaction — the opposite of a condensation reaction — must occur. Hydrolysis reactions take place when splitting proteins into peptide chains, or peptides into single amino acids. In hydrolysis, a water molecule is added to the peptide bond, causing the water to split. The hydroxyl group (-OH) attaches to the carboxyl group of one amino acid, and the hydrogen atom (H) attaches to the amino group of the other.
Peptide bonds are extremely stable, which means they are difficult to break. This is of particular importance for proteins, as they play vital roles in most lifeforms. For example, proteins called enzymes control almost all chemical reactions within living things. Also, the shape of a protein is of vital importance to how well that protein functions. The order of the amino acids that make up the protein, as well as the strength of the peptide bonds, are factors in determining and maintaining the shape of each particular type of protein.