What are Dipeptides?

A dipeptide is considered a mixture of two distinct amino acids. Since the amino acids are distinct, based on their composition, two dipeptide's isomers can be produced. Various dipeptides are biologically essential and are therefore crucial to industry. 

Amino Acids

Proteins are large molecules within our cells that have important roles in various cellular functions. All proteins are made up of individual building blocks called amino acids. Multiple amino acids can be strung together to make a protein molecule. There are 21 different amino acids, and each one is a little different from one another with an identical structural configuration. An amino acid contains an amine and a carboxyl group. An amine consists of a set of atoms composed of two hydrogen attached to nitrogen. The carboxyl group consists of an oxygen-binding carbon, which is also connected to a hydrogen with oxygen.  The R group is the component of the amino acid which differentiates this from many amino acids. There's a separate R-group to every amino acid, that could be as basic as hydrogen that occurs in glycine or it can be more complicated. The covalent association among amino acids is referred to as a peptide link and hence many covalently attached amino acids are labeled as polypeptides.

Peptide Bond

Peptides consist of amino acid monomeric units, connected in sequence by an amide link. The binding of one amino acid carboxy group to the amine group of next amino acid. Dipeptide is known as a peptide consisting of two amino acids. Tripeptide is a peptide containing three amino acids; if it contains four then it is called as tetrapeptide and so on.

Dipeptide Linkage

The protein can always be made from various amino acids linked around each other. The sequence wherein the amino acids are constructed decides which protein will be formed. A dipeptide is produced if two amino acids are combined. Two amino acids attached through a specific covalent bond known as peptide bond. Different amino acids, coupled together by polypeptide chains give rise to proteins. Similarly, the larger the protein molecule the greater number of amino acids present which in turn will have a greater number of peptide bonds. Dipeptides normally arise from polypeptides as a consequence of cleavage by a certain peptidase such as dipeptidyl peptidase a hydrolytic enzyme. The secretion of gastrin from g-cells into the stomach is stimulated by the dipeptides.

Few examples of dipeptides include carnosine, aspartame, bale nine and glycylglycine

"an image showing dipeptide formation. "

Process of Dipeptide Formation

The technique used to connect amino acids through establishing a peptide link results in dipeptide formation and it uses a mechanism named as dehydration reaction. During a dehydration process a molecule of water (H2O) is extracted to produce dipeptides. For this water molecule an oxygen and hydrogen from the carboxylic group present in one amino acid along with the hydrogen of amine group from other amino acid is removed. It can be represented as follows:

O-H (where O and H is taken from carboxyl group of one amino acid) + H (of amine group of another amino acid) = H2O.

Let’s consider the dehydration process of a simple amino acid glycine:

For the glycine molecule the R group is simple hydrogen molecule when two glycine molecules combine, they lose water molecule to form a dipeptide bond as shown in the diagram.

"An image formation of dipeptide bond in glycine molecule. "

Merrifield Synthesis

In the above method to synthesize a peptide of a considerable length such as ten residues it is necessary to have several steps, and to avoid unintended cross reactions, the product should be cautiously processed in between each phase. A sophisticated alteration of this technique has now been created in order to reduce the repetitive and time taking purification and also to significantly reduce the chemical loses in processing. This technique is termed Merrifield Synthesis after the founder R. Bruce Merrifield and it comprises the attachment of the peptide sequence C-terminus to a polymer solid, typically as small beads. The filtering and cleaning of the beads with suitable solvents simply completes segregation as well as filtration of the peptide molecule. For the next added peptide bond similarly add reagents and perform the cleaning procedures. It is possible to automate the whole process and commercially viable peptide synthesizing machines based on the Merrifield method.

"An image showing an overview of Merrifield process. "

Significance of Peptides

Peptides are chemicals that are essential both biologically and clinically. They exist normally in animals, and are also laboratory synthesized substances, once they are brought into an organism. Peptides are a cell and tissue structural feature and are also antibiotics, hormones, toxins, and enzymes. Examples include hormone such as oxytocin, glutathione that stimulates the tissue development, pancreatic insulin hormone and also glucagon. The World Anti-Doping Agency (WADA, the United States Anti-Doping Agency (USADA) and the Australia Sports Anti-Doping Authority have listed two different categories of peptides as Illegal Schedule 2(S2). The application of peptide hormones and secretagogue peptides is prohibited, whether or not competing, by professional athletes since these molecules work to improve results. The peptides that were prohibited are growth hormones, which enhance oxygenation in the blood, which enhances growth and recovery of the muscles and activate the secretion of hormones of endocrines including ovaries, testes, and thyroid.

Commercial Dipeptides

Commercial dipeptides occurring in moderate amount of which aspartame and carnosine are widespread. Astartame, the most widely used substitute for sweetness in foods, is an industrial non-saccharide sweetener that is sweeter than sugar (an adduct of two glucose molecules). In addition, Carnosine is an isomer of a dipeptide molecule consisting of amino acids including beta-alanine as well as histidine, highly found in muscle and brain tissue in a human body.

Common Mistakes

The naming of peptide is very important so the most common mistake can occur while naming a peptide bond and counting the number of monomer present in the bond formation. It should also be noted that the reaction of bond formation that is elimination of water molecule while bond formation should also be noted properly.

Context and Applications:

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for           

B.Sc. and M.Sc. Chemistry, Biochemistry and Molecular biology        

Bachelor of Molecular and cellular biology       

M.Sc. Biological science

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