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Peptides play an essential role in fundamental physiological processes and are necessary for many biochemical processes. A peptide is a short string of 2 to 50 amino acids, formed by a condensation reaction, joining together through a covalent bond.[1] Sequential covalent bonds with additional amino acids yield a peptide chain and the building block of proteins. Peptides are named based on the number of amino acid residues in the sequence. As peptide chains form between joining of the primary structure of amino acids, they may enlarge to become an oligopeptide when there are between 10 to 20 amino acids in the chain. In vivo, each amino acid is added to the amino-terminal of one amino acid to form a peptide chain.[1] When there are greater than 20 amino acids, the peptide is an unbranched chain deemed a polypeptide. Each amino acid comprising a peptide is called a “residue” since that is the portion remaining after the loss of water in the dehydration reaction. Amino acids are the organic starting molecule composed of a carboxyl-terminal and an amino group that makes up the foundation of a protein. Peptide synthesis depends on three main reactions: 1. an amino acid goes through a deprotection step, a preparatory reaction that adds the next amino acid to the chain, and lastly, a coupling reaction that forms the final peptide with functionality.[1] In the second step, the amino acid becomes activated with several reagents. Thes carboxylic acid in the amino acid will react to make the activated form, which will then enter into a coupling reaction. After one round of peptide synthesis, this process is repeatable to add more amino acids until creating the desired length of the peptide. Peptide bonds are resistant to conditions that denature proteins, such as elevated temperatures and high concentration of urea. Amino acids all have the same general structure, with a positive charge on nitrogen and negative on the carbonyl group.[1]

Pathophysiological processes related to peptides are very broad due to the ubiquitous nature of peptides in the body. This section will describe how peptides are involved in the pathophysiology of various metabolic processes. Peptide-Receptor Complex and Signaling Cascade Biologically active peptides are produced from genes that target specific proteins or protein-coupled receptors, such as G-protein-coupled-receptors (GPCRs).[22] The combination of this peptide-receptor complex can then switch on or off a series of cascading reactions through a multitude of mechanisms. These downstream reactions that are activated may include other G-proteins, tyrosine kinases, and a series of transcription events and thus control all cellular processing and functioning.[22] In some cases, when a peptide binding to a receptor causing a pathologic "on" state, unregulated transcription and proliferation may ensue, leading to an oncologic state. These cellular processes may be left unchecked and result in tumor growth. The design of synthetic peptides, created to act as endogenous peptides do and bind to a target receptor, can identify the location of tumor growth for identification and even for therapeutic purposes. Infection Peptides play a large endogenous role in humans and other species as a first-line barrier to fight infection. One of the components of the body's innate immune system includes the production of antimicrobial peptides (AMPs) in the epithelium.[23] In addition to the epithelium, AMPs are also produced by neutrophils, mast cells, and even adipocytes [24]. These AMPs can be post-translationally modified to fight a wide range of different infections, and with their cationic and interact with the negatively charged bacterial surface. A very important subset of AMPs is called defensins and cathelicidins.[24] Dermcidin is a known gene that encodes antimicrobial resistance peptides in the sweat glands that can survive at a high salt concentration and over a wide range of pH values.[25] It is known that some bacteria can even produce their AMPs and develop resistance mechanisms to endogenous AMPs so that they can proteolytically cleave the peptides and survive.[24]