Peptides
Two or more amino acids can join together into chains called peptides. The -NH3+ group on one end of an amino acid molecule reacts with the -COO– group on another, releasing a molecule of water and forming a new bond:
The linkage joining two amino acid units is called a peptide bond. Note that the product molecule still has a reactive -NH3+ group on the left and a reactive -COO– group on the right. These can react with additional amino acids to lengthen the peptide. The process can continue until thousands of units have joined, resulting in large proteins.
A chain consisting of only two amino acid units is called a dipeptide; a chain consisting of three is a tripeptide. For example, the following tripeptide consists of three amino acid units: serine, alanine, and cysteine. The amino acids are listed in order using their three-letter abbreviations: ser-ala-cys
The general term peptide refers to an amino acid chain of unspecified length. However, chains of about 50 amino acids or more are usually called proteins or polypeptides. In its physiologically active form, a protein may be composed of one or more polypeptide chains.
Space-filling model of bradykinin image description
A ball-and-stick model of a polypeptide chain is shown. The chain is depicted as a series of connected spheres representing atoms. These spheres are colored to indicate different elements: gray for carbon, white for hydrogen, red for oxygen, and blue for nitrogen. The atoms are connected by sticks, illustrating the chemical bonds between them. The polypeptide chain has a distinct, somewhat helical or wavy, three-dimensional structure, with various side chains branching off the main backbone. The overall arrangement suggests a flexible molecule. The model is presented against a plain white background, emphasizing the structure of the molecule itself. Several amino acid residues are visible within the chain, including those with aromatic side chains (represented by a cluster of gray spheres forming a phenyl group) and polar side chains containing nitrogen and oxygen atoms. The molecule appears to be a short peptide or a segment of a larger protein, likely representing its secondary or tertiary structure. The detailed representation allows for the visualization of atomic positions and bonding arrangements within the molecule.
For peptides and proteins to function correctly, it is not enough that they incorporate certain amounts of specific amino acids. The order, or sequence, in which the amino acids are connected is also of critical importance. Bradykinin is a nine-amino acid peptide produced in the blood that has the following amino acid sequence:
arg-pro-pro-gly-phe-ser-pro-phe-arg
This peptide lowers blood pressure, stimulates smooth muscle tissue, increases capillary permeability, and causes pain. When the order of amino acids in bradykinin is reversed,
arg-phe-pro-ser-phe-gly-pro-pro-arg
the peptide resulting from this synthesis shows none of the activity of bradykinin.
Just as millions of different words are spelled with our 26-letter English alphabet, millions of different proteins are made with the 20 common amino acids. However, just as the English alphabet can be used to write gibberish, amino acids can be put together in the wrong sequence to produce nonfunctional proteins. Although the correct sequence is ordinarily of utmost importance, it is not always absolutely required. Just as you can sometimes make sense of incorrectly spelled English words, a protein with a small percentage of “incorrect” amino acids may continue to function. However, it rarely functions as well as a protein having the correct sequence. There are also instances in which seemingly minor errors of sequence have disastrous effects. For example, in some people, every molecule of hemoglobin (a protein in the blood that transports oxygen) has a single incorrect amino acid unit out of about 300 (a single valine replaces a glutamic acid). That “minor” error is responsible for sickle cell anemia, an inherited condition that usually is fatal.
Concept Review Exercise
Describe the difference between an amino acid and a peptide.
Solution
A peptide is composed of two or more amino acids. Amino acids are the building blocks of peptides
Attributions
This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD, Openstax which is licensed under CC BY 4.0.
This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J Scott, Saylor which is licensed under CC BY-NC-SA 4.0.
