Polypeptide and peptides are the biological units used to create and maintain the cells of the body. When the body digests proteins, they are broken down into amino acids then recombined to create the specific peptides that the body needs. By combining amino acids into different protein structures, the body can produce the various tissues, fluids, hormones, and structures the body needs to survive. Polypeptides can be synthetically combined to create antibiotics, hormones and other materials as well.
The use of polypeptides and peptides for pharmaceutical applications is a growing field as researchers continue to develop new methods for manipulating these small chemical structures. By: Elly McGuinness. By: Jennifer Crump. Dictionary Dictionary Term of the Day. Workplace Testing Terms. Follow Connect with us.
Sign up. Each amino acid differs in terms of its "R" group. The "R" group of an amino acid is the r emainder of the molecule, that is, the portion other than the amino group, the acid group, and the central carbon. A peptide is two or more amino acids joined together by peptide bonds, and a polypeptide is a chain of many amino acids.
A protein contains one or more polypeptides. Therefore, proteins are long chains of amino acids held together by peptide bonds. As will be seen later in this unit, DNA is divided into functional units called genes. Therefore, it is commonly said that the order of deoxyribonucleotide bases in a gene determines the amino acid sequence of a particular protein. Since certain amino acids can interact with other amino acids in the same protein, this primary structure ultimately determines the final shape and therefore the chemical and physical properties of the protein.
The secondary structure of the protein is due to hydrogen bonds that form between the oxygen atom of one amino acid and the nitrogen atom of another. The polypeptide will then fold into a specific conformation depending on the interactions dashed lines between its amino acid side chains. Figure Detail. Figure 2: The structure of the protein bacteriorhodopsin Bacteriorhodopsin is a membrane protein in bacteria that acts as a proton pump. Its conformation is essential to its function.
The overall structure of the protein includes both alpha helices green and beta sheets red. The primary structure of a protein — its amino acid sequence — drives the folding and intramolecular bonding of the linear amino acid chain, which ultimately determines the protein's unique three-dimensional shape. Hydrogen bonding between amino groups and carboxyl groups in neighboring regions of the protein chain sometimes causes certain patterns of folding to occur.
Known as alpha helices and beta sheets , these stable folding patterns make up the secondary structure of a protein. Most proteins contain multiple helices and sheets, in addition to other less common patterns Figure 2. The ensemble of formations and folds in a single linear chain of amino acids — sometimes called a polypeptide — constitutes the tertiary structure of a protein.
Finally, the quaternary structure of a protein refers to those macromolecules with multiple polypeptide chains or subunits. The final shape adopted by a newly synthesized protein is typically the most energetically favorable one. As proteins fold, they test a variety of conformations before reaching their final form, which is unique and compact.
Folded proteins are stabilized by thousands of noncovalent bonds between amino acids. In addition, chemical forces between a protein and its immediate environment contribute to protein shape and stability.
For example, the proteins that are dissolved in the cell cytoplasm have hydrophilic water-loving chemical groups on their surfaces, whereas their hydrophobic water-averse elements tend to be tucked inside. In contrast, the proteins that are inserted into the cell membranes display some hydrophobic chemical groups on their surface, specifically in those regions where the protein surface is exposed to membrane lipids.
It is important to note, however, that fully folded proteins are not frozen into shape. Rather, the atoms within these proteins remain capable of making small movements. Even though proteins are considered macromolecules, they are too small to visualize, even with a microscope. So, scientists must use indirect methods to figure out what they look like and how they are folded. The most common method used to study protein structures is X-ray crystallography. With this method, solid crystals of purified protein are placed in an X-ray beam, and the pattern of deflected X rays is used to predict the positions of the thousands of atoms within the protein crystal.
In theory, once their constituent amino acids are strung together, proteins attain their final shapes without any energy input. In reality, however, the cytoplasm is a crowded place, filled with many other macromolecules capable of interacting with a partially folded protein. Inappropriate associations with nearby proteins can interfere with proper folding and cause large aggregates of proteins to form in cells. Cells therefore rely on so-called chaperone proteins to prevent these inappropriate associations with unintended folding partners.
Chaperone proteins surround a protein during the folding process, sequestering the protein until folding is complete. For example, in bacteria, multiple molecules of the chaperone GroEL form a hollow chamber around proteins that are in the process of folding. Molecules of a second chaperone, GroES, then form a lid over the chamber. Eukaryotes use different families of chaperone proteins, although they function in similar ways.
Chaperone proteins are abundant in cells. These chaperones use energy from ATP to bind and release polypeptides as they go through the folding process. Chaperones also assist in the refolding of proteins in cells. Folded proteins are actually fragile structures, which can easily denature, or unfold. Although many thousands of bonds hold proteins together, most of the bonds are noncovalent and fairly weak.
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