Amino acid peptide bonds joining

Figure 10.8 Structure of a dipeptide. The peptide bond joins glutamic acid and alanine by condensation of the a-carboxyl group of glutamic acid and the a-amino group of alanine. The resulting dipeptide is called glutamylalanine, which can be abbreviated to NH2-Glu-Ala-COOH or Glu-Ala.

Many biological molecules, such as proteins, are large and have complex structures. Proteins are composed of a sequence of units, called amino acids, which are joined by a strong covalent bond known as a peptide bond. But proteins also fold up into a certain shape, which is vital to their function—a protein that loses its shape cannot fulfill its function as an enzyme or a transporter. The sequence of amino acids determines the shape 20 different amino acids are found in proteins, and each one has slightly different chemical and physical properties. For example, some amino acids are hydrophilic, readily interacting... 

Three amino acids can be joined by two peptide bonds to form a tripeptide similarly, amino acids can be linked to form tetrapeptides, pentapeptides, and so forth. When a few amino acids are joined in this fashion, the structure is called an oligopeptide. When many amino acids are joined, the product is called a polypeptide. Proteins may have thousands of amino acid residues. Although the terms protein and polypeptide are sometimes used interchangeably, molecules referred to as polypeptides generally have molecular weights below 10,000, and those called proteins have higher molecular weights. 

Amino acids can be joined covalently through peptide bonds to form peptides and proteins. Cells generally contain thousands of different proteins, each with a different biological activity. 

Peptide bonds join the individual amino acids, attaching the a-amino group of one amino acid to the a-carboxyl group of another. Prolonged exposure to a strong acid or base at elevated temperatures is required to hydrolyze these bonds nonenzymatically. 

Since its discovery, isolation, and purification in the early twentieth century, insulin has been administered to diabetic patients exclusively by injection until the recent introduction of inhaled insulin. Insulin possesses certain physiochemical properties that contribute to its limited absorption from the gastrointestinal tract, and requires subcutaneous injection to achieve clinically relevant bioavailability. With a molecular size of 5.7 kDa, insulin is a moderately sized polypeptide composed of two distinct peptide chains designated the A chain (21 amino acid residues) and the B chain (30 amino acid residues) and joined by two disulfide bonds. Like all polypeptides, insulin is a charged molecule that cannot easily penetrate the phospholipid membrane of the epithelial cells that line the nasal cavity. Furthermore, insulin monomers self-associate into hexameric units with a molecular mass greater than 30 kDa, which can further limit its passive absorption. Despite these constraints, successful delivery of insulin via the nasal route has been reported in humans and animals when an absorption enhancer was added to the formulation. 

Proteins are polymers produced from amino acids that are joined by peptide linkages (amine-group carboxylic acid bonds). These compounds form the bulk of living tissue. Enzymes, those very selective and powerful catalysts, are also proteins. Enzymes may contain a group based on a metal atom, as may molecules from other classes of compounds (hemoglobin, chlorophyll, etc.). Some proteins serve special functions, such as hemoglobin, an oxygen carrier. 

Figure 3-1 Component Amino Acids of Proteins Joined by Peptide Bonds and Character of Side Chains. Source From Northern Regional Research Laboratory, U.S. Department of Agriculture.

The primary structure of proteins is the particular sequence of amino acids that is joined together by peptide bonds. The most important element of this primary stmcture is the amide bond. 

Now, let s consider an important class of catalysts— namely, enzymes. Enzymes are nature s catalysts and are made of proteins. The primary structure is the sequence of amino acids that are joined through peptide bonds (illustrated below) to create the protein polymer chain ... 

An amino acid may be converted to a compound that serves as a hormone (e.g., epinephrine, the thyroid hormones), or a series of amino acids may be joined by peptide bonds to produce a polypeptide hormone (e.g., insulin, prolactin). 

Special RNA molecules called transfer RNA (tRNA) actually perform the translation because they contain a three nucleotide sequence at one end, called an anticodon, that is complementary to the codon of the mRNA. At the other end of the tRNA, an amino acid (specific to the anticodon) is attached. The amino acid is covalently joined (via a peptide bond) to the growing polypeptide chain on the ribosome (See Figure 5.20). 

Figure 23.3 A peptide bond joins two amino acids to form a dipeptide. 

When the carboxyl group of one amino acid reacts with the amino group of another to give an amide linkage and eliminate water, a peptide bond is formed. In a protein, upward of a hundred amino acids are so joined to form a polypeptide chain. 

Proteins are the most abundant organic molecules in animals, playing important roles in all aspects of cell structure and function. Proteins are biopolymers of a-amino acids, so named because the amino group is bonded to the a carbon atom, next to the carbonyl group. The physical and chemical properties of a protein are determined by its constituent amino acids. The individual amino acid subunits are joined by amide linkages called peptide bonds. Figure 24-1 shows the general structure of an a-amino acid and a protein. 

Tripeptide A molecule containing three amino acid units, each joined to the next by a peptide bond. 

A polypeptide consists of at least ten amino acids that are joined together by peptide bonds. Peptide bonds are formed by coupling the carboxyl group of one amino acid to the amino group of another (see Fig. 2.2.15). The cross-over from polypeptides to proteins is not clearly defined. If more than 100 amino acids are linked a protein is formed. However, a protein often refers to a fully functional form and can consist of several polypeptide chains. 

Procarboxypeptidase is the inactive precursor of the pancreatic exopeptidase which hydrolyses the peptide bonds joining the amino acid in the C-terminal position to the rest of the chain. 

Protein synthesis begins when the mRNA combines with a ribosome. There, tRNA molecules, which carry amino acids, align with mRNA, and a peptide bond forms between the amino acids. After the first tRNA detaches from the ribosome, the ribosome shifts to the next codon on the mRNA. Each time the ribosome shifts and the next tRNA aligns with the mRNA, a peptide bond joins the new amino acid to the growing polypeptide chain. After all the amino acids for a particular protein have been linked together by peptide bonds, the ribosome encounters a stop codon. Because there are no tRNAs to complement the termination codon, protein synthesis ends and the completed polypeptide chain is released from the ribosome. Then interactions between the amino acids in the chain form the protein into the three-dimensional structure that makes the polypeptide into a biologically active protein (see Figure 18.24). 

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