polypeptide

Examples of polypeptide in the following topics:

• Protein Structure

  • A protein's primary structure is the unique sequence of amino acids in each polypeptide chain that makes up the protein.
  • Really, this is just a list of which amino acids appear in which order in a polypeptide chain, not really a structure.
  • For example, the pancreatic hormone insulin has two polypeptide chains, A and B.
  • Rarely does a single secondary structure extend throughout the polypeptide chain.
  • Proteins made from a single polypeptide will not have a quaternary structure.

• Amino Acids

  • An amino acid contains an amino group, a carboxyl group, and an R group, and it combines with other amino acids to form polypeptide chains.
  • The resulting chain of amino acids is called a polypeptide chain.
  • Each polypeptide has a free amino group at one end.
  • When reading or reporting the amino acid sequence of a protein or polypeptide, the convention is to use the N-to-C direction.
  • Although the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically any polymer of amino acids, whereas the term protein is used for a polypeptide or polypeptides that have folded properly, combined with any additional components needed for proper functioning, and is now functional.

• The Mechanism of Protein Synthesis

  • Catalyzing the formation of a peptide bond removes the bond holding the growing polypeptide chain to the P-site tRNA.
  • The growing polypeptide chain is transferred to the amino end of the incoming amino acid, and the A-site tRNA temporarily holds the growing polypeptide chain, while the P-site tRNA is now empty or uncharged.
  • This causes the polypeptide chain to detach from its tRNA, and the newly-made polypeptide is released.
  • The growing polypeptide chain is attached to the tRNA in the ribosome P site.
  • This creates a peptide bond between the C terminus of the growing polypeptide chain and the A site amino acid.

• The Central Dogma: DNA Encodes RNA and RNA Encodes Protein

  • These nucleotide triplets are called codons; they instruct the addition of a specific amino acid to a polypeptide chain.
  • Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery.
  • Translation is the process by which mRNA is decoded and translated to produce a polypeptide sequence, otherwise known as a protein.
  • In translation, a cell decodes the mRNA's genetic message and assembles the brand-new polypeptide chain.
  • The main function of tRNA is to transfer a free amino acid from the cytoplasm to a ribosome, where it is attached to the growing polypeptide chain. tRNAs continue to add amino acids to the growing end of the polypeptide chain until they reach a stop codon on the mRNA.

• The Protein Synthesis Machinery

  • For instance, ribosomes may consist of different numbers of rRNAs and polypeptides depending on the organism.
  • A ribosome is a complex macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides.
  • The peptidyl-tRNA carrying the growing polypeptide chain is held in the P site.
  • (More accurately, the growing polypeptide chain is added to each new amino acid bound in by a tRNA.)
  • (More accurately, the growing polypeptide chain is added to each new amino acid brought in by a tRNA.)

• Types and Functions of Proteins

  • These amino acids are covalently attached to one another to form long linear chains called polypeptides, which then fold into a specific three-dimensional shape.
  • Sometimes these folded polypeptide chains are functional by themselves.
  • Other times they combine with additional polypeptide chains to form the final protein structure.
  • Sometimes non-polypeptide groups are also required in the final protein.
  • For instance, the blood protein hemogobin is made up of four polypeptide chains, each of which also contains a heme molecule, which is ring structure with an iron atom in its center.

• Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

  • The structure of peptide hormones is that of a polypeptide chain (chain of amino acids).
  • The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland.
  • Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids.

• The Incorporation of Nonstandard Amino Acids

  • A protein (also called a polypeptide) is a chain of amino acids.
  • For instance, the peptide hormone insulin is cut twice after disulfide bonds are formed, and a propeptide is removed from the middle of the chain; the resulting protein consists of two polypeptide chains connected by disulfide bonds.
  • Also, most nascent polypeptides start with the amino acid methionine because the "start" codon on mRNA also codes for this amino acid.

• Denaturation and Protein Folding

  • At higher pHs pepsin's conformation, the way its polypeptide chain is folded up in three dimensions, begins to change.
  • It is often possible to reverse denaturation because the primary structure of the polypeptide, the covalent bonds holding the amino acids in their correct sequence, is intact.
  • The chaperonins clump around the forming protein and prevent other polypeptide chains from aggregating.

• Proteolytic Degradation

  • The breakdown of proteins into smaller polypeptides, or its respective amino acids, are necessary for metabolic and cellular homeostasis.
  • Polypeptides are commonly broken down via hydrolysis of the peptide bonds by utilizing a class of enzymes called proteases.