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.

• 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.

• The Relationship Between Genes and Proteins

  • The ribosome generates a polypeptide chain of amino acids using mRNA as a template.
  • The polypeptide chain folds up to become a protein.

• The Initiation Complex and Translation Rate

  • In ribosome assembly, the large and small ribosomal subunits and an initiator tRNA (tRNAi) containing the first amino acid of the final polypeptide chain all come together at the translation start codon on an mRNA to allow translation to begin.
  • The polypeptide synthesis begins and always proceeds from the N-terminus to the C-terminus, called the N-to-C direction.