proteins

• What is the definition of proteins?

  Proteins are naturally occurring, unbranched polymers.
• What are the monomers of proteins?

  Amino acids are the monomers of proteins.
• What is the most abundant substance in nearly all cells next to water?

  Proteins are the most abundant substance in nearly all cells next to water.
• Which elements are primarily found in proteins?

  Proteins contain carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and most also contain sulfur (S).
• What are amino acids?

  Amino acids are the building blocks for proteins.
• What functional groups do amino acids contain?

  Amino acids contain both —NH2 (amino) and —COOH (carboxylic acid) functional groups.
• What type of amino acids are found in proteins?

  The amino acids found in proteins are always alpha-amino acids.
• How many standard amino acids are normally present in proteins?

  There are 20 standard amino acids normally present in proteins.
• How are amino acids classified?

  Amino acids are classified according to the polarity of their side chains.
• What are essential amino acids?

  Essential amino acids are standard amino acids needed by the body that must be obtained from the diet.
• What is the mnemonic for essential amino acids?

  • PVT TIM HALL
  • Phenylalanine
  • Valine
  • Tryptophan
  • Threonine
  • Isoleucine
  • Methionine
  • Histidine
  • Arginine (conditionally essential)
  • Leucine
  • Lysine
• What is a complete dietary protein?

  A complete dietary protein contains all of the essential amino acids in the same relative amounts that the body needs.
• What type of protein sources are usually complete?

  Protein from animal sources are usually complete.
• What is an incomplete dietary protein?

  An incomplete dietary protein does not contain adequate amounts or does not contain all of the essential amino acids.
• What is a limiting amino acid?

  A limiting amino acid is an essential amino acid that is missing or present in inadequate amounts in an incomplete dietary protein.
• Which amino acid is absent in wheat, rice, oats, and corn?

  Lysine is absent in wheat, rice, oats, and corn.
• Which amino acid is absent in beans and peas?

  Methionine is absent in beans and peas.
• Which amino acid is absent in corn and beans?

  Tryptophan is absent in corn and beans.
• Which amino acid is absent in gelatin?

  Tryptophan is absent in gelatin.
• What is a chiral center in amino acids?

  A chiral center is a carbon atom that has four different attachments.
• How many standard amino acids possess a chiral center?

  Nineteen out of twenty standard amino acids possess a chiral center.
• What is the only achiral standard amino acid?

  Glycine is the only achiral standard amino acid.
• What is the common form of amino acids found in nature?

  Amino acids found in nature are generally L isomers, with some exceptions.
• What is the Fischer projection of amino acids?

  In the Fischer projection, —COOH is at the top, —R group is at the bottom, and —NH2 is positioned horizontally.
• What are the acid-base properties of amino acids?

  Amino acids are charged species both in solid state and in solution, with both acidic and basic groups present on the same carbon.
• What happens to amino acids in a neutral solution?

  In a neutral solution, —COOH tends to lose protons, becoming negatively charged, while —NH2 tends to accept protons, becoming positively charged.
• What is a zwitterion?

  A zwitterion is a molecule that has both a positive and a negative charge.
• How does the structure of a zwitterion change?

  The structure of a zwitterion changes when the pH of a solution changes.
• What is the isoelectric point?

  The isoelectric point is the pH at which the amino acid exists primarily in zwitterion form (>99%).
• Why are amino acids in their zwitterion form not attracted to an electric field?

  Amino acids in their zwitterion form are not attracted to an electric field because there is no net charge.
• What is isoelectric focusing used for?

  Isoelectric focusing is used in electrophoresis to separate proteins using a pH gradient.
• What are peptides?

  Peptides are unbranched chains of covalently-linked amino acids.
• How are peptides classified?

  Peptides are classified by the number of amino acid residues they contain.
• What is a dipeptide?

  A dipeptide consists of 2 amino acids.
• What is a tripeptide?

  A tripeptide consists of 3 amino acids.
• What is an oligopeptide?

  An oligopeptide consists of 10 to 20 amino acids.
• What is a polypeptide?

  A polypeptide consists of more than 20 amino acids.
• What is a peptide bond?

  A peptide bond is an amide bond that covalently links the —COOH group of one amino acid and the —NH2 group of another amino acid.
• Why do peptide chains have directionality?

  Peptide chains have directionality because they have two different ends: the N-terminal end (LEFT) and the C-terminal end (RIGHT).
• How is the sequence of amino acids indicated?

  The sequence of amino acids may be indicated using the standard 3-letter abbreviations.