Edexcel A (SNAB) A Level Biology:复习笔记2.3.4 Amino Acids & Peptide Bonds

Amino Acid: Structure

Proteins

  • Proteins are polymers (and macromolecules) made of monomers called amino acids
  • The sequence, type and number of the amino acids within a protein determines its shape and therefore its function
  • Proteins are extremely important in cells because they form all of the following:
    • Enzymes
    • Cell membrane proteins (eg. carrier)
    • Hormones
    • Immunoproteins (eg. immunoglobulins)
    • Transport proteins (eg. haemoglobin)
    • Structural proteins (eg. keratin, collagen)
    • Contractile proteins (eg. myosin)

Amino acids

  • Amino acids are the monomers of polypeptides
  • There are 20 amino acids found in proteins common to all living organisms
  • The general structure of all amino acids is a central carbon atom bonded to:
    • An amine (also called amino) group -NH2
    • A carboxylic acid group -COOH
    • A hydrogen atom
    • An R group (which is how each amino acid differs and why amino acid properties differ e.g. whether they are acidic or basic or whether they are polar or non-polar)

The general structure of an amino acid

The Peptide Bond

  • Peptide bonds form between amino acids
  • Peptide bonds are covalent bonds and so involve the sharing of electrons
  • In order to form a peptide bond :
    • A hydroxyl (-OH) is lost from the carboxylic group of one amino acid
    • A hydrogen atom is lost from the amine group of another amino acid
  • The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid
  • This is a condensation reaction so water is released
  • Dipeptides are formed by the condensation of two amino acids
  • Polypeptides are formed by the condensation of many (3 or more) amino acids
  • A protein may have only one polypeptide chain or it may have multiple chains interacting with each other
  • During hydrolysis reactions, the addition of water breaks the peptide bonds resulting in polypeptides being broken down to amino acids

 

Peptide bonds are formed by condensation reactions (releasing a molecule of water) and broken by hydrolysis reactions (adding a molecule of water)

Exam Tip

When asked to identify the location of the peptide bond, look for where nitrogen is bonded to a carbon which has a double bond with an oxygen atom, note the R group is not involved in the formation of a peptide bond.

Structures of specific amino acids are not required.

 

 

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