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2.3.12 · Biological Molecules

Amino Acids and the Peptide Bond — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 16 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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2.3.1 Amino Acids and the Peptide Bond — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of amino acids and the formation and breakage of the peptide bond.Polymers — proteins are macromolecules made of amino acid monomers.Shape and function — th…

  • General structure
  • Atoms lost
  • Type of reaction
  • Polypeptide
  • Hydrolysis
  • Amine group

Part 1 of 6Roles of proteins

Roles of proteins
Polymers
  • Polymers proteins are macromolecules made of amino acid monomers.
Shape and function
  • Shape and function the sequence, type and number of amino acids determine the shape and so the function.
Examples
  • Examples enzymes, membrane carrier proteins, hormones, immunoglobulins, haemoglobin, keratin, collagen and myosin.

Part 2 of 6Structure of an amino acid

Structure of an amino acid
Number
  • 20 amino acids are common to all living organisms.
General structure
  • General structure a central carbon bonded to an amine group (–NH2), a carboxyl group (–COOH), a hydrogen atom and an R group.
R group
  • R group it varies between amino acids, making them acidic or basic, polar or non-polar.

Part 3 of 6Forming a peptide bond

Forming a peptide bond
Atoms lost
  • Atoms lost a hydroxyl group (–OH) from the carboxyl group of one amino acid and a hydrogen from the amine group of another.
Bond
  • Bond the carbon (with C=O) of the first amino acid bonds to the nitrogen of the second.
Type of reaction
  • Type of reaction condensation, releasing one water molecule.
Products
  • Products two amino acids give a dipeptide, and many give a polypeptide.
R group
  • R group it is not involved in the peptide bond.

Part 4 of 6Polypeptides and proteins

Polypeptide
  • Polypeptide a chain of amino acids joined by peptide bonds.
Protein
  • Protein may contain one polypeptide chain or several interacting chains.

Part 5 of 6Breaking peptide bonds

  • Hydrolysis
    • adding water breaks peptide bonds and gives amino acids.

Part 6 of 6More from the specification

Peptide bond formation via condensation
  • Amino acid monomers join covalently via condensation reactions between amino (-NH2) and carboxyl (-COOH) groups, releasing a molecule of water.
Levels of protein structural organization
  • Polypeptides fold into secondary alpha-helices or beta-pleated sheets, stabilizing further into tertiary globular or fibrous configurations.
Bonds stabilizing tertiary protein structure
  • Tertiary protein folding is maintained by disulfide bridges, ionic bonds between charged R-groups, hydrogen bonds,
  • and hydrophobic interactions.
Tertiary protein folding and bonding
  • Tertiary protein folding and bonding tertiary structures are stabilized by ionic bonds, covalent disulfide bridges,
    • and hydrophobic interactions between non-polar R groups.
Alpha helix secondary structure
  • Alpha helix secondary structure hydrogen bonding between carbonyl oxygen atoms and amine hydrogens along polypeptide backbones maintains regular alpha helix geometry.
Fibrous proteins and prosthetic groups
  • Fibrous proteins and prosthetic groups structural fibrous proteins lack prosthetic groups,
  • while globular proteins like haemoglobin incorporate iron-containing haem prosthetic groups.
Triple helix
  • Three polypeptides wound together.
Hydrophobic
  • Water-hating; non-polar and unable to dissolve in water.
Specific
  • Able to act on only one type of substrate.
Fibrous protein
  • A long, insoluble structural protein, such as collagen.
Tertiary structure
  • 3D folding of a polypeptide held by R group interactions.
Prosthetic group
  • A non-protein component, such as haem in haemoglobin.
Primary structure
  • The sequence of amino acids in a polypeptide.
Hydrophobic interaction
  • A weak attraction between non-polar R groups.
Quaternary structure
  • Arrangement of multiple polypeptide subunits.

Key terms

TermMeaning
Amino acidThe monomer unit of proteins.
R groupThe variable side chain of an amino acid.
Peptide bondThe covalent bond between the C of one amino acid and the N of another.
DipeptideTwo amino acids joined by a peptide bond.
PolypeptideA chain of many amino acids.
Amine groupThe –NH2 functional group.

Avoid these mistakes

1
Including the R group in the peptide bond — the bond forms between the carboxyl and amine groups.
2
Saying peptide bond formation uses up water — water is released.
3
Saying all amino acids have the same R group — they differ only in their R groups.
4
Confusing glycosidic and peptide bonds — peptide bonds link amino acids.

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