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2.3.52 · Biological Molecules

Haemoglobin — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 20 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

2.3.5 Haemoglobin — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of haemoglobin and explain how it carries oxygen.Type — a globular protein found in huge amounts in red blood cells.Quaternary structure — four polypeptide subunits: two alpha-g…

  • Prosthetic group
  • Importance
  • Oxygen binding
  • Why haem is needed
  • Efficiency
  • Cooperative binding

Part 1 of 5Structure of haemoglobin

Structure of haemoglobin
Type
  • Type a globular protein found in huge amounts in red blood cells.
Quaternary structure
  • Quaternary structure four polypeptide subunits: two alpha-globins and two beta-globins.
Prosthetic group
  • Prosthetic group each subunit contains a haem group.
Bonding
  • Bonding the subunits are held together by bonds between R groups, including disulfide bonds.

Part 2 of 5Arrangement of R groups

Arrangement of R groups
Inside
  • Inside hydrophobic R groups face inwards, preserving the spherical shape.
Outside
  • Outside hydrophilic R groups face outwards, keeping it soluble.
Importance
  • Importance changing the amino acid sequence changes its properties, as in sickle cell anaemia.

Part 3 of 5The haem group

The haem group
Iron
  • Iron each haem contains an Fe2+ ion.
Oxygen binding
  • Fe2+ reversibly combines with an oxygen molecule to form oxyhaemoglobin, which is bright red.
Capacity
  • Capacity four haem groups carry four O2 molecules.
    • (eight oxygen atoms)
Why haem is needed
  • Why haem is needed none of the amino acids of the polypeptides binds oxygen well.

Part 4 of 5Function

Function
Transport
  • Transport it binds oxygen in the lungs and carries it to tissues for aerobic respiration.
Efficiency
  • Efficiency oxygen is poorly soluble in water, so binding to the soluble haemoglobin carries far more oxygen.
Cooperative binding
  • Cooperative binding each oxygen that binds alters the quaternary structure, increasing affinity for the next oxygen.

Part 5 of 5More from the specification

More from the specification
Fibrous vs Globular Protein Structure
  • Haemoglobin is a soluble globular protein specialized for metabolic gas transport,
  • whereas collagen is an insoluble fibrous protein that provides mechanical strength to connective tissues.
Collagen Triple Helix
  • Collagen consists of three helical polypeptide chains wound tightly together into a structural triple helix, reinforced by hydrogen bonding and covalent cross-links.
Haemoglobin Quaternary Conformation
  • Composed of four separate polypeptide chains, each bound to an iron-containing haem prosthetic group that binds oxygen molecules.
    • (two alpha and two beta chains)
Secondary and Tertiary Interactions
  • Alpha helices and beta pleated sheets in secondary structure are held by peptide backbone hydrogen bonds,
  • while tertiary folding is stabilized by ionic bonds, disulfide bridges,
  • and hydrophobic interactions.
Triple helix
  • Three polypeptides wound together.
Collagen
  • A protein in skin, gums and bones that needs vitamin C to form.
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.
Primary structure
  • The sequence of amino acids in a polypeptide.
Hydrophobic interaction
  • A weak attraction between non-polar R groups.
Secondary structure
  • Α-helix and β-pleated sheet held by backbone hydrogen bonds.
Amine group
  • The –NH2 functional group.

Key terms

TermMeaning
HaemoglobinA globular protein with four subunits that carries oxygen in red blood cells.
Haem groupA prosthetic group containing an Fe2+ ion.
OxyhaemoglobinHaemoglobin combined with oxygen.
SubunitOne polypeptide chain in a protein with quaternary structure.
GlobinThe polypeptide part of haemoglobin.
Cooperative bindingBinding of one molecule makes binding of the next easier.

Avoid these mistakes

1
Calling haemoglobin fibrous — it is globular and soluble.
2
Saying haemoglobin has one subunit — it has four polypeptide chains.
3
Forgetting the prosthetic group — haem is a non-protein part containing iron.
4
Saying haemoglobin binds oxygen with its amino acids — oxygen binds to Fe2+ in haem.

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