Haemoglobin and Collagen

AS · 11 min

Haemoglobin and collagen are the syllabus's two case studies of how protein structure determines function. Haemoglobin is a globular protein that picks up oxygen in the lungs and releases it in respiring tissues; collagen is a fibrous protein that gives skin, tendons, bone and artery walls their tensile strength. Both have quaternary structure, but they could hardly be more different in shape, solubility and role. Expect a 4 to 6 mark "describe the structure" or "relate structure to function" question on either, and a comparison of the two.

Haemoglobin: a globular protein

Haemoglobin is the oxygen-carrying protein inside red blood cells. Each red blood cell contains about 280 million haemoglobin molecules.

Structure

Key result

A haemoglobin molecule has quaternary structure, made of:

four polypeptide chains: two alpha (α) chains (α-globin, 141 amino acids each) and two beta (β) chains (β-globin, 146 amino acids each);
four haem groups, one attached to each polypeptide. A haem group is a non-protein prosthetic group: a ring-shaped molecule with an iron ion, $\ce{Fe^{2+}}$ , at its centre.

Each haem group can bind one molecule of oxygen ( $\ce{O2}$ ), so each haemoglobin molecule can carry up to four oxygen molecules.

Building up the levels of structure:

Primary structure. Each globin chain has a specific sequence of amino acids, determined by the α-globin and β-globin genes.
Secondary structure. Much of each chain is coiled into α-helices.
Tertiary structure. Each chain folds into a compact, specific 3-D shape. Hydrophobic R groups point inwards, forming a pocket that holds the haem group; hydrophilic R groups are on the outside of the molecule.
Quaternary structure. The four chains fit together into a roughly spherical molecule, held by hydrophobic interactions, hydrogen bonds and ionic bonds between the chains.

Relating the structure of haemoglobin to its function

Key result

Structural feature	How it relates to function
Iron ion ( $\ce{Fe^{2+}}$ ) in each haem group	the site where oxygen binds; one $\ce{O2}$ binds reversibly to each $\ce{Fe^{2+}}$ , so oxygen can be picked up in the lungs and released in the tissues
Four haem groups	each molecule carries four oxygen molecules, increasing the oxygen-carrying capacity of blood
Hydrophilic R groups on the outside	haemoglobin is soluble in the watery cytoplasm of the red blood cell, so it can be packed in at high concentration
Hydrophobic R groups lining the haem pocket	hold the haem group in place and keep water away from the iron, which helps the iron stay as $\ce{Fe^{2+}}$ so it can bind oxygen
Quaternary structure of four interacting chains	when one oxygen molecule binds, the shape of the molecule changes slightly, making it easier for the next oxygen molecules to bind (cooperative binding); this gives the S-shaped oxygen dissociation curve, so haemoglobin loads oxygen readily in the lungs and unloads it readily in respiring tissues
Specific sequence of amino acids	gives the precise shape needed; a single change can alter function (sickle cell haemoglobin)

When oxygen binds, haemoglobin becomes oxyhaemoglobin:

\ce{Hb + 4O2 <=> HbO8}

The reaction is reversible: in the lungs, where the partial pressure of oxygen is high, it moves to the right; in respiring tissues, where the partial pressure of oxygen is low, it moves to the left. This is developed fully in The oxygen dissociation curve.

The importance of iron

Iron is essential because it is the part of haemoglobin that binds oxygen. Without enough iron in the diet, a person cannot make enough functional haemoglobin, so the oxygen-carrying capacity of the blood falls. This is iron-deficiency anaemia, which causes tiredness and breathlessness because tissues receive less oxygen for aerobic respiration.

The iron must be in the $\ce{Fe^{2+}}$ (iron(II)) state to bind oxygen. Carbon monoxide is dangerous because it binds to the iron in haem far more strongly than oxygen does, and does not readily come off, so less haemoglobin is available to carry oxygen.

When the primary structure changes: sickle cell haemoglobin

In sickle cell anaemia, a mutation in the β-globin gene changes one amino acid in each β chain: glutamic acid (with a charged, hydrophilic R group) at position 6 is replaced by valine (with a non-polar, hydrophobic R group). This R group is on the outside of the molecule. At low oxygen concentrations, the hydrophobic valine on one molecule sticks to a hydrophobic region of another, so haemoglobin S molecules link up into long fibres. These distort the red blood cells into a sickle shape, making them less able to carry oxygen and likely to block capillaries. This is a striking example of how a change to one amino acid in the primary structure changes the properties and function of a protein (see Gene mutations).

Collagen: a fibrous protein

Collagen is the most abundant protein in mammals. It forms the tough, inelastic fibres of skin, tendons, ligaments, cartilage, bone, teeth, the cornea and the walls of blood vessels.

Structure of a collagen molecule

Key result

A collagen molecule is made of three polypeptide chains, each about 1000 amino acids long.
Each chain is coiled into a tight helix (a different, more extended helix from the α-helix).
The three chains are wound around each other to form a triple helix, like a three-stranded rope.
Every third amino acid is glycine, the amino acid with the smallest R group (a hydrogen atom). Glycine always lies on the inside of the triple helix, where there is room only for a very small R group, so the three chains can pack very closely together.
The three chains are held together by hydrogen bonds between them.
Collagen also contains a high proportion of proline and hydroxyproline, which help make the helix rigid.

The primary structure of each chain is therefore highly repetitive: glycine–X–Y, glycine–X–Y, and so on, where X is often proline and Y often hydroxyproline. This regular sequence is typical of fibrous proteins.

From molecule to fibre

Molecules. Each triple-helix collagen molecule is long and thin (about 300 nm long and 1.5 nm across).
Fibrils. Many molecules lie side by side, parallel to each other, linked by covalent cross-links between the R groups of amino acids (lysine and hydroxylysine) in neighbouring molecules. The molecules are staggered, so the ends of adjacent molecules do not line up; this avoids lines of weakness and gives the fibril its characteristic banded (striated) appearance in electron micrographs.
Fibres. Many fibrils bundle together to form collagen fibres, which are large enough to be seen with a light microscope.

Relating the structure of collagen to its function

Key result

Structural feature	How it relates to function
Three polypeptides wound into a triple helix	a rope-like structure that is very strong when pulled (high tensile strength)
Glycine every third amino acid (smallest R group)	allows the three chains to pack tightly together, making the triple helix compact and strong
Hydrogen bonds between the three chains	hold the chains firmly together
Covalent cross-links between neighbouring molecules	link molecules into fibrils that are very strong and stable
Molecules staggered, so ends do not coincide	no weak points along the fibril; strength is spread along its length
Fibrils bundled into fibres	further increases strength; fibres can be aligned in the direction of the force (parallel in tendons, in layers in skin)
Many hydrophobic R groups on the surface; very large molecules	insoluble in water, so collagen stays in place as a structural material and is not dissolved in tissue fluid
Collagen fibres are inelastic (do not stretch)	tendons transmit the pull of muscles to bones without stretching; in artery walls collagen prevents over-stretching and bursting under high blood pressure

Comparing haemoglobin and collagen

Feature	Haemoglobin	Collagen
Type of protein	globular	fibrous
Shape	compact, roughly spherical	long, rope-like
Number of polypeptides	four (2 α, 2 β)	three
Secondary structure	mainly α-helices	each chain a tight, extended helix; three wound into a triple helix
Primary structure	varied, non-repeating sequence	repetitive; glycine every third amino acid
Prosthetic group	yes: four haem groups with $\ce{Fe^{2+}}$	none
Bonds between molecules	none: separate molecules in solution	covalent cross-links between molecules
Solubility	soluble	insoluble
Role	physiological: transports oxygen	structural: tensile strength
Location	inside red blood cells	extracellular, in skin, tendons, bone, artery walls

Oxygen-carrying capacity (routine)

A red blood cell contains $2.8 \times 10^{8}$ haemoglobin molecules. Calculate the maximum number of oxygen molecules it can carry, and explain your reasoning.

Solution

Each haemoglobin molecule has four haem groups, each with one $\ce{Fe^{2+}}$ that binds one $\ce{O2}$ molecule, so each molecule carries up to four $\ce{O2}$ .

4 \times 2.8 \times 10^{8} = 1.12 \times 10^{9} \text{ oxygen molecules} \approx 1.1 \times 10^{9}

Describe the structure of haemoglobin (5 marks)

Describe the structure of a molecule of haemoglobin.

Solution

Marking points:

Globular protein with quaternary structure;
four polypeptide chains;
two α (α-globin) and two β (β-globin) chains;
each chain has a haem group (prosthetic group);
each haem contains an iron ion ( $\ce{Fe^{2+}}$ ), which binds one oxygen molecule;
chains have α-helix secondary structure; folded into specific tertiary structure;
hydrophobic R groups inside, hydrophilic R groups outside (soluble);
chains held together by hydrogen bonds, ionic bonds and hydrophobic interactions.

Relate the structure of collagen to its function (6 marks)

Explain how the structure of collagen is related to its function in tendons.

Solution

Marking points:

Function: tendons join muscle to bone and must transmit pulling forces without breaking or stretching, so they need high tensile strength and to be inelastic.
Each molecule is three polypeptide chains wound into a triple helix.
Glycine (smallest amino acid) is every third amino acid, allowing close packing of the chains.
Hydrogen bonds hold the three chains together.
Molecules are joined by covalent cross-links to form fibrils.
The ends of adjacent molecules are staggered, so there are no weak points.
Fibrils are grouped into fibres, and in tendons the fibres are arranged parallel, along the line of pull.
Collagen is insoluble, so it is not dissolved by tissue fluid.

Exam-hard: applying the ideas to an unfamiliar situation

Scurvy is a disease caused by a lack of vitamin C. Vitamin C is needed by the enzyme that converts proline into hydroxyproline in collagen. Symptoms include fragile blood vessels that bleed easily and wounds that do not heal. Suggest an explanation for these symptoms.

Solution

Without vitamin C, less hydroxyproline is made, so collagen chains contain less of it.
Hydroxyproline helps stabilise the triple helix (forming hydrogen bonds between the chains), so the collagen molecules are less stable and weaker.
Fewer stable molecules and cross-links means weaker fibrils and fibres with lower tensile strength.
Blood vessel walls contain collagen, which gives strength and resists the pressure of blood; with weak collagen the walls are fragile and rupture, causing bleeding.
Wound healing requires new collagen fibres to be laid down (in scar tissue); weak or insufficient collagen means wounds do not heal properly.

Credit is given for applying collagen's structure–function relationships to the new context, which is exactly what "suggest" questions test.

Watch out

Haemoglobin has four polypeptides and four haem groups; it carries four oxygen molecules (eight oxygen atoms). Saying "each haemoglobin carries one oxygen" or "two α and two β haem groups" is wrong: the haem groups are not α or β.
The iron in haem is $\ce{Fe^{2+}}$ , and oxygen binds to the iron, not to the globin.
Collagen is not made of α-helices wound together. Each chain is its own helix, and three chains form the triple helix.
In collagen, hydrogen bonds hold the three chains of one molecule together; covalent cross-links join neighbouring molecules into fibrils. Swapping these loses marks.
Collagen gives tensile strength, not elasticity. The elastic protein in artery walls is elastin.

Exam tip

The syllabus asks you to describe haemoglobin "including the formation of its quaternary structure from two alpha (α) chains (α-globin), two beta (β) chains (β-globin) and a haem group". Use those exact words.
"Relate structure to function, including the importance of iron": state that each $\ce{Fe^{2+}}$ binds one $\ce{O2}$ reversibly, so four per molecule; iron deficiency reduces oxygen transport.
For collagen, the most commonly credited points are: three chains, triple helix, glycine every third amino acid (small, close packing), hydrogen bonds between chains, covalent cross-links between molecules, staggered ends, fibrils form fibres, high tensile strength, insoluble.

Summary

Haemoglobin: globular protein; quaternary structure of 2 α-globin and 2 β-globin chains, each with a haem group containing $\ce{Fe^{2+}}$ .
Each $\ce{Fe^{2+}}$ binds one $\ce{O2}$ reversibly; one haemoglobin carries four $\ce{O2}$ .
Hydrophilic R groups outside make haemoglobin soluble; hydrophobic R groups inside hold the haem.
Binding of one $\ce{O2}$ changes the shape and makes further binding easier (cooperative binding).
Collagen: fibrous protein; three polypeptides in a triple helix; glycine every third amino acid allows close packing; hydrogen bonds between chains.
Collagen molecules are staggered and joined by covalent cross-links into fibrils, which form fibres: high tensile strength, insoluble, inelastic.

Practice questions

Question

State the number and type of polypeptide chains in a haemoglobin molecule.
Name the prosthetic group in haemoglobin and the ion it contains.
Explain why haemoglobin is soluble while collagen is insoluble. (3 marks)
Explain why iron is important in the diet. (2 marks)
Explain why glycine is important in the structure of collagen.
Name the bonds that (a) hold the three chains of a collagen molecule together and (b) link neighbouring collagen molecules.
State three places in the body where collagen is found and relate one to the function of collagen.
Compare the structures of haemoglobin and collagen. (5 marks)
In sickle cell haemoglobin, glutamic acid is replaced by valine on the surface of the β chains. Suggest why this change makes the molecules stick together at low oxygen concentration. (3 marks)
Explain how the structure of haemoglobin allows it to load oxygen efficiently in the lungs and unload it efficiently in respiring tissues. (5 marks)

Answers

Four: two α-globin chains and two β-globin chains.
Haem group; iron(II) ion, $\ce{Fe^{2+}}$ .
Haemoglobin is globular, with hydrophilic R groups on the outside that interact with water; collagen has many hydrophobic R groups on its surface and forms very large cross-linked fibrils, so it does not dissolve. (Also: collagen molecules are bonded to each other covalently into fibres.)
Iron is part of the haem group; it is where oxygen binds; without it less haemoglobin is made, reducing oxygen transport (anaemia).
Glycine has the smallest R group (H); every third amino acid is glycine, which lies in the centre of the triple helix, allowing the three chains to pack closely together, giving strength.
(a) Hydrogen bonds. (b) Covalent bonds (cross-links).
Any three of skin, tendons, ligaments, bone, cartilage, artery walls, cornea, teeth. Relation e.g.: tendons need high tensile strength and must not stretch to transmit muscle force to bone; artery walls need to resist high pressure without bursting.
Both proteins with quaternary structure, made of more than one polypeptide, held partly by hydrogen bonds. Haemoglobin globular, collagen fibrous; haemoglobin four chains, collagen three; haemoglobin has haem prosthetic groups with iron, collagen none; collagen has glycine every third amino acid (repetitive sequence), haemoglobin varied sequence; collagen chains form a triple helix, haemoglobin chains mainly α-helices folded into a compact shape; collagen molecules covalently cross-linked into fibrils, haemoglobin molecules separate; haemoglobin soluble, collagen insoluble.
Glutamic acid has a charged, hydrophilic R group that interacts with water; valine has a non-polar, hydrophobic R group; on the surface, it is repelled by water and forms hydrophobic interactions with hydrophobic regions on other haemoglobin molecules; so molecules stick together to form long fibres.
Four haem groups each with $\ce{Fe^{2+}}$ , each binding one $\ce{O2}$ reversibly; quaternary structure allows cooperative binding: when the first $\ce{O2}$ binds, the shape of the molecule changes, making it easier for the second and third to bind, so haemoglobin becomes highly saturated at the high partial pressure of oxygen in the lungs; as oxygen is released in tissues at low partial pressure, the shape change makes it easier for remaining oxygen to be released, so a small fall in partial pressure releases a lot of oxygen; haemoglobin is soluble so can be concentrated in red blood cells.

Haemoglobin: a globular protein

Structure

Relating the structure of haemoglobin to its function

The importance of iron

When the primary structure changes: sickle cell haemoglobin

Collagen: a fibrous protein

Structure of a collagen molecule

From molecule to fibre

Relating the structure of collagen to its function

Comparing haemoglobin and collagen

Practice questions

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