Globular and fibrous proteins

What are globular proteins?

• Compact, roughly spherical proteins that are soluble in water.

• Fold into spherical shapes due to:

  • Hydrophobic (non-polar) R groups orientating inwards.

  • Hydrophilic (polar) R groups orientating outwards, allowing interaction with water.

• Solubility allows globular proteins to function in metabolic reactions and transport within organisms.

Why are globular proteins soluble in water?

• Because polar R groups are on the outside and interact with water.

• Hydrophobic R groups are hidden inside, avoiding water.

• Water molecules form hydrogen bonds with outer hydrophilic regions.

What is the importance of the specific shape of globular proteins?

• The tertiary structure determines their specific function.

• Allows enzymes to catalyse specific reactions and antibodies to bind specific antigens.

What are conjugated proteins?

• Globular proteins that have a non-protein prosthetic group attached.

• Example: Haemoglobin, which has a haem group (contains Fe²⁺).

What is the structure of haemoglobin?

• Quaternary structure made of 4 polypeptide subunits:

  • 2 α-globin chains + 2 β-globin chains.

• Each subunit contains a haem group with an Fe²⁺ ion.

• Hydrophobic R groups face inward, hydrophilic R groups outward → soluble in blood plasma.

What is the function of haemoglobin?

• Found in red blood cells; transports oxygen.

• Each Fe²⁺ ion can reversibly bind one O₂ molecule → up to 4 O₂ molecules per haemoglobin.

• When O₂ binds, tertiary/quaternary structure changes, increasing oxygen affinity (cooperative binding).

• Oxygen is released in tissues where oxygen concentration is low.

How can changes in amino acid sequence affect haemoglobin?

• Mutation (e.g. sickle cell anaemia) causes valine (non-polar) to replace glutamic acid (polar).

• Reduces solubility → haemoglobin forms insoluble fibres → red blood cells distort shape.

What is the role of Fe²⁺ in haemoglobin?

• Allows reversible binding of oxygen molecules.

• Amino acids alone can’t bind O₂, so the prosthetic group is essential.

What are enzymes?

• Globular proteins that act as biological catalysts.

• Speed up chemical reactions by lowering activation energy.

• Have specific active sites that bind to substrates.

• Not used up or changed in the reaction.

What is insulin and its structure?

• Globular protein hormone made in the pancreas.

• Regulates blood glucose concentration.

• Consists of 2 polypeptide chains:

  • A chain: 21 amino acids

  • B chain: 30 amino acids

• Chains held by 3 disulfide bridges.

• Soluble → can be transported in blood.

how does the structure of insulin relate to its function

• Globular & soluble: Hydrophilic R groups face outwards → dissolves in blood for transport to target cells.

• Two polypeptide chains (A & B): Joined by disulfide bridges → maintains insulin’s 3D shape.

• Specific tertiary structure: the precise folding forms a specific active site, forms a complementary binding site to receptors on cell membranes → triggers glucose uptake, ensures the right response

What are fibrous proteins?

• Long, insoluble polypeptides with repetitive sequences.

• Have little or no tertiary structure (mainly secondary).

• High proportion of hydrophobic R groups → insoluble in water.

• Provide strength, elasticity, and support in tissues.

What is collagen and its structure and how does that relate to its function?

• Fibrous structural protein found in connective tissues: tendons, ligaments, skin, cartilage, bones.

• Each molecule = three polypeptide chains wound into a triple helix.

• Chains held by hydrogen bonds between every third glycine, allowing tight packing.

• Collagen molecules cross-link to form fibrils, which bundle into strong fibres.

• Covalent crosslinks between fibrils increase tensile strength.

• Insoluble because of length and hydrophobic R groups.

Why does collagen have high tensile strength?

• Many hydrogen bonds between chains in the triple helix.

• Covalent bonds between molecules in fibrils.

What are examples of fibrous proteins and their roles?

Protein	Role
Collagen	Connective tissue (skin, tendons, ligaments, bones)
Keratin	Hair, nails, horns, feathers – tough, protective
Elastin	Found in skin, lungs, blood vessels – allows stretch and recoil

What are key differences between globular and fibrous proteins?

Feature	Globular	Fibrous
Shape	Compact, spherical	Long, linear
Solubility	Soluble	Insoluble
Sequence	Irregular	Repetitive
Function	Metabolic (enzymes, hormones, transport)	Structural (tissue support, strength)
Example	Haemoglobin, insulin, enzymes	Collagen, keratin, elastin

What is the structure and function of keratin?

• Structure: Contains many cysteine residues → lots of disulphide bridges between polypeptides.

• Function: Provides strength and rigidity to hair, nails, and horns.

• Note: The more disulphide bonds, the less flexible the keratin.

What is the structure and function of elastin?

Elastin contains multiple crosslinks ( made from tropoelastin molecules) and coiling, giving it a strong

and extensible structure. It is found in living things where they need

to stretch and recoil as part of life processes. Elastin in our lungs

allows them to inflate and deflate.

what happens when a fibrous protein denatures compared to when a globular protein denatures

Fibrous proteins lose their structural strength when denatured, whereas

globular proteins become insoluble and inactive.

How does the structure of the enzyme catalase relate to its function?

• Type: Catalase is a globular enzyme found in nearly all living cells.

• Structure:

  • Has a quaternary structure made of four polypeptide subunits.

  • Each subunit contains a prosthetic haem group with an iron (Fe²⁺) ion at its centre.

• Function: Breaks down hydrogen peroxide (H₂O₂) — a toxic by-product of metabolism — into water and oxygen protecting cells from oxidative damage

• The globular shape makes catalase soluble, so it can function inside cells.

• The prosthetic haem groups enable redox reactions to occur quickly.