Proteins

What are proteins

Macromolecules composed of long chains of amino acids.

What are the roles of proteins

• Enzymes

• Antibodies

• Hormones

• Transport

• Within cell membranes

• Structural roles

What is an amino acid

A carbon bonded to an amine (amino) group, a carboxyl (carboxylic acid) group, a hydrogen atom, and a variable R group.

What are essential amino acids

Amino acids that must be ingested as they cannot be synthesised by the body.

How to plants obtain amino acids

Plants require a source of fixed nitrogen e.g. nitrate ions from the soil, to synthesise all the amino acids they need.

What is an amphoteric molecule

A molecule that can act as an acid or a base

How are amino acids amphoteric and what does this allow some amino acids to act as

The amine group can accept a hydrogen ion while the carboxylic group can donate a hydrogen ion. This allows some amino acids to act as buffers and resist a change in pH.

What is the bond between 2 amino acids called

Peptide bond

How does a peptide bond form

In a condensation reaction between the amine group of one amino acid and the carboxyl group of another amino acid.

Where does a polypeptide form

At a ribosome

What enzymes hydrolyse polypeptides

Proteases

Primary structure

Order of amino acids in a polypeptide chain, which is determined by the order of bases in a gene in DNA.

Secondary structure

Regular coiling or folding of the polypeptide chain to form an α helix or ß pleated sheet. This structure is held by hydrogen bonds.

Tertiary structure

Further irregular coiling and folding of the polypeptide chain to give a specific 3D shape due to bonds between R-groups.

Quaternary structure

Linking of more than one polypeptide chain to form a protein. This occurs in the Golgi body.

What are the 4 bonds (excluding peptide bonds) that maintain the structure of proteins. list them in descending order of strength.

• Disulfide bonds

• Ionic bonds

• Hydrogen bonds

• Hydrophobic interactions

Disulfide bonds

Covalent bond between sulfur atoms in R groups of 2 cysteine amino acids.

Ionic bonds

Electrostatic attraction between NH₃⁺ on a basic amino acid and COO⁻ on an acidic amino acid.

Hydrogen bonds

Weak electrostatic attraction between partial charges on electronegative oxygen atoms (in O-H and C=O) and electropositive hydrogen atoms (in NH and OH).

Hydrophobic interactions

These form between non-polar R groups.

What bonds maintain the primary structure

Peptide bonds only.

What bonds maintain the secondary structure

Hydrogen bonds only.

What bonds maintain the tertiary and quaternary structures

Disulfide, ionic, hydrogen bonds and hydrophobic interactions.

What is a conjugated protein

A protein that has a non-protein group attached to it (e.g. haem group on haemoglobin). This modification occurs in the Golgi body.

Properties of fibrous proteins

• Polypeptide chains arranged in parallel with little or no tertiary folding.

• Chain length may vary.

• Usually insoluble in water.

• Stable.

• Have a structural role.

Function of collagen

They have a high tensile strength to provide mechanical strength.

Locations of collagen

Tendons, bones, cartilage, the wall of arteries (to withstand pressure) and connective tissue.

Function of elastin

Elastin fibres are strong and extensible to allow stretch and recoil.

Locations of elastin

Skin, arteries, bronchioles and alveolar walls

Function of Keratin

Keratin filaments are insoluble and strong to provide mechanical protection and form a waterproof barrier.

Locations of Keratin

Nails, hair, fur, scales, hooves, claws, and feathers.

Properties of globular proteins

• Polypeptide chains have a tertiary structure and fold into a specific, compact spherical shape.

• Usually soluble in water.

• Usually less stable (denature easily).

• Often have a transport or metabolic role.

How are amino acids in globular proteins arranged

Hydrophobic R groups are located on the inside of the protein. They interact with each other (form hydrophobic interactions) and exclude water. Hydrophilic R groups are located on the outside of the protein, where the interact (form hydrogen bonds) with water molecules.

Hydrophilic R groups being located on the outside of globular proteins give them what property

Solubility in water

What is the exception to globular proteins being soluble in water

Integral membrane proteins would have hydrophobic R groups on the outside where they can interact with the hydrophobic phospholipid tails.

Why are globular proteins less stable than fibrous proteins

Because their tertiary structure is held by hydrogen and ionic bonds which can be broken by high temperature and extremes of pH so the protein denatures.

What are some examples of globular proteins

Haemoglobin and antibodies

Structure of haemoglobin

Quaternary structure composed of 2 α polypeptide chains and 2 ß polypeptide chains. It is also conjugated as each polypeptide chain has a covalently attached haem prosthetic group containing Fe²⁺.

What is the role of haemoglobin

To transport oxygen from the lungs to the tissues. The oxygen binds to the iron in each haem group meaning one molecule of haemoglobin can bind 4 molecules of oxygen.

Structure of antibodies

Quaternary structure composed of 2 light chains and 2 heavy chains held together by disulfide bonds. They are also conjugated as a carbohydrate is added to them.