[4] Proteins

What are proteins made from?

Amino acids, which are their monomers.

What is a polypeptide?

A polymer formed from many amino acids joined by peptide bonds.

What groups are attached to the central carbon of an amino acid?

Amino, carboxyl, hydrogen and R groups.

What varies between the 20 common amino acids?

The R group (side chain).

What does the R group determine?

The properties of an amino acid and its interactions in proteins.

How are dipeptides formed?

Two amino acids join by a condensation reaction.

What bond joins two amino acids?

A peptide bond.

Which groups form a peptide bond?

The carboxyl group of one amino acid and amino group of another.

What is removed during peptide bond formation?

Water, formed from OH and H.

How are peptide bonds broken?

By hydrolysis, which adds water.

What is primary protein structure?

The sequence of amino acids in a polypeptide.

Why is primary structure important?

It determines how the protein folds and therefore its function.

What can happen if one amino acid changes?

Protein shape and therefore function may change.

What is secondary protein structure?

Folding of the polypeptide into alpha helices and beta-pleated sheets.

What holds secondary structure together?

Hydrogen bonds between parts of the polypeptide backbone.

What are the two main secondary structures?

Alpha helix and beta-pleated sheet.

What is tertiary protein structure?

The further folding and twisting of secondary structures into 3D shape.

What determines tertiary structure?

The primary structure and interactions between R groups.

Which bonds can maintain tertiary structure?

Hydrogen bonds, ionic bonds and disulfide bridges.

How does pH affect ionic bonds in proteins?

Changes in pH can disrupt ionic bonds and alter protein shape.

Why is tertiary structure important?

It determines the protein's specific shape and therefore function.

What is quaternary protein structure?

The association of multiple polypeptide chains.

What else may occur in quaternary structure?

A non-protein prosthetic group may be associated with the protein.

Give an example of a prosthetic group.

The iron-containing haem group in haemoglobin.

What are fibrous proteins?

Long strands of polypeptide chains with cross-linkages.

Give two examples of fibrous proteins.

Keratin and collagen.

What are globular proteins?

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

Why are globular proteins soluble?

Their structure exposes hydrophilic regions to water.

What is the main role of fibrous proteins?

Structural roles.

What is the Biuret test used for?

Testing for proteins.

What is added first in the Biuret test?

Sodium hydroxide solution.

What is added after sodium hydroxide in the Biuret test?

A few drops of very dilute copper(II) sulfate solution.

What is a positive Biuret test result?

A purple/lilac colour.

What does the Biuret test detect?

Peptide bonds, indicating the presence of protein.

What is a negative Biuret test result?

The solution remains blue.

What is a conjugated protein?

A protein containing an additional non-protein prosthetic group.

How does protein structure relate to protein function?

The amino acid sequence determines folding, shape and therefore function.

What happens when a protein is hydrolysed?

Water is added to break peptide bonds.

What is a condensation reaction?

A reaction where two molecules join and water is removed.

What is hydrolysis?

A reaction where water is added to break a bond.

What is the difference between fibrous and globular proteins?

Fibrous proteins are long and structural; globular proteins are compact and generally soluble.

What maintains tertiary protein structure?

Hydrogen bonds, ionic bonds and disulfide bridges.

What happens if the primary structure of a protein changes?

The tertiary structure may change, altering the protein's function.