Proteins

What are amino acids?

monomers of proteins, 20 different types each with a different R-group

Structure of amino acids

central carbon, amino group, carboxyl group, R group, hydrogen atom

How is a peptide bond formed?

A condensation reaction between two amino acids, molecule of water is removed (-OH for carbonyl group and -H from amino group)

What is a dipeptide?

2 amino acids joined by a peptide bond

What is a tripeptide?

3 amino acids bonded together

How is the peptide bond broken?

Hydrolysis- will release constituent amino acids

What is a polypeptide?

polymer of amino acids, always one need with a free amino group called the N-terminus and one end with a free carboxyl group called the C-terminus

What determines the function of proteins?

The primary structure shape- changing a single amino acid will change the shape and therefore function

what is the primary structure of proteins?

The sequence of amino acids in a polypeptide chain, which determines its overall shape and function

Bonded by peptide bonds

Formed by condensation reactions

what is the secondary structure of proteins?

the sequence of amino acids causes parts of a protein molecule to bend into alpha-helix shapes or fold into beta-pleated sheets

these structures are held in place by hydrogen bonds

where are the hydrogen bonds in the secondary structure?

form between the C=O group of carboxyl group of one amino acid and the H in the amine group of another amino acid

what is the tertiary structure of a protein?

• the further folding of the secondary structure

• to form a unique, 3D shape

• held in place by ionic, hydrogen and disufide bonds

where do the ionic and disulfide bonds in the tertiary structure form?

between the R-groups of different amino acids

when do disulfide bonds form?

when there is a sulfur in the R-groups of both amino acids

what is the quaternary structure of a protein?

more than one polypeptide chain making up the protein

e.g. haemoglobin- made of 4 polypeptide chains

what does it mean if a protein is denatured?

the bonds which hold the tertiary and secondary structures in shape break, so the unique 3D shape is lost (especially ionic and hydrogen bonds break)

importance of primary structure

• if one amino acid in sequence is different the ionic/hydrogen/disulfide bonds form in a different location

• results in a different 3D shape

• happens if a mutation occurs

Structure of a globular protein?

• Compact, spherical molecules

• Highly irregular sequence

• Tertiary structure formed by folding of the polypeptide chain

What does the folding of the polypeptide chain in a globular protein do?

Brings hydrophobic amino acid residues inside and hydrophilic residues outside

Bonds in a globular protein?

• Hydrogen bonds between polar groups

• Ionic bonds between charged R-groups

• Disulphide bridges

• Hydrophobic interactions help maintain proteins shape

Are globular proteins soluble in water?

Yes as hydrophilic amino acids on outside

Functions of globular proteins

• act as proteins to speed up biochemical reactions

• Transport proteins

• Hormones

• Form part of immune system

• Store molecules/ions

Examples of globular proteins

• haemoglobin

• Myoglobin

• Insulin

• Amylase

• Antibodies

Structure of fibrous proteins

• long, narrow, thread-like shape

• Made of parallel polypeptide chains that form strong fibres/sheets

• Mostly secondary structure with little tertiary folding

• Regular sequence

What bonds are in fibrous proteins?

• hydrogen bonds stabilise their repetitive structures

• Cross linking (covalent bonds) like disulphide bridges provide extra strength and stabillity

• Hydrophobic interactions contribute to strength

Are fibrous proteins soluble?

No- insoluble as hydrophobic amino acids are exposed and structure is rigid

Functions of fibrous proteins

• provide structural support and strength to cells and tissues

• Form key components of connective tissues, skin, hair, nails and tendons

• Maintain shape and integrity of cells and organisms

Examples of fibrous proteins

• collagen- found in connective tissue tendons and bones as has high tensile strength

• Keratin

• Elastin