7.5 Proteins

Topic 7 Nucleic Acids & Proteins

7.5.1 Explain the four levels of protein structure, indicating the significance of each level.

(Quaternary structure may involve the binding of a prosthetic group to form a conjugated protein)

7.5.2 Outline the difference between fibrous and globular proteins, with reference to two examples of each type.

7.5.3 Explain the significance of polar and non-polar amino acids.

(Limit this to controlling the position of proteins in membranes, creating hydrophilic channels through membranes, and the specificity of active sites in enzymes)

7.5.4 State four functions of proteins, giving a named example

of each. (Membrane proteins should not be included)

Protein Structure Proteins have a complex structure. They are built up from amino acids. The sequence of amino acids will determine its shape and

ultimately its function. We can define four levels of structure in proteins:

Primary structure Secondary structure Tertiary structure Quaternary structure

Protein Structure Primary Structure:

Is the linear sequence of amino acids in a protein.

The amino acids are held together by peptide linkages - covalent bonds.

Bonding involved Covalent bonds between amino

acids – peptide bonds (linkages).

Protein Structure Secondary Structure:

These are regularly repeating structures.

These include -helix and -pleated sheets. -helix: where the polypeptide

chain is wound into a helix. -pleated sheets:

Bonding involved: Hydrogen bonding between

different amino acids in helix or pleated sheets.

Protein Structure Tertiary Structure:

Results in the overall three-dimensional shape of the protein

Can form either globular or fibrous proteins.

3D shape determines its functions. Bonding involved includes:

Covalent bonding – disulfide bridges Hydrogen bonding Ionic bonding Hydrophobic interactions

Protein Structure Quaternary Structure:

Is when two or more polypeptide chains are linked together.

Bonding involved includes: Covalent bonding – disulfide bridges Hydrogen bonding Ionic bonding Hydrophobic interactions

Shape of Proteins Proteins can be divided into two types according to their

shape: Fibrous

Have a long and narrow shape. Are mostly insoluble in water. eg: keratin, collagen

Globular Have a rounded shape. Are mostly soluble in water. eg: enzymes, antibodies

Polar & Non-polar Groups Amino acids can be divided into two types according to

the chemical characteristics of their R groups:

Polar amino acids have hydrophilic R groups. (12) Non-polar amino acids have hydrophobic R groups. (8) The distribution of polar and non-polar amino acids in a

protein influence where the protein is located in a cell and what function it can carry out.

Amine group

Carboxyl group

Polar & Non-polar Groups

Protein Functions Proteins have a huge range

Enzymes

eg: catalase Structural.

eg: collagen Transport.

eg: haemoglobin

of functions:

Movement. eg: myosin

Hormones eg: insulin

Defence. eg: immunoglobulin

7.5.1 Explain the four levels of protein structure, indicating the significance of each level.

(Quaternary structure may involve the binding of a prosthetic group to form a conjugated protein)

7.5.2 Outline the difference between fibrous and globular proteins, with reference to two examples of each type.

7.5.3 Explain the significance of polar and non-polar amino acids.

(Limit this to controlling the position of proteins in membranes, creating hydrophilic channels through membranes, and the specificity of active sites in enzymes)

7.5.4 State four functions of proteins, giving a named example

of each. (Membrane proteins should not be included)