Topic 7.5 Proteins

Question 1

7.5.1 Explain the primary level of protein structure, indicating the significance.

Answer 1

Primary Structure

• unique sequence of amino acids bonded by peptide bonds in protein
• 20 amino acids, arranged in any order (nucleotide base seqence in DNA determines order)
• significance: if order is changed, shape & function of protein are changed –> determines next 3 levels of protein organization

Question 2

7.5.1 Explain the secondary level of protein structure, indicating the significance.

Answer 2

Secondary Structure

• regular repeating structures stabilized by H-bonds between groups in the main chain of polypeptides
• common configurations of secondary structure are α-helix & β-pleated sheet (both have regular repeating patterns)
• created by formation of H-bonds between oxygen from carboxyl group of 1 amino acid & hydrogen from amino group of another
• doesn’t involve side chains, R groups

Question 3

7.5.1 Explain the tertiary level of protein structure, indicating the significance.

Answer 3

Tertiary Structure

• polypeptide chain bends & folds over itself b/c of interactions among R-groups & peptide backbone –> results in 3D shape
• significance: important in determining specificity of enzymes
• interactions causing tertiary organizations are:
  
  • disulfide (covalent) bonds - strong, often called bridges
  • hydrogen bonds b/w polar side chains
  • Van der Waals interactions among hydrophobic side chains of amino acids – strong b/c hydrophobic side chains are forced inwards when hydrophilic side chains interact w/ water towards outside of molecule
  • ionic bonds b/w (+) and (-) charged side chains

Question 4

7.5.1 Explain the quaternary level of protein structure, indicating the significance.

Answer 4

Quaternary Structure

multiple polypeptide chains combine to form single structure

• not all proteins have quaternary structure
• primary, secondary & tertiary bonds are involved on this level
• conjugated proteins exist at this level (include prosthetic or non-polypeptide groups)
  
  • e.g. Hemoglobin contains 4 polypeptide chains with heme (non-polypeptide group; contains iron atom that binds to oxygen) on each chain

Question 5

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

Answer 5

Fibrous

• many polypeptide chains, long narrow shape
• usually insoluble in water
• e.g. collagen (structural role in connective tissues of humans) & actin + myosin (major component of human muscle, involved in contraction)

Globular

• more 3D shape
• mostly water soluble
• e.g. hemoglobin (delivers O₂ to body tissues) & insulin (regulates blood glucose level in humans)

Question 6

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

Answer 6

Amino acids are grouped according to properties of their side chains (R-groups) –> important in determining enzyme specificity. Specific substrates only combine with particular enzyme’s active sites (combination possible when “fitting” occurs)

Polar

• hydrophilic
• found in regions of proteins exposed to water
• membrane proteins include polar amino acids towards interior & exterior of membrane – > creates hydrophilic channels in proteins through which polar substances can move

Non-polar

• hydrophobic
  
  • when in centre of water-soluble proteins, can stabilize structure
• found in regions of proteins linked to hydrophobic area of cell membrane

Question 7

7.5.4 State four functions of proteins, giving a named example of each.

Answer 7

Proteins: HAM In Im Am

• hemoglobin: protein containing iron that transports O₂ from lungs to all parts of body in vertebrates
• actin & myosin: proteins that interact to bring about muscle movement (contraction) in animals
• insulin: hormone secreted in pancreas that aids in maintaining blood glucose levels in vertebrates
• immunoglobulins: group of proteins that act as anti-bodies to fight bacteria & viruses
• amylase: digestive enzyme that catalyzes hydrolysis of starch