Proteins

Question 1

Formation of peptide bond (same as chem)

Answer 1

1. Peptide bond is formed between amino group of one amino acid and carboxyl group of another amino acid
2. via condensation
3. catalysed by peptidyl transferase

Question 2

Breaking of peptide bond

Answer 2

1. Peptide bond is broken between amino group of 1 amino acid and carboxyl group of another amino acid
2. via hydrolysis
3. catalysed by protease

Question 3

How protein structure is maintained

OR

Explain how different types of bonding hold protein molecules in shape

Answer 3

1. Primary structure: amino acids of a precise number, type and sequence in the polypeptide are held together by peptide bonds
2. The polypeptide chain coils and folds into (geometrically repeating subunits) a-helices and b-pleated sheets
3. stabilised by hydrogen bonds between C=O and -NH2 groups in the main chain of the polypeptide
4. Polypeptide chain is further bent, coiled and folded to form the tertiary structure, maintained by intramolecular hydrophobic interactions, hydrogen bonds, ionic bonds and disulfide bonds between R groups of amino acids
5. Quaternary structure (if >1 pp chain) : maintained by intermolecular hydrophobic interactions, hydrogen bonds, ionic bonds and disulfide bonds between R groups of the amino acids.

Question 4

Effect of temperature on protein structure

Answer 4

1. Increase in temperature increases kinetic energy of protein
2. High heat disrupts the hydrogen bonds and hydrophobic interactions maintaining the secondary and tertiary structure of the protein
3. Resulting in the loss of specific 3D conformation of the protein and protein denatures

Question 5

Effect of pH on protein structure

Answer 5

1. Change in pH alters ionic charge of the acidic and basic R groups of the amino acids of the protein
2. Ionic bonds and hydrogen bonds that maintain the secondary and tertiary structures of the protein are disrupted
3. Resulting in the loss of specific 3D conformation of the protein and protein denatures

Question 6

Structure and function of Haemoglobin

Answer 6

1. Consists of 2 a-chains and 2 b-chains and each polypeptide chain is coiled to form a-helices which is further bent and folded into a globular subunit. The 4 protein subunits are packed closely together, forming a more compact molecule

1a. allows for packing of more haemoglobin into the red blood cell, thus more O2 molecules can be transported

2. Each protein subunit contains a prosthetic haem group with an Fe2+

2a. Allows for reversible binding of oxygen molecules, enabling Hb to carry and readily release oxygen to respiring tissues

3. Hydrophilic R groups of amino acids on the surface of Hb molecule faces outwards and interacts with the aqueous medium

3a. soluble in an aqueous environment and can be transported in the blood while
carrying O2 from lungs to tissues vice versa

Question 7

Structure and function of collagen

Answer 7

1. 3 helical polypeptide chains wind tightly around each other, forming a tropocollagen
2. Ends of the parallel tropocollagen molecule are staggered and covalent cross-links form between these ends -> leading to formation of collagen fibrils, leading to greater tensile strength

Question 8

Fibrous vs globular protein

Answer 8

1. Fibrous insoluble in water while globular soluble in water
2. Globular: metabolic functions vs Fibrous: structural functions
3. Globular: held by HIDH vs Fibrous: held by hydrogen bonds and covalent cross-links

Question 9

Describe how structure of glycogen differs from structure of cellulose

Answer 9

1. Glycogen is a polymer of a-glucose while cellulose is a polymer of b-glucose
2. Glycogen is branched while cellulose is unbranched
3. Glycogen is linked by a-1,4 glycosidic bonds and a-1,6 glycosidic bonds while cellulose is linked by b-1,4 glycosidic bonds
4. Successive b-glucose residues of cellulose are rotated 180 degrees with respect to its adjacent residue while glycogen has no succesive rotation of a-glucose residues