Question

Question 8.  A transmembrane protein uses a β barrel structure to span the cell membrane.  How can amino acid mutations in a protein within this β barrel structure affect interactions with the membrane and why? (Up to 50 words)  

Question 9.  Haemoglobin is a protein that carries oxygen in our red blood cells.  Sickle cell anaemia is a genetic disease in which Glu at position 6 of haemoglobin is mutated to a Val, rendering haemoglobin non-functional.

1. What type of amino acid change is this?  (1 mark)

2. Give one reason why this could affect the function of this protein.  (1 mark)

Question 10.  Lectin is a protein that binds carbohydrates and is critical for biological recognition processes in living organisms.  Describe what bonds and forces could be involved in lectin and carbohydrate interactions. (Up to 50 words)  

Question 11.  A hexokinase is an enzyme that adds a phosphate to glucose after it enters the cell, which is considered the first step of glycolysis.  One enzyme, hexokinase A (HKA), has a K_m of 0.02 mM, whereas another, hexokinase B (HKB), has a K_m of 1.0 mM.  Explain why some types of fast growing cancer cells would use HKA instead of HKB. (Up to 50 words)  

Answer 1

Question 8.  A transmembrane protein uses a β barrel structure to span the cell membrane.  How can amino acid mutations in a protein within this β barrel structure affect interactions with the membrane and why? (Up to 50 words)  (4 marks)

A beta barrel structure is a secondary structure of protein which forms a closed barrel structure in which the first and the last strand of the protein are joined through hydrogen bonds. Beta barrels are involved in ion transport and thus can be found in channels. An example of beta barrel is porin channel which aids in absorption of water into cells. Typically the core of the beta barrel is formed of charged amino acids so that the ions entering through the core do not face any repulsion. On the other hand, the strands of beta-barrel which interact with membrane are formed of hydrophobic amino acids so that they can easily interact with the hydrophobic tails of phospholipids.

A mutation which results in the replacement of a hydrophobic amino acid, say valine, with a charge amino acid, say lysine,  in the region adjoining the membrane can disrupt the interactions with the hydrophobic tail. When in contact with the charged amino acid, the hydrophobic tail ( which means water fearing) will try to move away from it. Thus the interactions will be disrupted leading to the collapse of beta barrel structure.

Question 9.  Haemoglobin is a protein that carries oxygen in our red blood cells.  Sickle cell anaemia is a genetic disease in which Glu at position 6 of haemoglobin is mutated to a Val, rendering haemoglobin non-functional.

1. What type of amino acid change is this?  

When a polar charged amino acid such as Glutamate is replaced by a non-polar hydrophobic amino acid, it is called a radical replacement. A radical replacement of amino acid occurs when an amino acid is replaced by another with dissimilar properties.

1.Give one reason why this could affect the function of this protein.

Mutation occurs in the beta-chain of hemoglobin. Normally the hemoglobin molecules exist as isolated units inside the red blood cells whether they are oxygen bound or not. However in case of sickle-cell anemia, the hemoglobin molecules form long polypeptide chains when they are not oxygen bound. These rigid polypeptide chains change the disc-shaped structure of red blood cells into a sickle-cell shape which cannot easily migrate through the blood vessels resulting in diseased condition.

Question 10.  Lectin is a protein that binds carbohydrates and is critical for biological recognition processes in living organisms.  Describe what bonds and forces could be involved in lectin and carbohydrate interactions. (Up to 50 words)

Lectin - carbohydrate interaction is important for cell adhesion, signal transduction, pathogen detection ,etc. In majority of lectin-carbohydrate interactions, hydrogen bonding and charge transfer interactions occur. For example - In case of Type-C Lectin, calcium ion aids in the formation of coordinate covalent bond between the lectin and carbohydrate chain. The hydrogen bonds present in carbohydrate and lectin interact with the electronegative atoms to form hydrogen bonding.