Question

Explain an example of post-translational modification of enzyme activity using examples from plants? Why is post-translational modification of proteins useful in cells? (4) Explain how extremes of cold or heat affect the balance between the light and dark (light-independent) parts of photosynthesis (use a diagram) (5)

Answer 1

1. example: phosphorylation

It is the addition of phosphate group to a protein. Mainly to the serine, threonine or tyrosine residues. It is also known as phosphor regulation.Phosphorylation is included in cell cycle, growth, apoptosis and signal transduction pathways.

ATP + PROTEIN" PROT EIN KINASES DU NASP PHOSPHOPROTEIN + ADP

LHO 76 COOH serine ATP Ses - This kinose ser-Tho phosph- atase H2O ADP 1 COH. NH₂

Usefulness of post translational modification

• generate the heterogeneity in proteins
• help in utilizing identical proteins for different cellular functions in different cell types
• regulation of protein sequence in eukaryotic organisms.
• modify the end product of expression.
• translocation of proteins across biological membrane.

2. temperature depends on enzymes that function best between 0⁰ c and 35⁰ c

At very low temperature photosynthesis may stop entirely

At very high temperatures enzymes are denatured

Temperature Responoe of photosynthesis Light - 2000 -750 - 400 za 2000 - 750 .400 0 10 50 20 30 40 tempesatse (°C)