L21 - Proteins & enzymes

Question 1

What are proteins?

Answer 1

Large molecules composed of amino acids

Question 2

Amino acids

Answer 2

Most plant derived products contain some essential amino acids, but usually in low amounts.

20 amino acids → 9 are essential = cannot be produced by
humans

Nitrogen (N) main component

Question 3

Protein deposition in

plant organs

Answer 3

• The seed is the best studied example of protein deposition
and storage
• Amount of protein in seeds: 10 40%
• Storage proteins play a major role in protein deposition

Formed at the ER and transported to
protein storage vacuoles and protein
bodies (PB)

Classified into:
- Vegetative storage proteins (VSPs)
- Seed storage proteins (SSPs) : 
 Source of nutrients during germination
 Protective role in plant
defence

Question 4

Enzymes. Are all enzymes proteins?

Answer 4

All enzymes are proteins, but NOT ALL PROTEINS are enzymes

Question 5

Enzymatic browning

Answer 5

1. Hydroxylation of
   monophenols to diphenol
2. Oxidation of diphenols to o quinone

Enzymatic browning
requires exposure to O2 after:
• Bruising,
• Cutting,
• Compression, ...

Wounding allows
chloroplastic PPO enzyme to interact with
vacuolar substrates.

Question 6

Example

Answer 6

o-quinone

• highly unstable yellow
compounds that form complexes
with amino acids or proteins
• polymerization of these intermediates and condensation of o quinones produce melanins (reddish/brown coloured polymers)

Phenylalanine ammonia lyase: 
Key enzyme that catalyses the
first step in the synthesis of a
variety of polyphenyl
compounds

Question 7

Household trick against enzymatic browning?

Answer 7

• Coating in sugar/syrup: reduce oxygen diffusion
• Lemon juices: antioxidants + lower pH
• Heating: inactivating PPO

Question 8

Cell wall degrading enzymes

Answer 8

• Hemicellulases
• Pectinases
• Cellulases

Question 9

Important enzymes are: PPO, PAL, PG and PME