L21 - Proteins & enzymes Flashcards
What are proteins?
Large molecules composed of amino acids
Amino acids
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
Protein deposition in
plant organs
• 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
Enzymes. Are all enzymes proteins?
All enzymes are proteins, but NOT ALL PROTEINS are enzymes
Enzymatic browning
- Hydroxylation of
monophenols to diphenol - 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.
Example
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
Household trick against enzymatic browning?
- Coating in sugar/syrup: reduce oxygen diffusion
- Lemon juices: antioxidants + lower pH
- Heating: inactivating PPO
Cell wall degrading enzymes
- Hemicellulases
- Pectinases
- Cellulases
Important enzymes are: PPO, PAL, PG and PME