Peptidases in Plant Tissue


Peptidases are a diverse group of enzymes with many different catalytic mechanisms. They constitute up to 2% of the plant proteome and function in multitude of ways to ensure survival. The study of seed development and germination has revealed many peptidases that are crucial to the correct deposition of seed storage proteins in monocotyledonous and dicotyledonous plants. Under the correct environmental conditions, germination of the seed is initiated. The organised synthesis and action of peptidases ensure the hydrolysis of the storage proteins which then provides the necessary amino acids for the seedling's growth. These events have profound implications to the survival of the plant species and to the survival of ecosystems that feed on the seeds and plant material generated after germination. Peptidase activity (which can be monitored using coloured substrates) is also linked to plant senescence, programmed or regulated cell death, to the 26S proteasome which functions in normal cellular events to regulate the total cell protein and also to provide defence against pathogen invasion. The activity of these peptidases can be regulated by naturally occurring inhibitors and the differential location of substrate and enzyme. Plant peptidases (e.g. papain) are of commercial interest particularly in the meat processing industry because of their wide pH optima, thermostability and their ability to work on heterogeneous substrates.

Key Concepts

  • Peptidases are the principal enzymes in the catabolism of proteins and peptides.
  • Seed germination is an event in which the stored carbohydrate, lipid and protein in the seed is hydrolysed and used for the synthesis of new macromolecules vital for the emerging plant's growth.
  • Plant programmed cell death is the death of a cell responding to a variety of extracellular and intracellular signals.
  • Endomembrane flow is the dynamic movement of lipid and protein from the endoplasmic reticulum through the Golgi bodies to the plasma membrane or storage vesicles.
  • Signal peptides are small specific sequences at the NÔÇÉterminus of a proteins sequence to designate which organelle the protein should be directed to.

Keywords: peptidase; cysteine; serine; metallo; aspartate; MEROPS; germination; cell death; proteasome; signal peptidase

Figure 1. Two methods to hydrolyse proteins into amino acids; (a) the use of concentrated acids and (b) the sequential addition of peptidase enzymes.
Figure 2. (a) The activated nucleophilic sulphur (aided by histidine) on the enzyme's active site targets the carbon in the peptide bond within the target protein's structure. Asparagine helps to stabilise the cys and histidine+ ion pair at the active site. Papain is a typical plant cysteine peptidase (the amino acid numbers refer to papain's catalytic triad) with a pH optimum between 5.0 and 7.0. The enzyme has a broad specificity but preferentially cleaves to the carboxy side of lysine or arginine when they are preceded with a hydrophobic amino acid. Adapted from Vernet et al., . (b) The covalent attachment of the cysteine peptidase to the amino peptide fragment of the target protein. The formation of thioester intermediate on the active site of the enzyme is hydrolysed by a water molecule. (c) The water molecule releases the amino terminal peptide of the target protein. The net result is the generation of two peptide fragments from the hydrolysed target protein. Serine/threonine peptidases have a similar two‐step catalytic mechanism involving an activated hydroxyl on the active site serine's function group.
Figure 3. Storage proteins isolated from Vicia faba seeds at different stages of germination subjected to nondenaturing (native) PAGE in a 7.5% gel showing increased electronegativity due to the deamidation of glutamine residues in the storage proteins. Bonner . Reproduced with permission of Taylor & Francis.


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Further Reading

Aehle W (2004) Industrial enzymes: enzymes in food applications. In: Aehle W (ed.) Enzymes in Industry: Production and Applications. Wiley: UK.

Barrett AJ, Rawlings ND and Woessner JF (eds) (2012) Handbook of Proteolytic Enzymes, 3rd edn. Academic Press: UK.

Barrett A, Rawlings N and Woessner JF (eds) (2013) Handbook of Proteolytic Enzymes, 3rd edn. Academic Press: UK.

Birk Y (2010) Plant Protease Inhibitors: Significance in Nutrition, Plant Protection, Cancer Prevention and Genetic Engineering. Springer‐Verlag: DE.

Guevara MG and Daleo GR (eds) (2018a) Biotechnological Applications of Plant Proteolytic Enzymes. Springer: UK.

Guevara MG and Daleo GR (eds) (2018b) Biotechnological Applications of Plant Proteolytic Enzymes. Springer Verlag: UK.

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Bonner, Philip LR(Feb 2020) Peptidases in Plant Tissue. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0027980]