10.8 Digestion of protein
Proteinases are peptide hydrolases; a group of enzymes which hydrolyse the peptide bonds of proteins and peptides, cleaving the substrate molecule into smaller fragments and, eventually, into amino acids. This is a complex group of enzymes, varying greatly in physicochemical and catalytic properties. Proteolytic enzymes are produced intra- and extracellularly, playing important roles in regulatory processes of the cell as well as contributing to nutrition through degradation of protein food sources. Intracellular proteolysis seems to be the responsibility of large multicatalytic complexes of proteinases which are called proteasomes (CLICK HERE for a reminder).
Extracellular proteinases are involved mainly in the hydrolysis of large polypeptide substrates into the smaller molecules which can be absorbed by the cell. Extracellular proteinases are produced by many species of fungi but most is known about protein utilisation and proteinase production by Aspergillus species and Neurospora crassa. The Basidiomycota Agaricus, Coprinopsis, and Volvariella have been shown to be able to use protein as a sole source of carbon about as efficiently as they can use the sugar glucose, and can also use protein as a source of nitrogen and sulfur (Kalisz, Moore & Wood, 1986). Mycorrhizal fungi have been shown to use protein as a source of both nitrogen and carbon and some ectomycorrhizas supply nitrogen derived from proteins in soil to their plant hosts.
In Aspergillus species, proteinase production is controlled by derepression; the Neurospora crassa proteinase is controlled by induction and repression. In neither case is proteinase produced in the presence of ammonia; which appears, therefore, to be the primary source of nitrogen for these fungi. In sharp contrast, production of extracellular proteinases in Basidiomycota is regulated mainly by induction; as long as substrate protein is available the proteinases are produced, even in the presence of adequate alternative supplies of ammonia, glucose and sulfate. So in this case the protein might be presumed to be the ‘first choice’ substrate (Kalisz, Wood & Moore, 1987, 1989).
Protein is probably the most abundant nitrogen source available to plant-litter-degrading organisms in the form of plant protein, lignoprotein and microbial protein. Many pathogenic microorganisms secrete proteinases which are involved in the infection process and some, including the apple pathogenic fungus Monilinia fructigena, are known to utilise host proteins for nutrition. The virulence of a few pathogenic fungi is correlated with their extracellular proteinase activity. Several species release specific proteinases which can hydrolyse structural and other proteins resistant to attack by most other proteinases, such as insect cuticles. The animal dermatophytes Microsporum and Trichophyton produce collagenases, elastases and keratinases.
Enzymes that degrade proteins form two major groups: peptidases and proteinases (Kalisz, 1988). Exopeptidases remove terminal amino acids or dipeptides and are subdivided according to whether they act at the carboxy terminal end of the substrate protein (carboxypeptidases); the amino terminal end (aminopeptidases), or on a dipeptide (dipeptidases). Proteinases cleave internal peptide bonds; they are endopeptidases. The proteinase catalytic mechanism can be determined indirectly by study of response to inhibitors which react with particular residues in the active site of the enzyme. This leads to subclassification into four groups:
- Serine proteinases are the most widely distributed group of proteolytic enzymes. They have a serine residue in the active site, are generally active at neutral and alkaline pH, and show broad substrate specificities.
- Cysteine proteinases occur in few fungi though extracellular cysteine proteinases have been reported in Microsporum sp., Aspergillus oryzae, and Phanerochaete chrysogenum (Sporotrichum pulverulentum).
- Aspartic proteinases show maximum activity at low pH values (pH 3 to 4) and are widely distributed in fungi.
- Metalloproteinases have pH optima between 5 and 9 and are inhibited by metal-chelating reagents, such as ethylenediamine tetra acetic acid (EDTA). In many cases the EDTA-inhibited enzyme can be reactivated by zinc, calcium or cobalt ions. Metalloproteinases are widespread, but only a few have been reported in fungi and most of these are zinc-containing enzymes, with one atom of zinc per molecule of enzyme.
Updated December 17, 2016