10.8 Digestion of protein

10.8 Digestion of protein

Proteases (also called proteinases or peptidases) 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 proteases which are called proteasomes (CLICK HERE for a reminder).

Extracellular proteases are involved mainly in the hydrolysis of large polypeptide substrates into the smaller molecules which can be absorbed by the cell. Extracellular proteases are produced by many species of fungi but most is known about protein utilisation and protease 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 et al., 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, protease production is controlled by derepression; the Neurospora crassa protease is controlled by induction and repression. In neither case is protease 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 proteases in Basidiomycota is regulated mainly by induction; as long as substrate protein is available the proteases 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 et al., 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 proteases 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 protease activity. Several species release specific proteases which can hydrolyse structural and other proteins resistant to attack by most other proteases, 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 proteases (Kalisz, 1988; Nirmal et al., 2011; Monteiro de Souza et al., 2015). 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). Proteases cleave internal peptide bonds; they are endopeptidases. The protease catalytic mechanism can be determined indirectly by study of response to inhibitors which react with specific residues in the active site of the enzyme. This leads to subclassification into four groups:

  • Serine proteases 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. Most eukaryotic lineages that have been sequenced encode a set of 13 to 16 serine proteases, suggesting that all fungal serine protease families evolved in the eukaryote lineage before animals, plants and fungi diverged from one another (Muszewska et al., 2017).
  • Cysteine proteases occur in few fungi though extracellular cysteine proteases have been reported in Microsporum sp., Aspergillus oryzae, and Phanerochaete chrysogenum (Sporotrichum pulverulentum). Cysteine proteases are involved in many functions in animals and replacement enzymes (which take control of the host function) are often produced by their parasites and pathogens so the enzymes from these sources may become good pharmacological targets in several major diseases of humans (Verma et al., 2016).
  • Aspartic proteases show maximum activity at low pH values (pH 3 to 4) and are widely distributed in fungi, performing important functions related to nutrition and pathogenesis. In addition, their high activity and stability at acid pH make them attractive for industrial application in the food industry; they are used as milk-coagulating agents in cheese production and are used to improve the flavour profile of some foods (Mandujano-González et al., 2016).
  • Metalloproteases 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. Metalloproteases 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. Some (known as fungalysins) are able to degrade the human extracellular matrix proteins elastin and collagen and are thought to act as virulence factors in diseases caused by fungi (Fernandez et al., 2013).

Updated July, 2018