Anaerobic Degradation of Protein

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Proteins are biological macromolecules, either soluble or solid (e.g., feathers, hair, nails). Outside the cell at an acid pH or in the presence of enzymes, soluble proteins precipitate, e.g., precipitation of casein by addition of rennet enzyme. The reaction sequences necessary for protein degradation in a methanogenic ecosystem are outlined in Figure 1.5. Hydrolysis of precipitated or soluble protein is catalyzed by sev-

Anaerobic Degradation
Fig. 1.4 Anaerobic degradation of starch under low- and high-loading conditions.
Fig. 1.5 Anaerobic degradation of proteins.

eral types of proteases that cleave membrane-permeable amino acids, dipeptides, or oligopeptides. In contrast to the hydrolysis of carbohydrates, which proceeds favorably at a slightly acid pH, optimal hydrolysis of proteins requires a neutral or weakly alkaline pH (McInerney, 1988). In contrast to the fermentation of carbohydrates, which lowers the pH due to volatile fatty acid formation, fermentation of amino acids in wastewater reactors does not lead to a significant pH change, due to acid and ammonia formation. Acidification of protein-containing wastewater proceeds optimally at pH values of 7 or higher (Winterberg and Sahm, 1992), and ammonium ions together with the CO2-bicarbonate-carbonate buffer system stabilize the pH. Acetogenesis of fatty acids from deamination of amino acids requires a low H2 partial pressure for the same reasons as for carbohydrate degradation. This can be maintained by a syntrophic interaction of fermentative, protein-degrading bacteria and acetogenic and methanogenic or sulfate-reducing bacteria. Except for syntroph-ic interaction of amino acid-degrading bacteria with methanogens for maintenance of a low H2 partial pressure, clostridia and presumably also some other sludge bacteria may couple oxidative and reductive amino acid conversion via the Stickland reaction. One amino acid, e.g., alanine (Eq. 4) is oxidatively decarboxylated and the hydrogen or reducing equivalents produced during this reaction are used to reductive-ly convert another amino acid, e.g., glycine, to acetate and ammonia (Eq. 5).

CH3-CHNH2-COOH + 2 H2O ^ CH3-COOH + CO2 + NH3 + 2 H2 (4)

For complete degradation of amino acids in an anaerobic system therefore, a syn-trophism of amino acid-fermenting anaerobic bacteria with methanogens or sulfate reducers is required (Wildenauer and Winter, 1986; Winter et al., 1987; Orlygsson et al., 1995). If long-chain amino acids are deaminated (Eqs. 6-9), fatty acids such as propionate, i-butyrate, or i-valerate are formed directly. The fatty acids require acet-ogenic bacteria for their degradation.

leucine + 2 H2O ^ i-valerate + CO2 + NH3 + 2 H2 (7)

i-leucine + 2 H2O ^ 2-methylbutyrate + CO2 + NH3 + 2 H2 (8)

glutamate + 2 H2O ^ propionate + 2 CO2 + NH3 + 2 H2 (9)

In contrast to carbohydrate degradation, where the necessity for propionate- and butyrate-degrading acetogenic bacteria can be circumvented by substrate limitation (Fig. 1.4a), during protein degradation these fatty acids are a product of deamina-tion, and their formation cannot be avoided by maintaining a low H2 partial pressure. In the methanogenic phase there is no difference in methanogenic activity whether carbohydrates or proteins are fermented, except that the methanogens in a reactor fed with protein need to be more tolerant to ammonia and higher pH.

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