Examples of hydrolases are alpha-amylase (which breaks down glucose polymers),
cellulase (for cellulose), proteolytic exoenzyme (breaks down peptide bonds in
protein molecules), and lipase (which hydrolyzes fats and other esters).
Without digestive enzyme, only low molecular weight substances gain entrance
into cells. There are indications that hydrophilic groups containing OH, COOH,
NH 2 , sulfonate, etc. with 12 carbons or under can pass through cell membranes,
whereas hydrophobic groups with more than 8 carbons cannot diffuse into cells.
Nonhydrolyzed, hydrophobic compounds gain entrance in a different manner.
According to Mickinney [3], these hydrophobic compounds are attracted to the
lipid fraction of the cytoplasmic membrane where they are soluble. By penetrating
into the lipid fraction, the remainder of the molecule can be brought into the cell.
Not all diffusible substance can penetrate into cells. The cell can absorb and retain
certain substances selectively while excluding or excreting others. Although all cells
have this same general property, different organisms differ markedly from each other
in their ability to accept certain specific organic nutrients from the wastewater for use
in their metabolism. The selective nature of a cytoplasmic membrane is caused in
part by its lipoprotein composition. There are specific “combining sites” in the
membrane that effect the selective transport of particular compounds and ions into
the cell. The possession of specific penetration or transport mechanisms plays an
important role in the substrate specificity of many bacteria. Stainer [4] cites an
example in which many bacteria are unable to oxidize citrates as organic nutrients
simply because these compounds do not enter the cell. The same bacteria, however,
possess all the enzymes necessary for citrate oxidation and produce citric acid
constantly as an intermediate metabolite.
1.2.3 Intracellular Enzymatic Actions
Inside the cell, chemical transformations take place with the help of intracellular
enzymes. The CoA portion of the enzymes react with the carboxyl group of short
chain acids, amino acids or hydroxy acids, to form a CoA-acid complex. A number
of reactions will follow. Most often, β-oxidation takes place in which enzymes
remove H 2 and add H 2 O to the organic molecules. The resulting acetyl-CoA will
then enter the tricarboxylic acid cycle for terminal oxidation. Acetate metabolism is
very common with microorganisms, and it is the key intermediate for energy and
synthesis according to Mickinney [5]. The tricarboxylic acid cycle, which is well
accepted as the terminal oxidation scheme for acetate, is presented in Fig. 3.1. Six
CO 2 molecules are produced in three different steps to show the fate of the organic
carbon in glucose.
With short chain alcohols, aldehydes, ketones, and amines, the reactions are
directed toward conversion of the hydrophilic group to a carboxyl group so that
reaction with CoA is possible. Chemical changes of organic substances in wastewater by microorganisms are summarized in Fig. 3.2.
3 Biological Processes
81
cellulase (for cellulose), proteolytic exoenzyme (breaks down peptide bonds in
protein molecules), and lipase (which hydrolyzes fats and other esters).
Without digestive enzyme, only low molecular weight substances gain entrance
into cells. There are indications that hydrophilic groups containing OH, COOH,
NH 2 , sulfonate, etc. with 12 carbons or under can pass through cell membranes,
whereas hydrophobic groups with more than 8 carbons cannot diffuse into cells.
Nonhydrolyzed, hydrophobic compounds gain entrance in a different manner.
According to Mickinney [3], these hydrophobic compounds are attracted to the
lipid fraction of the cytoplasmic membrane where they are soluble. By penetrating
into the lipid fraction, the remainder of the molecule can be brought into the cell.
Not all diffusible substance can penetrate into cells. The cell can absorb and retain
certain substances selectively while excluding or excreting others. Although all cells
have this same general property, different organisms differ markedly from each other
in their ability to accept certain specific organic nutrients from the wastewater for use
in their metabolism. The selective nature of a cytoplasmic membrane is caused in
part by its lipoprotein composition. There are specific “combining sites” in the
membrane that effect the selective transport of particular compounds and ions into
the cell. The possession of specific penetration or transport mechanisms plays an
important role in the substrate specificity of many bacteria. Stainer [4] cites an
example in which many bacteria are unable to oxidize citrates as organic nutrients
simply because these compounds do not enter the cell. The same bacteria, however,
possess all the enzymes necessary for citrate oxidation and produce citric acid
constantly as an intermediate metabolite.
1.2.3 Intracellular Enzymatic Actions
Inside the cell, chemical transformations take place with the help of intracellular
enzymes. The CoA portion of the enzymes react with the carboxyl group of short
chain acids, amino acids or hydroxy acids, to form a CoA-acid complex. A number
of reactions will follow. Most often, β-oxidation takes place in which enzymes
remove H 2 and add H 2 O to the organic molecules. The resulting acetyl-CoA will
then enter the tricarboxylic acid cycle for terminal oxidation. Acetate metabolism is
very common with microorganisms, and it is the key intermediate for energy and
synthesis according to Mickinney [5]. The tricarboxylic acid cycle, which is well
accepted as the terminal oxidation scheme for acetate, is presented in Fig. 3.1. Six
CO 2 molecules are produced in three different steps to show the fate of the organic
carbon in glucose.
With short chain alcohols, aldehydes, ketones, and amines, the reactions are
directed toward conversion of the hydrophilic group to a carboxyl group so that
reaction with CoA is possible. Chemical changes of organic substances in wastewater by microorganisms are summarized in Fig. 3.2.
3 Biological Processes
81