Three concepts of metabolism may be considered: substrate breakdown, energy production, and macromolecule synthesis. These three phenomena, however, are so closely related that complete delineation cannot be achieved. This chapter will discuss carbohydrate and protein breakdown, most of which is fermentative.
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Many steps are involved in the fermentation of carbohydrates. Early worker thought that breakdown was chemical. A few metabolic patterns encompass the fermentation pathways utilized by most microorganisms.
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Hexoses are usually phosphorylated at one or two positions, and the energy utilized in phosphorylation is derived from the change of adenosine triphosphate (ATP) to adenosine diphosphate (ADP).
Diphosphorylated hexoses are characteristically broken down into two triose units. Each triose produces two high energy bonds in its conversion to pyruvate. One high energy bond results from phosphate esterified onto a carbon with an oxygen double-bond attachment, and the other results from a carbon that is double boned to another carbon.
In each case, energy is released by further metabolism, and ADP forms ATP. This energy change, termed substrate phosphorylation, produces four total high energy bonds per mole of hexose.
Since two bonds were utilized in hexose phosphorylation, only two are gained. Two hydrogens are given off by hexose breakage into two trioses, and hydrogens thus produced are picked up by nicotinamide adenine dinucleotide phosphate (NADP).
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Pyruvate may be broken down into acetaldehyde and carbon dioxide, as in the alcoholic fermentation, and the acetaldehyde is reduced to ethanol by hydrogens given off when the hexose molecule broke. Glycerols are also produced in small amounts in yeast fermentations, and large amounts can be produced by inhibiting alcoholic fermentation steps.
Yeast and other microorganisms degrade hexoses according to the process described, which is termed the Embden-Meyerhof (EM) pathway. Hexoses may be converted by other pathways, however, and pentose is involved after primary decarboxylation.
Only one high energy bond is derived per mole of hexose in this system. Pentose, as well as hexoses, may be broken down by some microorganisms, or hexoses may be broken down by way of pentoses—or the pentose shunt system.
Ethanol, lactate, formate, acetoin, carbon dioxide, water, and other compounds may result from carbohydrate degradation. Ethanol may be further oxidized to acetate in vinegar preparations, or many compounds may be prepared by fermentations that produce ethanol.
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Butanediol, which is utilized in the preparation of synthetic rubber, results from the reduction of acetoin. Acetate, butyrate, acetone, and other industrially important compounds are prepared from fermentations. Ascorbic acid (vitamin C) is prepared from sorbose, which results from microbial action on sorbitol.
Disaccharides and polysaccharides are broken into monosaccharides by hydrolysis, and further metabolism proceeds as with hexoses. Starch and cellulose, for example, have different linkages of hexoses, and organisms must possess proper hydrolytic enzymes in order to break down polysacchardes.
Most microorganisms, when washed, will not readily attach protein. Active growth appears to be necessary to hydrolyze protein. Amino acids result from protein breakdown, and amino acids, in turn, are degraded by bacterial action.
Amino acids are further metabolized by bacterial enzymes. Principal changes include decarboxylation, oxidation, reduction, and transamination. The final products of amino acid metabolism are carbon dioxide, water, and ammonia.