Catabolism refers to the metabolic events whereby a foodstuff is broken down so as to extract energy in the form of adenosine triphosphate (ATP), as well as reducing power (customarily generated primarily in the form of nicotinamide adenine dinucleotide (NADH, reduced form) but utilised as nicotinamide adenine dinucleotide phosphate (NADPH, reduced form) to fuel the reactions (anabolism) wherein cellular constituents are fabricated (Fig. 1.10).
In focusing on the organotrophs, and in turn even more narrowly (for the most part) on those that use sugars as the main source of carbon and energy, we must first consider the Embden-Meyerhof-Parnas (EMP)
Fructose 6-phosphate ATP
Glyceraldehyde 3-phosphate '
2C3 1,3-Diphosphoglyceric acid 2 ADPv
Fig. 1.11 The EMP pathway.
3-Phosphoglyceric acid g
2-Phosphoglyceric acid ^H2O
Phospho-enolpyruvic acid 2 ADP~
2 ATP-« Pyruvic acid pathway (Fig. 1.11). This is the most common route by which sugars are converted into a key component of cellular metabolism, pyruvic acid. This pathway, for example, is central to the route by which alcoholic fermentations are performed by yeast. In this pathway, the sugar is 'activated' to a more reactive phosphorylated state by the addition of two phosphates from ATP. There follows a splitting of the diphosphate to two three-carbon units that are in equilibrium. It is the glyceraldehyde 3-phosphate that is metabolised further, but as it is used up, the equilibrium is strained and dihydroxyace-tone phosphate is converted to it. Hence we are in reality dealing with two
identical units proceeding from the fructose diphosphate. The first step is oxidation, the reducing equivalents (electrons, hydrogen) being captured by NAD. En route to pyruvate are two stages at which ATP is produced by the splitting off of phosphate - this is called substrate-level phosphorylation. As there are two three-carbon (C3) fragments moving down the pathway, this therefore means that four ATPs are being produced per sugar molecule. As two ATPs were consumed in activating the sugar, there is a net ATP gain of two.
In certain fermentations, the Entner-Doudoroff pathway (Fig. 1.12) is employed by the organism, a pathway differing in the earliest part insofar as only one ATP is used. Meanwhile, in certain lactic acid bacteria, there is the quite different phosphoketolase pathway (Fig. 1.13).
A major outlet for pyruvate is into the Krebs cycle (tricarboxylic acid cycle; Fig. 1.14). In particular, this cycle is important in aerobically growing cells. There are four oxidative stages with hydrogen collected either by NAD or flavin adenine dinucleotide (FAD). When growing aerobically, this reducing power can be recovered by successively passing the electrons across a sequence of cytochromes located in the mitochondrial membranes of eukaryotes or the plasma membrane of prokaryotes (Fig. 1.7), with the resultant flux of protons being converted into energy collection as ATP through the process of oxidative phosphorylation (Fig. 1.15). In aerobic systems, the terminal electron acceptor is oxygen, but other agents such as sulphate or nitrate can serve the function in certain types of organism. An example of the latter would be the nitrate reducers that have relevance in certain meat fermentation processes (see Chapter 13).
In classic fermentations where oxygen is not employed as a terminal electron acceptor and indeed the respiratory chain as a whole is not used, there needs to be an alternative way for the cell to recycle the NADH produced in the EMP pathway, so that NAD is available to continue the process. Herein lies the basis of much of the diversity in fermentation end products, with pyruvate being converted in various ways (Fig. 1.16). In brewer's yeast, the end product is ethanol. In lactic acid bacteria, there are two modes of metabolism. In homofermentative bacteria, the pyruvate is reduced solely to lactic acid. In heterofermentative lactic acid bacteria, there are alternative end products, most notably lactate, ethanol and carbon dioxide, produced through the intermediacy of the phosphoketolase pathway.
As noted earlier, higher molecular weight molecules that are too large to enter into the cell as is are hydrolysed by enzymes secreted from the organism. The resultant lower molecular weight materials are then transported into the cell in the same manner as exiting smaller sized materials. The transport is by selective permeases, which are elaborated in response to the needs of the cell. For example, if brewing yeast is exposed to a mixture of sugars, then it will elaborate the transport permeases (proteins) in a defined sequence (see Chapter 2).
Pyruvate Glyceraldehyde 3-phosphate y .As per Embden-Meyerhof-Parnas
Fig. 1.12 The Entner-Doudoroff pathway.
ADP-* Glucose 6-phosphate
Ribulose 5-phosphate u
Glyceraldehyde 3-phosphate Acetyl phosphate +
I^NAD Lactic acid
Fig. 1.13 The phosphoketolase pathway.
*ADP Ribose 5-phosphate u
Acetyl phosphate ADP
Glyceraldehyde 3-phosphate +
Lactic acid h2ü
CH2 I 2
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