Info

CH I

c/s-Aconitate

COO-I

CH2 I 2

Isocitrate COO-

a-Keto-glutarate

Succinate

Succinyl CoA

0 II

COO-

CoASH

CoASH

CoASH

COO-I

COO-

Fig. 1.14 The tricarboxylic acid cycle.

VJJOO^

Fig. 1.15 Oxidative phosphorylation. The passage of electrons through the electron transport chain is accompanied by an exclusion of protons (H+) from the cell (or mitochondrion for a eukaryote). The energetically favourable return passage of protons 'down' a concentration gradient is linked to the phosphorylation of ADP to produce ATP.

Fig. 1.15 Oxidative phosphorylation. The passage of electrons through the electron transport chain is accompanied by an exclusion of protons (H+) from the cell (or mitochondrion for a eukaryote). The energetically favourable return passage of protons 'down' a concentration gradient is linked to the phosphorylation of ADP to produce ATP.

Glucose

Pyruvate

CO2+

Acetaldehyde

Ethanol

Alcoholic fermentation

Pyruvate + Acetyl phosphate

Acetyl CoA

Acetaldehyde

Lactate

Ethanol

Pyruvate

Pyruvate

Lactate

Heterolactic fermentation

Homolactic fermentation

Pyruvate

Acetyl CoA Formate Acetaldehyde

Acetate Ethanol 2CO

Mixed acid fermentation

Fig. 1.16 Alternative end products in fermentation.

NADPH NADPH

> Serine

Sulphate Activated ^^ Sulphite SulphideCysteine^-Methionine sulphate

NADP NADP

Fig. 1.17 The assimilation of sulphur.

Anabolism

The above-named pathways are examples of how cells deal with sugars, thereby obtaining carbon, hydrogen and oxygen. As observed earlier, cells must also secure a supply of other elements from the medium. Nitrogen may be provided as amino acids (e.g. in the case of brewing yeast), urea or inorganic nitrogen forms, primarily as ammonium salts (often used in wine fermentations).

Sulphur can variously be supplied in organic or inorganic forms. Brewing yeast, for example, can assimilate sulphate, but will also take up sulphur-containing amino acids (Fig. i.i7).

The major structural and functional molecules in cells are polymeric. These include

(1) Polysaccharides - notably the storage molecules such as glycogen in yeast, which has a structure closely similar to the amylopectin fraction of starch (see later), and the structural components of cell walls, for example, the mannans and glucans in yeast and the complex polysaccharides in bacterial cell walls.

Lipids t

Glycerol-P

Cytochromes t

Haems«- Porphyrins

Isoleucine

Glucans

Glucose

DNA, RNA, ATP, NAD, coenzyme A

Nucleotides Histidine

Lipids t

Glycerol-P

Cytochromes t

Haems«- Porphyrins

Isoleucine

Shikimate

Chorismate

Tryptophan Tyrosine Phenylalanine Polyisoprenes Quinones

Lipids

Threonine ■*- Aspartate

Asparagine Methionine Pyrimidines

DNA RNA

Succinate

Fig. 1.18 A simplified overview of intermediary metabolism.

Shikimate

Chorismate

Tryptophan Tyrosine Phenylalanine Polyisoprenes Quinones

Lipids

Threonine ■*- Aspartate

Asparagine Methionine Pyrimidines

DNA RNA

Succinate

Sterols ». Citrate a-Ketoglutarate

Glutamate •> Glutamine Proline •• Arginine

Fig. 1.18 A simplified overview of intermediary metabolism.

(2) Proteins - notably the enzymes and the permeases.

(3) Lipids - notably the components at the heart of membrane structure.

A greatly simplified summary of cellular metabolism, incorporating the essential features of anabolic reactions is given in Fig. 1.18. It is sufficient in the present discussion to state that pyruvate is at the heart of the metabolic pathways. There are clearly various draws on it, both catabolic and anabolic. Of particular note is the draw off from the tricarboxylic acid cycle to satisfy biosynthetic needs, meaning that there is a failure to regenerate the oxaloac-etate needed to collect a new acetyl-CoA residue emerging from pyruvate. Thus, cells have so-called anaplerotic pathways by which they can replenish necessary intermediates such as oxaloacetate. The best-known such pathway is the glyoxylate cycle (Fig. 1.19).

It is essential that the multiplicity of reactions, which as a whole constitute cellular metabolism, are controlled so that the whole is in balance to achieve the appropriate needs of the cell under the prevailing conditions within which it finds itself. It is outside the scope of this book to dwell on these regulatory mechanisms, but they include coarse controls on the synthesis of the necessary permeases and enzymes (the general rule being that a protein is only synthesised as and when it is needed) and fine controls on the rate at which the enzymes are able to act. Examples of the impact of these control strategies

Acetate

Acetyl coenzyme A

Acetate

Acetyl coenzyme A

Oxaloacetate

Citrate

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