Oxidationreduction reactions

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Many metabolic reactions involve the transfer of electrons from one molecule to another; these are called oxidation-reduction or redox reactions. When a molecule (or atom or ion) loses an electron, it is said to be oxidized. (Note, that despite the terminology, oxygen does not necessarily take part in the reaction.) Conversely, when an electron is gained, the recipient is reduced (Figure 6.14).

Many metabolic reactions involve the loss of a hydrogen atom; since this contains one proton and one electron, the reaction is regarded as an oxidation, because an electron

*Not all of the energy is converted into ATP. A proportion of it is lost as heat, some of which allows the enzyme-mediated reactions to proceed at a faster rate.

Figure 6.13 Adenosine triphosphate (ATP). ATP is a nucleotide such as similar to those depicted in Figures 2.20 and 2.21. Note the extra phosphate groups

has been lost:

OH O

Lactate Pyruvate

The lactate in the example above, by losing two hydrogen atoms, has automatically lost two electrons and thus become oxidised to pyruvate. Oxidation reactions are

Oxidation

Oxidation

Reduction

Figure 6.14 Oxidation-reduction reactions. When one molecule is oxidised, another is simultaneously reduced. In the example shown, 'X' loses an electron and thus becomes oxidised. By receiving the electron, 'Y' becomes reduced

Reduction

Figure 6.14 Oxidation-reduction reactions. When one molecule is oxidised, another is simultaneously reduced. In the example shown, 'X' loses an electron and thus becomes oxidised. By receiving the electron, 'Y' becomes reduced

Box 6.1 Coupled reactions

Many reactions in metabolic pathways, including glycolysis, can only take place if they are coupled to a secondary reaction. The oxidising power necessary for the conversion of glyceraldehyde-3-phosphate into 1,3-diphosphoglycerate in step 6 of glycolysis for example, is provided by the coenzyme NAD+. In the coupled reaction, this becomes oxidised to NADH:

NAD+ NADH Glyceraldehyde-3-phosphate-► 1, 3-diphosphoglycerate always associated with the transfer of energy from the oxidised substance to the reduced substance.

Two important molecules that we shall encounter a number of times in the following pages are the coenzymes nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate; you will be relieved to learn that they are nearly always referred to by their abbreviations, NAD+ and NADP+, respectively! Both are derivatives of the B vitamin niacin, and each can exist in an oxidised and a reduced form:

Oxidised Reduced

They are to be found associated with redox reactions (see Box 6.1), acting as carrier molecules for the transfer of electrons. In the oxidation of lactate shown above, the oxidising power is provided by the reduction of NAD+, so the full story would be:

Lactate--► Pyruvate

As the lactate is oxidised, so the NAD+ in the coupled reaction is reduced. It is said to act as the electron acceptor. NAD+/NADH is generally involved in catabolic reactions, and NADP+/NADPH in anabolic ones.

As the equation above shows, there can be no oxidation without reduction, and vice versa; the two are irrevocably linked. The tendency of an compound to lose or gain electrons is termed its redox potential (Eo) (Box 6.2).

In the following section, we shall examine chemoheterotrophic metabolism, used by the majority of microorganisms to derive cellular energy from the oxidation of carbohydrates. Other groups have evolved their own systems of energy capture, and these will be considered later in the chapter.

Figure 6.15 provides a summary of the catabolic (breakdown) pathways used by heterotrophs. Complex nutrients such as proteins and polysaccharides must be

Box 6.2 Redox potentials

Substances vary in the affinity they have for binding electrons; this can be measured as their oxidation--reduction potential or redox potential, relative to that hydrogen. The flow of electrons in the electron transport chain (see Fig. 6.21) occurs because the carriers are arranged in order of their redox potentials, with each having a greater electron affinity (more positive redox potential) than its predecessor. Thus electrons are donated to carriers with a more positive redox potential.

enzymatically broken down and converted to substances that can then enter one of the degradative pathways that lead to energy production.

Glucose is the carbohydrate most widely used as an energy source by cells, and the processes by which it is broken down in the presence of oxygen to give carbon dioxide and water are common to many organisms. These have been very thoroughly studied and can be summarised:

Strongly negative redox potential (Good electron donors)

Reduction Potential

+ Strongly positive redox potential (Good electron acceptors)

C6H12O6 + 6O2 6CO2 + 6H2O

Polysaccharide -Sugars

Pyruvate

Protein

Nucleic acid

Peptides

Purines

Pyruvate

Pyrimidines-Dihydrouracil

+ Pentoses

Lipid

Fatty acids + Glycerol

Figure 6.15 Catabolic pathways in heterotrophs. Pathways for the catabolism of proteins, nucleic acids and lipids as well as carbohydrates can all feed into the tricarboxylic acid cycle

Glucose (6C)

Glucose (6C)

2ATP

'Sowing' phase

2ADP

Glyceraldehyde 3-phosphate (3C)

Dihydroxyacetone phosphate (3C)

NADH

Glyceraldehyde 3-phosphate (3C)

2ATP

'Reaping' phase

Pyruvate

Figure 6.16 Glycolysis. Two molecules of ATP are 'spent' in the first stage of glycolysis, in which glucose is converted into the 3-carbon compounds glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. During the second stage, four ATPs are produced per molecule of glucose, so there is a net gain of two ATPs. In addition, reducing power is generated in the form of NADH (two molecules per molecule of glucose). See Figure 6.17 for a more detailed depiction of the reactions involved in glycolysis

What this equation, crucially, does not show is that as a result of this process, energy is released, and stored in the form of 38 molecules of ATP, so for completeness, we need to add to the respective sides:

The release of the energy contained within a molecule of glucose does not occur in a single reaction, but happens gradually, as the result of numerous reactions linked together in biochemical pathways, the first of which is glycolysis (Figure 6.16). Glycolysis can occur with or without oxygen, and is common to both aerobic and anaerobic organisms. Oxygen is essential, however, for aerobic respiration, by which ATP is generated from the products of glycolysis. Anaerobes proceed down their own pathways following glycolysis as we shall see, but these lack the ATP-generating power of the aerobic process.

Why glucose ?

By concentrating on glucose catabolism in this way, you may think we are ignoring the fate of other nutrient molecules. If you take another look at Figure 6.15, however, you will notice that the breakdown products of lipids, proteins and nucleic acids also find their way into our pathway sooner or later, having undergone transformations of their

38ADP + 38Pi 38ATP (Pi = inorganic phosphate group)

own.

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