Coenzymes and metals in oxidation and reduction reactions

In its simplest form, oxidation is the combination of a molecule with oxygen. Thus, if a carbohydrate, such as glucose (C6H12O6), is burned in air (or metabolized in the body), it is oxidized to carbon dioxide and at the same time oxygen is reduced to water:

Oxidation reactions need not involve the addition of oxygen; oxidation is the removal of electrons from a molecule. Thus, the conversion of the iron Fe2+ ion to Fe3+ is also an oxidation, although in this case there is no direct involvement of oxygen.

Table 2.1 The major coenzymes

Source

Functions

CoA

Coenzyme A

Pantothenic acid

Acyl transfer reactions

FAD

Flavin adenine dinucleotide

Vitamin B2

Oxidation reactions

FMN

Flavin mononucleotide

Vitamin B2

Oxidation reactions

NAD

Nicotinamide adenine dinucleotide

Niacin

Oxidation and reduction

reactions

NADP

Nicotinamide adenine dinucleotide

Niacin

Oxidation and reduction

phosphate

reactions

PLP

Pyridoxal phosphate

Vitamin B6

Amino acid metabolism

There are a number of other coenzymes,which are discussed as they are relevant to specific metabolic pathways. In addition to those shown in this table, most of the other vitamins also function as coenzymes (see Chapter 11).

There are a number of other coenzymes,which are discussed as they are relevant to specific metabolic pathways. In addition to those shown in this table, most of the other vitamins also function as coenzymes (see Chapter 11).

In many reactions, the removal of electrons in an oxidation reaction does not result in the formation of a positive ion — hydrogen ions (H+) are removed together with the electrons. This means that the removal of hydrogen from a compound is also oxidation. For example, a hydrocarbon such as ethane (C2H6) is oxidized to ethene (C2H4) by removing two hydrogen atoms onto a carrier:

CH3-CH3 + carrier

Reduction is the reverse of oxidation — the addition of hydrogen, or electrons, or the removal of oxygen are all reduction reactions. In the reaction above, ethane was oxidized to ethene at the expense of a carrier, which was reduced in the process. The addition of hydrogen to the carrier is a reduction reaction. Similarly, the addition of electrons to a molecule is a reduction, so, just as the conversion of Fe2+ to Fe3+ is an oxidation reaction, the reverse reaction, the conversion of Fe3+ to Fe2+, is a reduction.

Most of the reactions involved in energy metabolism involve the oxidation of metabolic fuels, while many of the biosynthetic reactions involved in the formation of metabolic fuel reserves and the synthesis of body components are reductions.

In some metabolic oxidation and reduction reactions the hydrogen acceptor or donor is a prosthetic group, e.g. haem (section 2.4.1.1) or riboflavin (section 2.4.1.2). In other cases, the hydrogen acceptor or donor acts as a substrate of the enzyme (e.g. the nicotinamide nucleotide coenzymes; section 2.4.1.3).

2.4.1.1 Metal ions

The electron acceptor or donor may be a metal ion that can have two different stable electron configurations. Commonly, iron (which can form Fe2+ or Fe3+ ions) and copper (which can form Cu+ or Cu2+ ions) are involved.

In some enzymes, the metal ion is bound to the enzyme protein; in others, it is incorporated in an organic molecule, which in turn is attached to the enzyme. For example, haem is an organic compound containing iron that is the coenzyme for a variety of enzymes, collectively known as the cytochromes (section 3.3.1.2). Haem is also the prosthetic group of haemoglobin, the protein in red blood cells that binds and transports oxygen between the lungs and other tissues, and myoglobin in muscle. However, in haemoglobin and myoglobin the iron of haem does not undergo oxidation; it binds oxygen but does not react with it.

Diabetes Sustenance

Diabetes Sustenance

Get All The Support And Guidance You Need To Be A Success At Dealing With Diabetes The Healthy Way. This Book Is One Of The Most Valuable Resources In The World When It Comes To Learning How Nutritional Supplements Can Control Sugar Levels.

Get My Free Ebook


Post a comment