The nicotinamide nucleotide coenzymes NAD and NADP

The vitamin niacin (section 11.8) is important for the formation of two closely related compounds, the nicotinamide nucleotide coenzymes - nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). As shown in Figure 2.16, they differ only in that NADP has an additional phosphate group attached to the ribose. The whole of the coenzyme molecule is essential for binding to enzymes, and most enzymes can bind and use only one of these two coenzymes, either NAD or NADP, despite the overall similarity in their structures.

The functionally important part of these coenzymes is the nicotinamide ring, which undergoes a two-electron reduction. In the oxidized coenzymes there is a positive charge associated with the nitrogen atom in the nicotinamide ring, and the oxidized forms of the coenzymes are usually shown as NAD+ and NADP + . Reduction involves the transfer of two electrons and two hydrogen ions (H+) from the substrate to the

Nucleotide Coenzymes Coenzyme
Figure 2.14 Riboflavin and the flavin coenzymes, riboflavin monophosphate and flavin adenine dinucleotide.

coenzyme. One electron neutralizes the positive charge on the nitrogen atom. The other, with its associated H+ ion, is incorporated into the ring as a second hydrogen at carbon-2.

The second H+ ion removed from the substrate remains associated with the coenzyme. This means that the reaction can be shown as

where X-H2 is the substrate and X is the product (the oxidized form of the substrate). Note that the reaction is reversible, and NADH can act as a reducing agent:

where X is now the substrate and X-H2 is the product (the reduced form of the substrate).

Oxidized Riboflavin Reduced
Figure 2.15 Oxidation and reduction of the flavin coenzymes. The reaction may proceed as either a single two-electron reaction or as two single-electron steps by way of intermediate formation of the riboflavin semiquinone radical.

The usual notation is that NAD and NADP are used when the oxidation state is not relevant, and NAD(P) when either NAD or NADP is being discussed. The oxidized coenzymes are shown as NAD(P)+ and the reduced forms as NAD(P)H.

Unlike flavins and metal coenzymes, the nicotinamide nucleotide coenzymes do not remain bound to the enzyme, but act as substrates, binding to the enzyme, undergoing reduction and then leaving. The reduced coenzyme is then reoxidized either by reaction with another enzyme, for which it acts as a hydrogen donor, or by way of the mitochondrial electron transport chain (section 3.3.1.2). Cells contain only a small amount of NAD(P) (of the order of 400 nmol/g in liver), which is rapidly cycled between the oxidized and reduced forms by different enzymes.

In general, NAD+ is the coenzyme for oxidation reactions, with most of the resultant NADH being reoxidized by the mitochondrial electron transport chain, while NADPH

Nad Nicotinamide Nucleotide

Figure 2.16 The nicotinamide nucleotide coenzymes, NAD and NADP. In the oxidized coenzyme there was one hydrogen at carbon-4, but this is not shown when the ring is drawn. In the reduced coenzymes both hydrogens are shown, with a dotted bond to one hydrogen and a bold bond to the other, to show that the ring as a whole is flat, with one hydrogen at carbon-4 above the plane of the ring and the other below.

Figure 2.16 The nicotinamide nucleotide coenzymes, NAD and NADP. In the oxidized coenzyme there was one hydrogen at carbon-4, but this is not shown when the ring is drawn. In the reduced coenzymes both hydrogens are shown, with a dotted bond to one hydrogen and a bold bond to the other, to show that the ring as a whole is flat, with one hydrogen at carbon-4 above the plane of the ring and the other below.

is the main coenzyme for reduction reactions (e.g. the synthesis of fatty acids; section 5.6.1).

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Responses

  • zahra goytiom
    How do nad and nadp act as coenzymes in oxidation?
    6 years ago
  • tristan
    What does the oxidized and reduced form of NADP?
    5 years ago
  • scarlett ross
    Are coenzymes nad and nadp associated with metal ions?
    3 years ago

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