17.1.1: Cofactors and Helpers in Metabolic Reactions

Enzymes and Hormones Regulate Metabolism

The human body uses enzymes to facilitate many metabolic reactions. Several of these enzymes require cofactors to function; some textbooks refer to them as coenzymes. To keep things simple, we are including these coenzymes in our definition of cofactors. Cofactors can be either organic or inorganic molecules that are required by enzymes to function. Many organic cofactors are vitamins or molecules derived from vitamins. Most inorganic cofactors are minerals. Cofactors can be oxidized or reduced for the enzymes to catalyze the reactions.

Vitamin Cofactors - examples NAD and FAD

Two common cofactors that are derived from the B vitamins, niacin and riboflavin: they are nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. The structure of NAD and FAD are shown below.

Figure 9.2.1 Structure of NAD¹ upside down. The atoms are circled to help orient this structure with Figure 9.2.3

Figure 9.2.2 Structure of FAD². The nitrogens are circled to help orient this structure with Figure 9.2.4

Both of these cofactors can be reduced; NAD is reduced to form NADH, while FAD is reduced to form FADH₂ as shown in the 2 figures below.

Figure 9.2.3 The reduction of NAD to form NADH³

Figure 9.2.4 The reduction of FAD⁴ to FADH₂

Accordingly, without adequate niacin and riboflavin in the diet, the coenzyme forms NAD + NAD+ and FAD would not be made in the body and metabolism would come to a stop,5 which in turn can lead to serious illness.

Mineral Cofactor - example 'iron' or Fe

An example of a mineral that serves as a cofactor is Fe²⁺ for proline and lysyl hydroxylases. We will discuss later in detail why vitamin C (ascorbic acid) is needed to reduce iron to Fe²⁺ so that it can serve as a cofactor for proline and lysyl hydroxylases.

Figure 9.2.5 Iron (Fe²⁺) is a cofactor for proline and lysyl hydroxylases

Hormones that support metabolic functions

Hormone	Produced by	Type of Reaction	Control of Carbohydrate Metabolism	Control of Fat Metabolism	Control of Protein Metabolism
Cortisol	Adrenal glands	Catabolic	Stimulates gluconeogenesis	In acute states, no effect. Chronically elevated levels lead to lipogenesis	Stimulates protein degradation
Epinephrine	Adrenal glands	Catabolic	Stimulates glycogenolysis	Stimulates lipolysis	May stimulate protein degradation
Glucagon	Pancreas	Catabolic	Stimulates glycogenolysis	Stimulates lipolysis	Stimulates protein degradation
Insulin	Pancreas	Anabolic	Stimulates glycogen synthesis	Stimulates lipogenesis	Stimulates protein synthesis

Table 9.2 Major Hormones with influence on metabolism (modified from5)

Check your knowledge:

True or False: Enzymes catalyze metabolic reactions with the aid of cofactors, especially forms of niacin and riboflavin, as well as hormones such as insulin, glucagon, epinephrine, and cortisol.

True or False: Insulin and glucagon are produced in the pancreas, cortisol and epinephrine are produced in the adrenal glands.

Contributors

Brian Lindshield and Claudia Kelley

References & Links

1. en.Wikipedia.org/wiki/File:NAD%2B_phys.svg
2. en.Wikipedia.org/wiki/File:Fl...nucleotide.png
3. en.Wikipedia.org/wiki/File:NA..._reduction.svg
4. en.Wikipedia.org/wiki/File:FA...equlibrium.png
5. Blake, J. S., Munoz, K. D., & Volpe, S. (2019). Nutrition: From Science to You (4th ed.). Pearson.