Regulation of metabolic pathways at the cell level

Subchapter content

 * 1) General principles of regulation of metabolic pathways at the cell level
 * 2) Compartmentation of metabolic events
 * 3) Change in absolute enzyme concentration
 * 4) Modulation of the activity of an already existing enzyme
 * 5) Regulation of individual metabolic pathways

General principles of regulation of metabolic pathways at the cell level
The activity of metabolic pathways must be constantly changed by the organism. It is regulated both according to the immediate needs and possibilities of a particular cell, and according to the needs of the organism as a whole.

At the level of the organism, regulation is mediated by signals that come to the cells from the outside. They are transferred across the cell membrane and inside they connect with the regulatory events of the cell itself. Regulations tend to have a number of steps that fall into cascades. It often ends by changing the activity of the enzyme, which thus acts as an effector of regulation. Usually only one enzyme of the metabolic pathway is subject to regulation, the so-called key or regulatory enzyme. Key enzymes usually catalyze the slowest reactions of metabolic pathways.

The speed of a certain metabolic pathway as a whole is determined by its slowest step. As we have already stated, it is this step that is usually subject to regulation. Both the number of key enzyme molecules and their activity (i.e. catalytic efficiency ) can change. Key enzymes often catalyze practically irreversible, e.g. strongly exergonic reactions.

Events that change the speed of metabolic pathways can be divided into three groups:


 * 1) Regulation that uses compartmentalization of metabolic events
 * 2) Enzyme concentration control
 * 3) Modulation of activity (catalytic efficiency) of enzyme molecules

Compartmentation of metabolic events
A eukaryotic cell is divided into several compartments by semipermeable membranes. They differ from each other in, for example, enzymatic equipment or membrane transporters. The pH values ​​are also different - enzymes often have different pH optima. If there was only one space in the cell, some of the enzymes would probably not be functional or the catalysis mediated by them would not be efficient enough.

See Compartmentation of Metabolic Pathways (FBLT) for more detailed information .

Overview of metabolic pathways according to the compartments in which they take place

We also observe a different distribution of substrates and products in different compartments of the cell. Even some coenzymes cannot pass freely between compartments, e.g. molecules of NADH or coenzyme A do not pass through the inner mitochondrial membrane. Many enzymes need a suitable coenzyme for their catalytic function. By changing the concentration of a coenzyme in a certain compartment, a certain metabolic pathway can be turned on or off. Compartmentation also facilitates the regulation of conflicting events.

E.g. the synthesis of fatty acids takes place in the cytoplasm, while their degradation takes place in the mitochondria. The speed of reactions depends on


 * supply of substrates, or cosubstrates (coenzymes),
 * from the previous steps of the metabolic pathway,
 * by transport from other compartments,
 * pumping out products
 * further steps of the metabolic pathway,
 * by transport to other compartments