Control of Metabolic Pathways Higher Human Biology Unit 1 – Section 6 Metabolic Pathways

Learning Intentions By the end of this section you should be able to: Explain the regulation of gene expression, enzyme production and enzyme function. Describe the role of extra- and intracellular signal molecules.

What is a Metabolic Pathway? A metabolic pathway consists of several stages, each of which involves conversion of one metabolite to another. Each step in the pathway is controlled by a specific enzyme.

Each step in a metabolic pathway is driven by a specific enzyme. Each enzyme is coded for by at least one gene. If one enzyme is absent, the metabolic pathway stops. Metabolic pathways can therefore be regulated through gene expression.

Switching genes On and Off. Some metabolic pathways (e.g. glycolysis reactions in respiration) operate continuously. So the genes which code for these enzymes are always expressed and ‘switched on’. However, other enzymes are only produced when required by the cell, thereby saving resources and energy.

Lactose Metabolism in Escherichia coli Lactose is a sugar that it found in milk. It is made up of a molecule of glucose and galactose E.coli can only make use of the glucose for respiration if it is released from the galactose. The enzyme responsible for breaking up lactose is B – galactosidase. E.coli has a gene that codes for B – galactosidase but it only produces this enzyme when lactose is present. Somehow the gene that codes for B – galactosidase is switched on in the presence of lactose. The process of switching on a gene only when the enzyme that it codes for is needed is called enzyme induction.

Control of growth is under genetic control with genes being switched on and off at specific times. One example of a genetic switch was discovered by Jacob and Monod who demonstrated the method by which a gene making the enzyme ß- galactosidase could be regulated to ensure that :- a)the enzyme was only switched on when its substrate lactose was present. b)valuable raw materials were not wasted. c)valuable energy in the form of ATP was not wasted. Genetic Control Genes can work together to help regulate the production of an enzyme 1.Structural gene is transcribed and translated into the enzyme ß- galactosidase which breaks down the sugar lactose 2. Operator gene switches on the structural gene 3. Regulator gene controls the functioning of the operator through the production of a ‘repressor protein’ All three genes working together are known as an operon

The production of the enzyme B – galactosidase by E. coli bacteria. In the absence of lactose (a sugar) the lactose digesting enzyme ‘B – galactosidase’ is not produced by the bacteria. But when lactose is present, the lactose digesting enzyme ‘B – galactosidase’ is produced.

And here is an animation:-

Since the lactose sugar starts the production of the enzyme it is known as the inducer Mutations of the operon genes 1.Regulator The repressor protein would be faulty or not made so the operator would be free to switch on the structural gene all the time. Therefore the enzyme would be made continuously. Therefore wasting resources and energy. 2.Operator The operator would not be able to switch on the structural gene. Therefore the enzyme would not be made. Therefore cell could not use lactose as an energy source 3.Structural The enzyme would be faulty or not be made. Therefore cell could not use lactose as an energy source.

Regulation of gene expression by ‘signal molecules’. The activation of some enzymes is controlled by signal molecules. In the Lac operon system the signal molecule is lactose sugar. It combines with the repressor molecule, allowing the structural gene for B – galactosidase to be expressed.

Intra- and Extracellular Signal Molecules. The molecules that effect a cell’s metabolism and originate from out with the cell (e.g. lactose) are called extracellular signal molecules. Hormones such as Adrenaline are also examples of extracellular signal molecules.

Intra- and Extra-cellular Signal Molecules. Molecules that effect a cell’s metabolism and originate from inside the cell itself are called intracellular signal molecules.