Metabolic regulation Metabolic regulation: -Genetic level -Cellular level: - enzyme activity - Cell surface receptors

Metabolic regulation Genetic level regulation: Control which protein is synthesized through adjusting the rate of transcription of that gene: - feedback repression: the end product of enzymatic activity accumulates and blocks transcription. a repressor protein bound to the end product (co- repressor) can bind to the operator region and hinder RNA polymerase binding.

m-RNA RNA polymerase repressor PromoterOperatorGene 1Gene 2Gene 3 Normal Transcription RNA polymerase repressor PromoterOperatorGene 1Gene 2Gene 3 corepressor Transcription Blocked DNA template inactive active DNA template

In Procaryotes Operon: A set of genes, encoding proteins with related functions, under the control of a single promoter-operator. m-RNA RNA polymerase PromoterOperatorGene 1Gene 2Gene 3 repressor DNA template

Genetic organization of the Tryptophan operon DNA template encoding repressor encoding related enzymes for tryptophan synthesis

Metabolic regulation Genetic level regulation: -Induction: a metabolite ( often a substrate for a pathway) accumulates and acts as an inducer of transcription. The inducer will bind the repressor protein, and the complex is inactive as a repressor.

m-RNA RNA polymerase PromoterOperatorGene 1Gene 2Gene 3 Transcription Permitted RNA polymerase repressor PromoterOperatorGene 1Gene 2Gene 3 Transcription Blocked repressor Inducer DNA template

Catabolite Repression (Glucose Effect) e.g. The lactose operon controls the synthesis of three proteins (Lac z (lactase), lac y, lac a ) involved in lactose utilization as a carbon and energy source in E. coli. m-RNA RNA polymerase PromoterOperatorLac zLac yLac a repressor allolactose

Example Inducer: allolactose modified from lactose in the cell. Induction of allolactose might not be sufficient for maximum transcription if a carbon-energy source (e.g. glucose) preferred to lactose is present. Only when glucose is depleted, the cell will expend energy to create a pathway to utilize the less favorable carbon-energy source lactose.

Metabolic regulation Catabolite repression (glucose effect) When the cell has an energetically favorable carbon-energy source available, it will not expend significant energy to create a pathway for utilization of a less favorable carbon-energy source; it will not transcript the related enzyme for such reaction.

Metabolic regulation Genetic level regulation: -Some genes are regulated. -others are not (constitutive): their gene products are made at a relatively constant rate irrespective of changes in growth conditions. ( enzymes are expected to use under almost any conditions such as that involved in glycolysis)

Metabolic regulation Cellular level- metabolic pathway control: -The metabolic pathway can be controlled by enzyme activity. -The activity of allosteric enzymes can be controlled by effectors including inhibitors and activators. -Most often the first reaction in the pathway is inhibited by accumulation of the product: feedback inhibition or end-product inhibition.

What are the differences between feedback repression and feedback inhibition?

Reduced enzyme activity Operator on DNA template occupied by the complex Effect End product + enzyme End product + repressor Complex formed The respective reaction is inhibited. Blocked Transcription Consequence Cellular: Activity of enzyme Genetic: RNA transcription Regulation level Feedback inhibition Feedback repression

Metabolic regulation Cellular level- metabolic pathway controls: The activities of a group of enzymes (pathway) can be controlled. -Isozymes -Concerted feedback -Sequential feedback -Cumulative feedback

Metabolic regulation Cellular level- metabolic pathway controls through: -Isozymes -A number of separate enzymes initially carry out the same conversion, each of which is sensitive to inhibition by a different end product.

Metabolic regulation -Isozymes GlucoseGlucose + ATP → glucose-6-phosphate + ADPATPADP Glucokinase and hexokinase I, II, III Glucokinase is not inhibited by glucose-6-phosphate while other three enzymes are.

Metabolic regulation - Concerted feedback inhibition More than one end product or all end products must be present in excess to repress the first enzyme.

Metabolic regulation - Sequential feedback inhibition the common steps are inhibited by the product before the branch, and the first enzyme of each branch is inhibited by the branch product. High levels of P1 and P2 inhibit enzyme E4 and E5, respectively → M3 will accumulate →the pathway is inactivated if both P1 and P2 are high. M1M2M3 M4 M5 P1 P2 X X X E1 E2 E4 E3

Sequential feedback inhibition

Concerted Metabolic regulation -Cumulative feedback inhibition or cooperative feedback inhibition - A single allosteric enzyme may have effector sites for - several end products of a pathway; - each effector causes only partial inhibition. - Full inhibition is a cumulative effect. Cumulative

Inosine 5-mono-phosphate (IMP)

Metabolic regulation Cellular level- how cell senses its extracellular environment -Mechanisms to transport small molecules across cellular membrane: -Energy-independent uptake: - passive diffusion - facilitated diffusion - Energy-dependent uptake - Active transport - Group translocation

Metabolic regulation Energy-independent uptake: - passive diffusion: Molecules move down a concentration gradient from high to low concentration. The cytoplasmic membrane consists of lipid core with very small pores: - water and oxygen uptake - charged and large molecules can not cross.

Metabolic regulation Energy-independent uptake: -Facilitated transport: Molecules move down a concentration gradient from high to low concentration with a carrier molecule (protein). - The protein is considered embedded in the membrane. - The transport can result in the exit and entry of the targeted molecule. e.g. Sugar and low-molecular-weight organics in Eucaryotes. Glycerol in procaryotes.

Metabolic regulation Energy-dependent uptake: -Active transport is similar to facilitate transport but against concentration gradient. - carrier proteins embedded in the cellular membrane are necessary components. - energy is required. pH (proton motive force) or ion gradients (hydrolysis of ATP) between inside and outside cells. e.g. K + transport

Metabolic regulation Energy-dependent uptake: -Group translocation : chemical modification of the substrate during the process of transport. - energy is required: - the substrate is modified when cross the membrane and trapped inside the cell irreversibly. e.g. Phophotransferase system: Energy source: phosphoenolpyruvate (PEP) sugar (extracellular) + PEP(intracellular) → sugar-P(intracellular) + pyruvate (extracellular)

Metabolic regulation Cell level-role of cell receptors in metabolism and cellular differentiation -Almost all cells have receptors (protein) on their surfaces providing a cell with information about its environment. -Surface receptor can bind a chemical in the extracellular space which control the direction of cell movement responding to the gradient of chemical, light or oxygen. -Quorum sensing molecule can sense cell concentration depending on intracellular receptor protein -Some surface receptor of higher organism can respond to steroids and growth factors (proteins). - Some receptors attach cells to surfaces.

Summary of Metabolic Regulation Metabolic regulation: Genetic level: control transcription of genes (repression, induction and glucose effect) Cellular level: - enzyme activity: feedback inhibition Isoenzyme, concerted feedback, sequential and cumulative feedback inhibition - Cell surface receptors: - Control the cell movement. - Sense the cell concentrations. - Respond to steroids and growth factors (proteins). - Attach cells to surfaces