Metabolic Regulation in Human Physiology

Regulation and Integration of Metabolism

• Lecture M5/5, presented by Jane Carré, lecturer in Human Nutrition and Metabolism, SoBS
• Focuses on the regulation and integration of metabolic pathways in the body

Putting it all together

• Students need to consider main metabolic pathways and their roles in energy metabolism
• Identify which pathways are catabolic and anabolic
• Analyze the impact of high pathway activity on:
  • Reduction potential (NAD(P)H/NAD(P)+)
  • Phosphorylation potential (ATP/[ADP + Pi])
• Determine which pathways are active during the fed state and the fasted state, and in which organs these pathways operate

Accumulation of metabolites

• Key metabolites indicate the energy state of the liver
• High energy charge state, typically characterized by:
  • Sufficient substrate (full TCA cycle intermediates)
  • High reducing power (NAD(P)H/NAD(P)+)
  • High phosphorylating power (ATP/ADP)
• Low energy charge state, typically characterized by:
  • Insufficient substrate (depleted TCA cycle intermediates)
  • Low reducing power (NAD(P)H/NAD(P)+)
  • High phosphorylating power (ATP/ADP)

Metabolic convergence

• Major fuels (glucose, fatty acids, and amino acids) converge on the typical metabolic pathways
• These fuels are processed through glycolysis, the TCA cycle, and oxidative phosphorylation (OXPHOS) to produce ATP

Learning outcomes

• Students will be able to explain the need for metabolic pathway flux regulation.
• Students will be able to describe that control of metabolic flux is shared by multiple enzymes in a pathway. Enzymes with high control represent critical regulatory steps
• Students will understand that the concept of energy metabolism is primarily regulated by metabolic demands
• Students will outline the multitude of mechanisms used to regulate metabolism.
• Students will be able to describe the important roles of hormonal regulation.
• Students will be able to explain the metabolic role of insulin when the body is in the fed state
• Students will be able to identify key metabolic intermediates that are used to assess cellular energy status

Key metabolic roles of different organs

• Liver: Key roles include:
  • Storage of carbohydrates (glycogen)
  • Glucose output
• Muscle: Key roles include:
  • Storage of carbohydrates (glycogen)
  • Reserve of amino acids
  • Amino acid output
• Adipose: Key roles include:
  • Storage of fat (TAG)
  • Fatty acid output

The need for regulation of metabolism

• Nutrient intake is sporadic in humans. The body needs to be able to intake, store, and oxidize macronutrients when needed.
• Anabolic and catabolic pathways are active at different times based on nutrient availability and overall energy needs.
• Different organs have varied fuel needs, with different pathways active at differing times based on energy demands

Metabolic pathway flux is controlled

• Metabolic pathway flux, or overall rate of metabolite flow, is most strongly controlled by certain enzymes in the pathway.
• Small changes in the activity of certain enzymes can greatly alter pathway flux compared to other enzymes.
• These enzymes have high control and the ability to regulate flux.
• Changing enzyme activity may:
  • Restrict or stimulate metabolite flow through a pathway.
• Enzymes may be regulated through changes in their activity

Energy metabolism is controlled

• Energy metabolism is largely regulated by energy demand and responding to changes in cytosolic ATP/ADP
• Changes in energy demands (eg. exercise) more significantly influence metabolic flux than changes in energy supply
• Communication of metabolic demands is through cytosolic ATP/ADP and mitochondrial NADH/NAD+

Enzymes with most metabolic control

• Enzymes with high metabolic control often are:
  • The first unique step in a reaction pathway.
  • Located after a branchpoint in a reaction pathway.
  • Enzymes far from equilibrium in pathways with irreversible steps.
  • Control processes responsible for intermediate availability.

Concepts in metabolic regulation

• Metabolic pathways respond to environmental cues and the specific needs of different tissues.
• Methods for metabolic regulation include:
  • Compartmentalization of metabolic enzymes
  • Activation/inhibition of enzymes through different isozymes
  • Regulation in response to the cellular environment or metabolic load
  • Enzyme activity regulation, such as product inhibition/activation, substrate restriction/activation and covalent modification
  • Hormonal regulation

Short and long-acting regulation of enzymes

• Short-acting regulation of enzyme activity (minutes to hours). Short-term regulation strategies may include:
  • Variation in enzyme expression, such as variations in isozymes.
  • Substrate availability
  • Feedback/feedforward mechanisms
  • Fast-acting hormones
  • covalent modifications
• Long-acting regulation of enzyme activity (hours to days). Long-term regulation strategies may include:
  • Slow-acting hormones
  • Rate of enzyme synthesis

Compartmentalisation of metabolism

• Tissue-specific expression of metabolic pathways
• Compartmentalisation of enzymes within cells
  • Gluconeogenesis is restricted to specific cells and tissues.
  • Fatty acid synthesis occurs in different cellular compartments to fatty acid breakdown.
  • Urea cycle is confined to the liver.

Alternative enzymes to catalyze 'reverse' process

• Gluconeogenesis is not a straightforward reversal of glycolysis.
• Different enzymes are needed to overcome the irreversible steps.
• The same holds true for fatty acid synthesis compared to fatty acid breakdown

Gluconeogenesis - pathway

• Gluconeogenesis is the essentially reverse of glycolysis, except for certain irreversible steps.
• The key irreversible steps in glycolysis require distinct enzymes to proceed in the reverse direction, as the original enzymes cannot be used.
• Acetyl CoA cannot be directly converted to glucose.

Regulation by different isozymes

• Isozymes are different versions of the same enzyme, resulting from an alternate gene or splicing mechanisms.
• Isozymes may differ in kinetic and/or regulatory properties, reflecting varying tissue- specific requirements.
• Hexokinase and glucokinase are key examples of isozymes with different kinetic properties, reflected in the way the liver and muscle use glucose.

Reminder: enzymes

• Enzyme activity depends on substrate concentration
• Certain metabolic enzymes have a Michaelis constant Km, the substrate concentration at which an enzyme reaches half of its maximum reaction rate.
• Enzymes can be modulated through product inhibition or allosteric activation/inhibition by molecules that bind to a regulatory site other than the active site.

Different isozymes with various properties

• Different isozymes exhibit varying regulatory properties:
  • Different substrate affinities (Km)
  • Distinct kinetic and regulator responses
• Hexokinase (muscle cell) displays high substrate affinity at physiological glucose levels.
• Glucokinase (pancreatic beta-cells and liver) is effective at higher glucose concentrations

Different ISO enzymes with varying properties

• Muscle cells use hexokinase; and pancreatic beta-cells use glucokinase.
• The regulatory potential of different isozymes influences cell-speciifc physiological functions.
• High glucose levels in the blood stimulate pancreatic insulin release, which is required for glucose uptake by muscle tissue and other target tissues.

Feedback and feed-forward regulation

• Metabolite concentrations can regulate metabolic pathway flux through allosteric mechanisms
• Metabolites upstream or downstream of a pathway's enzymes can regulate the enzyme's activity.
• Allosteric regulation involves an effector binding to a site other than the active site, affecting the enzyme's conformation and thus activity.

Regulation of fatty acid synthesis and breakdown

• The liver and adipose tissue should not use synthetic and breakdown pathway simultaneously
• Fatty acid oxidation and fatty acid synthesis occur in separate cellular compartments and utilize different enzyme sets.
• Metabolic products of one pathway (eg malonyl CoA) can serve as regulatory signals to prevent the other pathway from operating simultaneously.

Regulation of CPT1

• Substrate availability regulates CPT1, influencing the co-transport of carnitine and fatty acid across the inner mitochondrial membrane.
• High CoA levels stimulate continued uptake of carnitine and co-transport of fatty acids (during high metabolic rates), and low levels of CoA prevent it (at rest).

Allosteric feedback regulation of CPT1 by malonyl CoA

• Malonyl CoA, an intermediate in fatty acid synthesis, is a critical regulator of CPT1, inhibiting its activity by preventing fatty acyl CoA uptake into the mitochondria and preventing the use of fatty acids as fuel. -The allosteric regulation of CPT1 by malonyl CoA helps maintain a balance between fatty acid synthesis and breakdown

Allosteric regulation

• Allosteric regulation of phosphofructokinase and glycolysis
  • Phosphofructokinase plays a key role in regulating the glycolysis pathway
  • PFK activity is modulated based on energy state within the cell
  • Inhibitor effect of ATP and activator effect of AMP.
  • Citrate acts as a feedback inhibitor of PFK
• Inactivation of phosphofructokinase is also triggered through elevated levels of ATP.

Role of adenylate kinase and AMP

• AMP and ADP act as cellular signals of energy status, regulating metabolic processes.
• Adenylate kinase converts ADP to AMP in response to high energy demand, stimulating further ATP production and using fuel reserves.
• This regulatory mechanism helps maintain cellular energy homeostasis.

Allosteric regulation of AMP kinase

• AMP-activated kinase (AMPK) is a key regulator of energy metabolism.
• Elevated AMP levels, indicating low energy charge, trigger AMPK activation.
• Phosphorylation of target proteins by AMPK results in metabolic responses such as changes in fuel mobilization and mitochondrial function

Hormones

• The release of hormones is stimulated by external or internal cues, such as metabolic fuels, other hormones, and neuronal control. The hormones released stimulate receptors on target cells, driving cellular response.
• Hormones can be classified as fast-acting (affecting existing enzymes) or slow-acting (affecting synthesis rates of enzymes).

Hormone binding and cellular responses

• Hormone binding to receptors initiates a signaling cascade influencing cellular responses.
• Fast-acting hormonal regulation primarily involves changes in existing enzyme activity, typically achieved via covalent modification.
• Slow-acting hormonal regulation alters the rate of enzyme synthesis primarily through second messengers influencing transcription rates for relevant genes.

Key hormones regulating macronutrient metabolism

• Insulin, glucagon, incretins, catecholamines, cortisol, growth hormone, thyroid hormone, and leptin are key hormones that regulate macronutrient metabolism in the body.

Integrating metabolism

• Metabolism of carbohydrates and fats are interconnected - one can be elevated while the other is suppressed
• Metabolic interactions and regulation occur throughout the body and are regulated by hormonal or metabolites signalling.

Summary

• Summary of learning outcomes

ATP yields from fuel sources

• ATP yield from oxidation of different fuel sources.
• Theoretical maximum yeild versus actual yield, factors contributing to actual yields lower than theoretical maximum yield.

Metabolic flexibility

• The capacity of cells to adapt to different conditions by utilizing various energy-yielding pathways (anaerobic and aerobic pathways).
• Aerobic pathways yield more ATP than anaerobic ones, for instance, with a slower turnover.
• Rapid or slow turnovers are either efficient or inefficient based on energy.

Integrating metabolism (continued)

• Metabolism of carbohydrates and fats is related
• One pathway will typically be favored based on current metabolic status

Active consolidation

• Compare and contrast aerobic and anaerobic glycolysis.
• Similar key information on fatty acid metabolism

Other suggestions for active consolidation

• Draw a more visual representation of the main metabolic connections of the major metabolites discussed including regulating molecules
• Focus on key enzymes, regulatory signals, and metabolic conditions in various organs.

MCQs

• MCQ questions testing understanding of concepts concerning metabolism and the ways in which the body regulates pathways and balances energy states

Additional MCQs

• Further MCQ questions focused on metabolism

Active transport of calcium ions, example MCQ

• Understanding of the role of ATP during active transport.

Uncoupling of OXPHOS in BAT in response to adrenaline

• Understanding of how adrenaline stimulates uncoupling proteins in brown adipose tissue and how this affects energy expenditure.

Inter-organ crosstalk between fat and CHO metabolism

• Understanding of how various organs work together to regulate energy through fuel exchange and metabolism

Test your knowledge on the metabolic roles of insulin, glycogen storage, and cellular energy status. Explore the intricate regulatory mechanisms involving key intermediates and enzymes like AMP-activated kinase and phosphofructokinase. This quiz will challenge your understanding of metabolic processes in the fed state.