Metabolism, Citric Acid Cycle, and Nitrogen Metabolism

Lipid Metabolism Overview

• Recap of yesterday's overview of lipid metabolism.

• Plan to integrate isolated metabolism components.

Metabolic Distribution and Adaptation

• Discussion on metabolism distribution across tissues in animals.

• Focus on metabolic activity sharing.

• Examples of adaptive metabolism changes over time in different physiological conditions.

Biological Specimen Analysis

• Class to analyze metabolites in dog urine and feces.

• Less about measuring metabolites, more about discussion.

• Consider metabolite profiles in different physiological and clinical states.

Formative Assessment: Case Studies

• Case study discussions as formative assessment.

• To be shared next week, requiring integrative & adaptive metabolism thinking.

• Structured exercises with scaffolded questions to guide thinking.

• Potential tweaks to case studies.

Overall Plan

• Bring all metabolism topics together.

• Shift to endocrine regulation of metabolism afterward for the semester's end.

Shared Pathways

• Citric acid cycle.

• Nitrogen metabolism within cells.

Citric Acid Cycle

• Also known as Tricarboxylic Acid Cycle/Krebs Cycle (named after its proposer).

Location and Significance

• Occurs within mitochondria.

• Accounts for significant oxidation of carbon compounds, releasing carbon dioxide.

• Main site of carbon dioxide liberation from metabolism.

Products

• Acetyl CoA enters the cycle.

• Carbon dioxide produced as waste.

Energy Production

• Significant energy released in the form of NADH, FADH, and GTP.

• Site of reducing power liberation and high-energy electrons for oxidative phosphorylation.

Activated Molecules

• GTP functions like ATP, using high-energy phosphine hydro bonds.

• FADH operates similarly to NADH/NADPH, taking up high-energy electrons & protons using nitrogen-containing ring structures.

Common Pathway in Catabolism

• Central pathway for proteins, carbohydrates, and lipids via acetyl CoA.

• Products (carbon dioxide & reducing power) used in oxidative phosphorylation for ATP generation.

Acetyl CoA Formation

• Formed directly from fatty acid catabolism or indirectly through pyruvate oxidation.

Other Substrates

• Amino acid catabolism yields glucogenic products used in gluconeogenesis.

• These products break down into citric acid cycle intermediates/pyruvate.

Ruminant Metabolism

• Volatile fatty acids (propionate, acetate, butyrate) from fermentation.

• Acetate/butyrate contribute to acetyl CoA formation.

• Propionate contributes to succinate, a citric acid cycle intermediate.

Pyruvate Oxidation to Acetyl CoA

• Key irreversible step committing to oxidation.

• Limits interchangeability of energy-yielding nutrients.

• Catalyzed by pyruvate dehydrogenase complex (3 enzymes).

• Releases carbon dioxide and energy as NADH.

• Carbohydrates can be used for gluconeogenesis, but fatty acids cannot.

Irreversible Reaction

• Fatty acids break down to acetyl CoA and are thus ketogenic (cannot be substrates for gluconeogenesis).

• Amino acids that break down to pyruvate can be used in gluconeogenesis.

Starvation state

• Breakdown (catabolism) of protein and lipids

Pyruvate Dehydrogenase Complex

• Oxidation of pyruvate to acetyl CoA.

• Diagram shows the conversion from 3-carbon pyruvate to 2-carbon acetyl CoA, with carbon dioxide liberation and NADH production.

Citric Acid Cycle Details

• Eight steps, cyclical pathway.

• Starts and ends with oxaloacetate.

• Two carbons from acetyl CoA are not substrates for gluconeogenesis.

Key Reactions and Products

• Step 1: Condensation of oxaloacetate + acetyl group → citrate (6 carbons).

  • Coenzyme A liberated back to the mitochondrial matrix.

• Carbon Dioxide Release: Liberation of carbon dioxide and energy, leading to 5-carbon intermediates.

• Further Liberation: Subsequent liberation of carbon dioxide to 4-carbon molecule. More energy liberated.

• Steps 5 Onward: GDP, FADH, and NADH are created.

  • Four high energy electrons liberated.

• Net effect: 2 carbons in, 2 carbons out, oxaloacetate retained, energy produced for oxidative phosphorylation.

Structural Details and Intermediates

• Structures show intermediates like alpha-ketoglutarate (role in oxo-deamination) and succinyl CoA (from propionate catabolism).

• Intermediates can be used for amino acid biosynthesis.

Tracking Carbons

• Carbons from acetyl CoA aren't lost in their initial cycle but eventually cycle out.

• The carbon at the top of oxaloacetate will be liberated as carbon dioxide in the next cycle.

• Net chemical reaction maintains oxaloacetate concentration (4 carbons in start and end).

Common Misconceptions

• Oxygen for carbon dioxide formation comes from the organic molecules, not from breathed molecular oxygen.

• Breathed molecular oxygen is converted into water during oxidative phosphorylation.

Overall Reaction

• Acetyl CoA + Water → NADH + FADH + GTP + Carbon dioxide.

• Shows how the cycle uses energy stored in acetyl groups.

Regulation of Citric Acid Cycle

• Cycle activity is significantly regulated, not just happening passively.

Key Regulators

• Oxaloacetate Concentration: Influences cycle activity and ketogenesis.

• Feedback Loops: Reactants (like pyruvate), products (like ATP, NADH, acetyl CoA) influence cycle activity.

• Energetic Status of the Cell: High energy availability reduces pyruvate breakdown.

Kinase Activation

• Phosphorylation and dephosphorylation regulate pyruvate dehydrogenase complex activity.

Isocitrate Dehydrogenase

• Step three, regulated by energetic status (ADP stimulates, high energy inhibits).

• Concentrations of the activated carrier molecule availability influence cycle activity.

Secondary Signalling

• Influx of ions like calcium and magnesium into mitochondria influences cycle activity.

Citric Acid Cycle as Biosynthetic Hub

• Key central pathway for biosynthesis, with intermediates serving as substrates for various molecules.

Biosynthetic Connections

• Ketones: Influenced by cycle activity and oxaloacetate concentration.

• Fatty Acids: Citrate transport system moves acetyl groups from mitochondria to cytoplasm; Shares pathway with citric acid cycle.

• Amino Acids: Intermediates for amino acid biosynthesis.

• Nucleotides: A couple intermediates used for nucleotide base formation.

• Gluconeogenesis: Oxaloacetate is a substrate.

• Core metabolism: glycolysis, pyruvate oxidation to acetyl CoA, and the citric acid cycle.

Nitrogen Metabolism and Urea Cycle

• Focus on the urea cycle: nitrogen sources, importance, and cycle function.

• Highlight disruptions to nitrogen metabolism and their impacts.

Objectives

• Urea cycle: components and interaction with other metabolic pathways.

• Nitrogen sources utilized.

• Inherited disorders of nitrogen metabolism.

Krebs Connection

• Krebs also identified urea cycle as a potential metabolic cycle.

Nitrogen Excretion Forms

• Ureotelic: Excrete urea (terrestrial vertebrates).

• Uricotelic: Excrete uric acid (birds, reptiles).

• Ammonotelic: Excrete ammonia directly (fish).

Amino Acid Catabolism

• Amino acids catabolized by removing the amino nitrogen group (oxidative deamination).

• Carbon skeletons (alpha-keto acids) are degraded into metabolic intermediates (pyruvate, acetyl CoA, Krebs cycle intermediates).

Oxidative Deamination

• Transfer of amino nitrogen:

  • Amino nitrogen + alpha-ketoglutarate → glutamate + byproduct.

• Glutamate returns to alpha-ketoglutarate, releasing ammonium ions.

• Amino acid → glutamate → ammonium. The next stage is urea.

Why Eliminate Urea?

• Ammonia = a toxic intermediate, especially for neurons.

Toxicity Mechanisms

• Ammonia: readily permeates cell membranes; diffuses into the brain and mitochondria.

• If high ammonium ions:

  • Formation of glutamate.

  • Alpha-ketoglutarate decreases.

  • The citric acid cycle drops in activity.

• Conversion to ammonium ions can impact the proton gradient → affecting oxyphosphorylation.

• Increase glutamine and glutamate in neural cells significantly changes osmotic potential → cells swell (cerebral edema).

• Decrease in glutamate which also causes neurotransmitter function to change.

Homeostasis

• Animals must manage amino nitrogen groups/ammonium concentrations.

• Primarily achieved through the urea cycle.

Urea Cycle

• The two lots of amino nitrogen come from:

  • Amine and oxygen plus carbon dioxide: form carbonyl phosphate.

  • Amino acid aspartate.

Key Metabolic Steps

• Amino nitrogen and carbon from carbamoyl phosphate + amino nitrogen from aspartate → urea.

Process Steps

• First, carbon dioxide + ammonium + energy yield carbamoyl phosphate.

• Carbamoyl phosphate + ornithine → citrulline (tracking carbon and oxygen components).

• Citrulline + aspartate + Energy + Arginiosuccinate.

Importance

• Urea is then finally lost through urine and faeces.

Cellular Compartmentalization

• The urea cycle occurs across multiple compartments.

  • Ammonia → carbamoyl phosphate = mitochondrial matrix.

  • Other amino acids and other oxygen moves through the plasm, with the urea liberated in the plasm.

Transport Coordination

• Movement across the inner mitochondrial membrane is an active, coordinated process.

• Ornithine and citrulline are concurrently trafficked.

Citric Acid Cycle Relationship

• There is a relationship between aspartate and the liberated product fumarate.

The Cycle Interacts

• Fumarate, through the late stages in the citric acid cycle.

• Oxaloacetate + amine from oxygen = apsartate.

  • Homegrown amine and oxygen are needed with urea formation.

  • Interrelated with the activity of the citric acid cycle and urea cycle.

Amino Acid Interrelationships

• There is a need to maintain a one-to-one requirement, the source of the amino nitrogen.

• Must interconvert nitrogen and the apatate:

  • For example, if there is lots of ammonium, some can be trafficked or turned into carbon phosphate.

  • There must interconversion (Balance) between the sources of urea.

Energy Cost

• Four high-energy phosphate groups from three different ATP molecules are utilized in urea formation.

• It costs around 15% of the energy to eliminate amino acids.

• There is an energetic use, using amino acid catabolism as an energy source.

Other Urea Cycle Facts

*Urea must be low in concentrate to stop urea from falling out.
This requires a lot of water to be used. So there is a need to scavenge water (ruminants can offset the loss.).
Vertebrates carry bladders to carry around all the water.
*Animals that cannot carry much cannot scavenge, like birds.

Urea Cycle Disruption

Sytralinemia

*inherited disorder.
occurs occasionally, is in Holstein Friesian cattle.
Single gene change to enzyme which caused a disruption.
Single gene change =hyperammonia.
*There for there is Single nucleotide substitution in one enzyme gene.

Symptoms

*behavioral changes.
*Head pressing on cells
*loss of consciousness and death

Animals Disruption

*There are other simple gas exchange is used, such as in aquatic animals that exchange gills from ammonia.

Bird's Solution

** birds and reptiles, and is as uric acid.
Has a significant energetic cost to form uric acid.
doesn't need to have a lot of water,