Introduction to Metabolism

Introduction to Metabolism

• Complex substances are broken down for energy, required metabolites, and structural components.
• Cells synthesize new complex substances.
• Thousands of reactions occur simultaneously in a single cell.
• These reactions occur with minimal side products, energy loss, or interference, at reasonable temperatures, pH, and pressure.
• Control and regulation ensures optimum efficiency for all reactions.
• Metabolism encompasses all chemical reactions.
• Catabolism breaks down complex substances.
• Cellular metabolism involves:
  • First breaking down excess carbohydrates.
  • Then lipids.
  • Finally amino acids, if energy from carbohydrates is insufficient.
• Nutrients not used for energy are utilized to construct structures, stored, or excreted.
• 40% of the energy released in catabolism is captured in Adenosine Triphosphate (ATP); the remainder is released as heat.
• Anabolism synthesizes complex substances from simpler ones.
• It is responsible for:
  • Structural maintenance.
  • Growth.
  • Secretion production.
  • Nutrient reserve building.
• Pyruvate, Acetyl CoA, and citric acid cycle intermediates serve as starting materials.
• It depends on the supply of energy (ATP/GTP) and reducing equivalents (NADPH+ + H+).
• NADPH is a reduced form of nicotinamide adenine dinucleotide phosphate, a coenzyme used in anabolic reactions like lipid and nucleic acid synthesis.

Digestion of Dietary Carbohydrates

• Starch constitutes the primary carbohydrate source.
• Several digestive enzymes break down carbohydrates.
• α-Amylase initiates digestion by cleaving α-1,4 bonds but not α-1,6 bonds.
• Completion of digestion requires other enzymes, including α-glucosidase and α-dextrinase.
• Common disaccharides like sucrose and lactose are digested by sucrase and lactase respectively.
• Glucose and galactose are transported into the intestine via the sodium-glucose linked transporter.
• Fructose entry is mediated by the GLUT5 transporter.

Carbohydrate Metabolism

• Glucose is the central molecule in carbohydrate metabolism.
• Fructose, galactose, and mannose enter the pathways.
• All cells utilize glucose for energy production.
• Normal fasting blood glucose levels range from 70-100 mg/dl.
• The liver is central to carbohydrate metabolism; it monitors and stabilizes blood glucose levels.
• Entry of glucose into cells is usually mediated by insulin and transporters (GLUT-1 to GLUT-5 and GLUT-7).
  • GLUT-1 is abundant in red blood cells.
  • GLUT-4, in skeletal muscle and adipose tissue.
• Glucose uptake is insulin-independent in hepatocytes, erythrocytes, and the brain.
• Glucose uptake relies on insulin in muscle and adipose tissue.
• Absorbed glucose prioritizes:
  • Glycogen storage in muscle and liver.
  • Energy provision through oxidation to ATP.
  • Storage as fat (triglycerides) in adipose tissue, but only if it's excess glucose.
• High blood glucose triggers insulin release from the pancreas, resulting in glucose absorption by muscle, adipose cells, and glycogen storage.

Glycolysis

• It's a ten-step metabolic pathway.
• It converts glucose (or glycogen) into two molecules of pyruvate and two molecules each of NADH and ATP.
• All catabolized carbohydrates must start with the glycolytic pathway.
• Glycolysis is central to generating both energy and metabolic intermediaries.
• It features an energy investment phase (reactions 1-5), requiring 2 ATP to convert glucose to two glyceraldehyde 3-phosphate molecules.
• The energy payoff phase (reactions 6-10) generates two pyruvate molecules and four ATP molecules from two glyceraldehyde 3-phosphate molecules.
• A net of two ATP and two NADH molecules are produced.
• NADH: Reduced form of Nicotinamide adenine dinucleotide, a coenzyme found in all living cells.
• Glycolysis occurs in all cells' cytoplasm in the body.
• Glycolysis is observed in anaerobic or aerobic conditions.
  • Lactate is the end-product under anaerobic conditions.
  • $Glucose + 2ADP + 2Pi → 2 Lactate + 2ATP$
  • Pyruvate formed under aerobic conditions is converted into $CO_2$ and $H_2O$
  • $Glucose + 2 ADP + 2 Pi → 2 Pyruvate + 2 ATP + 2 H_2O$
• It serves as a major pathway for ATP generation in tissues lacking mitochondria (e.g., RBCs, cornea).
• Glycolysis is essential for the brain, requiring glucose oxidation via glycolysis before oxidation to $CO_2$ and $H_2O$.
• It represents a central metabolic pathway with intermediates useful for amino acid and fat synthesis.
• Reversal of glycolysis will result in gluconeogenesis.
• Glucose utilization pathways provide or consume ATP.
• The glycolytic pathway key aspects are for energy metabolism and branch points diverting carbon for NADPH generation, glucosyl unit storage in glycogen, and neuromodulator, amino acid and nucleotide biosynthesis.
• Major reactions and net ATP yields/consumption from the glycolytic pathway are indicated in color-coded boxes.

Enzyme Classification

• Dehydrogenase oxidizes substrate using cofactors as electron acceptors or donors (e.g., pyruvate dehydrogenase).
• Reductase adds electrons from reduced cofactors (e.g., enoyl ACP reductase).
• Kinase phosphorylates substrates (e.g., hexokinase).
• Hydrolases use water to cleave molecules:
  • Phosphatase hydrolyzes phosphate esters (e.g., glucose-6-phosphatase).
  • Esterase (lipase) hydrolyzes esters, acting on lipid esters (e.g., lipoprotein lipase).
  • Thioesterases hydrolyze thioesters.
• Thiolase uses thiol to assist thioester formation (e.g., β-ketothiolase).
• Isomerases interconvert isomers (e.g., triose phosphate isomerase).

Reactions of Glycolysis

• A sequence of reactions converts glucose into pyruvate
• It produces a relatively small amount of direct energy
• It occurs in cytoplasm, not requiring oxygen.
• It happens in three distinct phases:
  • Energy investment.
  • Splitting.
  • Energy generation.
• Glucose is phosphorylated to glucose 6-phosphate by hexokinase or glucokinase, an irreversible reaction.
• It is dependent on ATP and $Mg^{2+}$.
• Hexokinase exists present in all tissues, catalyzing phosphorylation of other hexoses (fructose, mannose).
• It is irreversibly inhibited by Glu-6-phos.
• Glucokinase, present only in the liver, catalyzes phosphorylation of glucose and is not inhibited by Gluc-6-phos.
• Glucokinase is used in high levels of glucose after intake, and Hexokinase is used in low concentrations.
• Glucose 6-phosphate is impermeable to the cell membrane.
• It is a central molecule involved in glycolysis, glycogenesis gluconeogenesis and the pentose phosphate pathway.
• Glucose 6-phosphate undergoes isomerization to fructose 6-phosphate in the presence of phosphoglucoisomerase.
• Fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate by phosphofructokinase, an irreversible and regulatory step.
• 6-carbon fructose 1,6-bisphosphate is split into two 3-carbon compounds.
• These are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate by the enzyme aldolase.
• Triosephosphate isomerase catalyzes the reversible reactions of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.
• 2 molecules of glyceraldehyde 3-phosphate are formed from 1 molecule of glucose.
• Glyceraldehyde 3-phosphate dehydrogenase converts glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate.
• It passes through an electron transport chain and is oxidatively phosphorylated if NADH passes through.
• Energy production is affected by the catalytic enzyme, phosphoglycerate kinase involved in the reaction to produce 3-phosphoglycerate:
  • This enzyme also has a role in with the following functions:
    • ATP Synthesis
    • Substrate level phosphorylation:
      • ATP is synthesized from the substrate without the need of the electron transport chain
    • Reversible effect:
      • A rarity amongst kinds of reactions.
• 3 phosphoglycerate is then converted to 2-phosphoglycerate via phosphyglycerate mutase
  • Note: It is an Isomerization reaction.
• Enolase then converts this 2-phosphoglycerate to a high energy compound by the name phosphoenol pyruvate (Requires Mg2+).
  • It is inhibited by fluoride. That enzyme requires $Mg^{2+}$
    • In the lab, the fluoride levels are increased in order to prevent glycolysis by the cells to insure that blood glucose levels are correctly estimated.

Reactions of Glycolysis part 2

• Pyruvate kinase catalyzes phosphate transfer from phosphoenol pyruvate to ADP.
• It synthesizes ATP by phosphorylation of pyruvate, which happens in the following steps
  1. A balance sheet:
     • 2 ATP is expended.
     • 4 ATP units are eventually produced in each and any of the 36 fragments
     • Note that these are generated from 2 different glucose units
     • As such the net production stands at 2 ⁓P bonds of ATP per glucose used.
• Glycolysis’ Pathway (omitting H±):
  • $glucose ± 2 NAD⁺ = ⁺ 2 ADP + 2Pi - pyruvate= ⁺ 2NADH ⁺ 2 ^TP.
  • There are 3 kinds of irreversible kinase reactions that primarily drive glycolysis forward:
    • Hexokinase or Glucokinase:
      • Phosphorylation Of Glucose.
      • It is inhibited by product, 6-phosphate.
      • It is response to slowed Glycolysis pathway
• Phosphofructokinase.
  • Primarily a rate limiting and regulatory enzyme for glycolysis
    • This is a allosteric regulatory Enzyme.
    • It measures the current adaquacy of energy levels
      • Inhibitors: ATP and Citrate in conditions of high energy.
      • Activators: ADP and AMP.
      • Fructose when present with low energy
• Pyruvate Kinase:
  • Tetramer allosteric in nature.
    • Inhibitor when with ATP, Acetyle CoA, Fatty Acids are found present.
    • An activator when fructose is present.
      • Considered a strong "feed-forward" stimulant for Pyruvate Production (pyruvate kinase).

Pyruvate Metabolism

• After Glycolysis, pyruvate may undergo three different fates.
• Conversion to lactate (anaerobic pathway).
• Conversion to alanine (amino acid production).
• Entry into the TCA cycle via the pyruvate dehydrogenase pathway.
• Conversion of pyruvate to lactate occurs as follows:
  • In Anaerobic conditions the pyruvate, is converted to lactate by catalytic lactate dehydrogenase.
    • In this condition there is a product known as lactate dehydrogenase.
    • As such there is a product obtained from the reaction involved.
    • The reaction is activated by glyceraldehyde-3-Phosphate hydrogenase.
    • Also involves a regenerative compound known
      • Regenerative NAD: Is an enzyme that can be reused to proceed the glycosis if there isn’t enough oxygen to facilitate ATP production.
        • It is important for skeletal muscles and intense workouts by uninterrupted glycolysis.
    • Glycolysis in Red Blood Cells.
      • Can directly lead to lactate production as the mitochondria.
      • The muscle utilizes ATP to generate contraction
      • Thus as such the lactate can be used in muscle.
      • That lactate is converted to Glucose molecules for recovery.
      • NAD -NADH has to go back to NAD through Lactic Acid production
      • Lactate is converted to liver based pyruvate
      • Lactate can be transported via blood and liver.
      • That same lactate is converted back to the pyruvate in the liver.
      • Conversion to alanine (amino acid).
      • Converts to Alanine via export to blood (via glutamine and ketoglutarate).
        • Note that this uses Keto Acid, transferase enzymes and amino acids.
        • This reaction happens with a number key steps which are shown to occur during aerobic situations which are used in
        • The Preparation of the entry of the Citric Cycle TCA Cycle with enzymes
        • Also produces Acetyl COA.
        • Electron transport chain.
        • Some special conditions:
          • Increases Glucose consumption
          • Can increases the transcription factor, Hypoxia (HIF) which also impacts the synthesis of glucose transporters that are used for tumor creation

Aerobic vs Anaerobic Metabolism

• Aerobic Conditions result of inhibition of glycolysis from access to oxygen
  • This inhibits glycolysis in three steps which are indicated as:
    • In Aerobic Conditions - there is a high level of glucose 1.6-bisphosphate synthesis.
    • This indicates that Pasteurs effect impacts phosphofructokinase.
    • High ATP is found to cause Pasterus affect.
• Conversely this result in the reverse reactions known as
  • Addition of glucose to high aerboic glycolysis.
    • Opposite affect of Pasteurs effect
• Glycolisis, in the cells can result in large effects in the cell and is dependent upon aspects including:
  • Glucose metabolism
  • Large branches leading to synthesis.
  • NADOH production
  • glycogen.
    • Gluconeogenesis, or Glycolytic Production
    • Glycogen synthesis
    • Neuro - Modulation
    • There is however a net ATP/GLyc yield which ranges from G to G2.
    • Consumed = Net ATP or G2 + 1
    • ATP /C is produced within the Glucagon and is consumed in other conditions. Citric acid. In aerobic conditions Pyruvate molecules undergo a transition to AcetylCoA in a process that can only occur under aerobic conditions
• Pyruvate dehydrogenase:
  • This catalytic reaction is multi-component involving enzymes, and complexes:
    • It is only found in the mitochondia.
    • Can effect large actions within the kidney, Cardiac Muscle.
    • Is activated by 5 different cofactors such as :
      • Theyamine Pyrophosphate (TTP).
      • Lipoaminde (Lypoic acid is linked to eAmino group of Lysin).
      • Flavin Adenine dinucleotide (FAD).
      • Conenzyme A, NAD
      • Synthesize to Acetyl - COA from Pervuc ACid requires is
  • It requires enzymes such as:
    • De carboxylase.
      • Involves ionized for enzymes.
    • Oxidized
      • The Hydroxide group (found w thin TPP oxidized to form Acetyle group via the lipoic derivatives:
        • is activated with enzymes
        • This occurs with the removal of hydroxy groups at the reactive disulfide bonds.
  • Requires Enzyme:
    • Dihydrolipoamide Dehydrogenase reactions to occur, it must be re -oxidized.
  • However note : If issues arise, such as:
    • TPP, issues can occur
      • Thiamine deficiencies.
      • Pyruvic acuminsulation occurs
      • Alcoholics (cause issues to occur which increase LActate production)
      • Is inhibited by heavy metals like arsenic and mercury

Acetyl CoA

• Acetyl groups enter the TCA as acetyl-coenzyme A.
• Coenzyme A (CoASH, or CoA) -mercaptoethylamine group bonded via amide linkeage to the vitamin pantothenic acid.
• Pantothenic acid attached to a 3: phosphate bridge.
• Acetyl group bonded as a thioester to the sulfhydrul .
• CoA functions as a catrier with ACety, COA.
• A high energy compound
• Acyl COA.

Citric Acid Cycle

• Oxidation:

  • Transports the Hydroxy Ethyl Grouo TTP.

• Also known as Krebs cycle to produce energy supply that occurs with oxidation pathways for fats and proteins. It is the bodies critical pathway to 2-3x of O2 from the body it provides intermediate energy in for heme in blood. Citric acid cycle- Krebs with CO2 + H2O C04- C6 cycle NADH FADH2 NAD NADH.

  • The enzymes found around the mitochondrial matrix increase closeness
  • Krebs Cycle
  • 2 carbon molecules, 4 to 6 of Citrate.
  • Catalytic roles such as citric acid

The Citric Acid Cycle

• Formation: Citrate
  • Acetly CoA combines to by the process of citrate.
• Isomerization: It turns
• Dehydration:
  • Remove +Add H20-
• Isolate-CO2 NADH released is isocitrate
• Cycle is for energy ATP
  • The citric acid and Malate can increase energy levels
  • Note at his point in time.
• Formation at 1-4:
• Aconitase and ketolurate to succinyl to GDP with h phosphate. The last of malates, hydrogenase the three products released by.