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Biochemistry I - Glycolysis - Page 1

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Biochemistry I - Glycolysis

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Biochemistry I - Glycolysis - Page 1 preview imageStudy GuideBiochemistry I–Glycolysis1. Glycolysis: ATP and NADH ProductionGlycolysis is a central metabolic pathway that breaks down glucose to produce energy. The energy-yielding phase of glycolysis involves a series of reactions between3-carbon moleculesthat generateATPandNADH, the cell’s key energy carriers.1.1Energy-Yielding Phase of GlycolysisThe first molecule that directly produces energy in this phase isglyceraldehyde-3-phosphate (G3P).This molecule reacts with:•ADP•Inorganic phosphate (Pᵢ)•NAD⁺The reaction is catalyzed by the enzymeglyceraldehyde-3-phosphate dehydrogenase.
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Biochemistry I - Glycolysis - Page 2 preview imageStudy Guide1.2Mechanism of the Glyceraldehyde-3-Phosphate ReactionThis reaction occurs in several important steps:1.Athiol groupfrom the enzyme attacks thealdehyde carbonof glyceraldehyde-3-phosphate.2.This forms athiohemiacetal intermediate.3.NAD⁺then acceptstwo electrons, becomingNADH.4.The aldehyde group of the substrate isoxidizedto the level of acarboxylic acid.5.Inorganic phosphate replaces the thiol group, forming1,3-bisphosphoglycerate.This step is crucial because it linksoxidationtoenergy capture.1.3ATP Formation by Substrate-Level PhosphorylationThe next step transfers a phosphate group from1,3-bisphosphoglycerateto ADP, producing ATP.This reaction is catalyzed byphosphoglycerate kinase.At this point:•Two ATP molecules wereused earlierin glycolysis•Two ATP molecules are nowregeneratedSo, theenergy balance returns to zero.1.4Rearrangement of the Carbon SkeletonNext,3-phosphoglycerateis converted into2-phosphoglycerateby the enzymephosphoglyceratemutase.This rearrangement prepares the molecule for the next energy-releasing step.
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Biochemistry I - Glycolysis - Page 3 preview imageStudy Guide1.5Formation of Phosphoenolpyruvate (PEP)The enzymeenolaseremoves:•A hydroxyl group from carbon 3•A hydrogen from carbon 2This dehydration reaction forms a double bond, producingphosphoenolpyruvate (PEP).1.6Why PEP Has High EnergyPEP is anenol, and enols are usually less stable thanketo compounds.PEP cannot easily convert to its keto form because of the phosphate group.When the phosphate is released:•PEP converts to theketo form (pyruvate)•Alarge amount of free energyis releasedThis energy is used to make ATP.1.7Formation of ATP and PyruvateThe enzymepyruvate kinasetransfers the phosphate from PEP to ADP, forming ATP andpyruvate.
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Biochemistry I - Glycolysis - Page 4 preview imageStudy GuideThis step:•Is highly favorable•Producestwo ATP molecules per glucose•Brings glycolysis into apositive energy balanceOverall Reaction of GlycolysisOverall equation of glycolysis:Glucose + 2 ADP + 2 Pᵢ+ 2 NAD⁺→ 2 Pyruvate + 2 ATP + 2 NADH + 2 H⁺+ 2 H₂O1.8Regeneration of NAD⁺: An Essential StepFor glycolysis to continue,NADH must be converted back to NAD⁺.This regeneration can occur in two ways:•Anaerobically(without oxygen)•Aerobically(with oxygen)1.9Anaerobic Regeneration: Lactate FormationIn animal muscle cells, NAD⁺is regenerated by transferring electrons topyruvate, forminglactate.This reaction is catalyzed bylactate dehydrogenase.
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Biochemistry I - Glycolysis - Page 5 preview imageStudy GuideThis reaction oxidizes NADH back to NAD⁺.This pathway is also used bylactic acid bacteriain yogurt production.1.10Anaerobic Regeneration: Ethanol FormationIn yeast and some microorganisms, pyruvate is converted intoethanolin two steps:1.Pyruvate decarboxylaseremoves CO₂, formingacetaldehyde2.Alcohol dehydrogenasereduces acetaldehyde to ethanol using NADHWhen ethanol is produced, the reaction of glycolysis becomes:
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Biochemistry I - Glycolysis - Page 6 preview imageStudy Guide1.11Biological and Practical Significance of Ethanol Formation•High sugar content in grapes leads to higher ethanol production•Ethanol concentrations above ~14% inhibit fermentation•This explains limits in natural wine productionAlcohol dehydrogenase also works in thereverse directionin humans:•Found in liver and intestines•Converts ethanol back to acetaldehyde•Acetaldehyde contributes to alcohol toxicity•Differences in intestinal enzyme levels explain why people respond differently to alcohol1.12Pyruvate to Acetyl-CoA: Entry into the TCA CycleUnder aerobic conditions, pyruvate isoxidatively decarboxylatedto formacetyl-CoA, which enterstheTCA cycle.This step connects glycolysis to oxidative metabolism.1.13Why Aerobic Metabolism Produces More ATPLouis Pasteur observed that oxygeninhibits fermentationin yeast.This is because:•Oxygen allowscomplete oxidationof glucose carbons to CO₂•More free energy is released•More ATP is produced than in fermentationHigh ATP levels theninhibit key glycolytic enzymes, slowing glycolysis.Key Takeaway•Glycolysis produces ATP and NADH from glucose•Substrate-level phosphorylation generates ATP directly•NAD⁺must be regenerated for glycolysis to continue•Anaerobic pathways form lactate or ethanol•Aerobic metabolism yields more ATP through complete oxidation
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Biochemistry I - Glycolysis - Page 7 preview imageStudy Guide2.Glycolysis RegulationCells carefully control glycolysis so that energy is producedwhen neededand conservedwhenenergy is plentiful. This control is achieved by regulating specific steps in the pathway rather thanevery reaction.2.1General Principle of Metabolic RegulationA key rule in metabolism is thatregulation usually occurs at the first committed stepof a pathway.A committed step is one after which the molecule hasonly one possible fate.However, glycolysis is connected to many other pathways. Some glycolytic intermediates are used tomake:•Lipids•Amino acids•NucleotidesBecause of this, glycolysis is regulated atmore than one step, allowing the cell to coordinate energyproduction with biosynthesis.2.2Why Multiple Control Points Are NeededFor example:•Dihydroxyacetone phosphate (DHAP)can be used to makeglycerol, which is needed forfat (triglyceride) synthesis.•In a well-fed state, animals store energy as fat.•Even when ATP demand is low, glucose must still be converted into triose phosphates.Therefore, glycolysis must sometimes continuepartially, without fully breaking glucose down to CO₂.
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Document Details

University
Texas A&M University
Course
Biochemistry I
Subject
Biochemistry

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