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Overview and Energetics of the Glycolytic Pathway

• Definition and General Pathway Function:

  • Glycolysis (historically referred to as the Embden-Meyerhof-Parnas pathway) is a central metabolic pathway responsible for the breakdown of the six-carbon monosaccharide glucose into two molecules of the three-carbon compound pyruvate.
  • The complete sequence encompasses ten distinct, enzyme-catalyzed cytosolic reactions partitioned into two main functional stages: the Energy Investment Phase (Fase 1: Gasto de energía) and the Energy Generation Phase (Fase 2: Generación de energía).

• Global Energetic Balance:

  • Energy Investment Phase: Consumes a total of $2\,\text{ATP}$ molecules per mole of glucose to phosphorylate intermediates and commit hexoses to metabolic breakdown.
  • Energy Generation Phase: Produces a total of $4\,\text{ATP}$ molecules via substrate-level phosphorylation and $2\,\text{NADH}$ equivalents of reducing power via oxidative phosphorylation reactions.
  • Net Energetic Yield:$\text{Net ATP} = 4\,\text{ATP (produced)} - 2\,\text{ATP (consumed)} = +2\,\text{ATP}$$\text{Net Reducing Power} = +2\,\text{NADH}$

Phase 1: Energy Investment Phase (Gasto de Energía)

• Overall Goal of Phase 1:

  • To activate hexose sugars through two successive, irreversible ATP-dependent phosphorylations, preventing the intermediates from diffusing across the plasma membrane and preparing the carbon skeleton for cleavage into two phosphorylated trioses.
  • Total energy input required across this phase is $2\,\text{ATP}$.

• Step 1: Phosphorylation of Glucose (Glucosa):

  • Substrate: Glucose (Glucosa).
  • Enzymes: Hexokinase (Hexoquinasa) or Glucokinase (Glucoquinasa).
    • Hexokinase is present in most non-hepatic tissues, possesses a low $K_m$ for glucose (high affinity), and is inhibited by its product, glucose-6-phosphate.
    • Glucokinase (Hexokinase IV) is found predominantly in hepatocytes and pancreatic beta cells, exhibits a significantly higher $K_m$ (lower affinity), is not inhibited by glucose-6-phosphate, and operates under conditions of high blood glucose availability.
  • Reaction Mechanism and Energetics: Transfer of a terminal phosphoryl group from ATP to the C-6 hydroxyl group of glucose:         $\text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-Phosphate} + \text{ADP} + \text{H}^+$
  • Energy Investment: Consumes $1\,\text{ATP}$.
  • Product: Glucose 6-phosphate (Glucosa 6 fosfato).

• Step 2: Isomerization of Glucose 6-Phosphate:

  • Substrate: Glucose 6-phosphate (Glucosa 6 fosfato).
  • Enzyme: Phosphoglucose isomerase / Phosphoglucoisomerase (Fosfoglucosa isomerasa).
  • Reaction Type: Reversible isomerization of an aldohexose to a ketohexose:         $\text{Glucose-6-Phosphate} \rightleftharpoons \text{Fructose-6-Phosphate}$
  • Product: Fructose 6-phosphate (Fructosa 6 fosfato).
  • Biochemical Logic: Moving the carbonyl oxygen from carbon-1 to carbon-2 creates a free primary alcohol at C-1, which is necessary for the subsequent phosphorylation step and ensures a symmetrical cleavage in later steps.

• Step 3: Phosphorylation of Fructose 6-Phosphate:

  • Substrate: Fructose 6-phosphate (Fructosa 6 fosfato).
  • Enzyme: Phosphofructokinase-1 (Fosfofructoquinasa 1 / PFK-1).
  • Reaction Type: Irreversible phosphorylation of the C-1 hydroxyl group:         $\text{Fructose-6-Phosphate} + \text{ATP} \rightarrow \text{Fructose-1,6-Bisphosphate} + \text{ADP} + \text{H}^+$
  • Energy Investment: Consumes $1\,\text{ATP}$.
  • Product: Fructose 1,6-bisphosphate (Fructosa 1,6 bifosfato).
  • Pathway Significance: This constitutes the primary committed and rate-limiting regulatory step of glycolysis. PFK-1 is tightly regulated by allosteric effectors, being inhibited by high levels of ATP and citrate, and activated by AMP and fructose-2,6-bisphosphate.

• Step 4: Aldol Cleavage of Fructose 1,6-Bisphosphate:

  • Substrate: Fructose 1,6-bisphosphate (Fructosa 1,6 bifosfato).
  • Enzyme: Aldolase (Aldolasa / Fructose-bisphosphate aldolase).
  • Reaction Type: Reversible retro-aldol condensation cleaving a six-carbon bisphosphorylated sugar into two interconvertible three-carbon phosphorylated sugars (triose phosphates):         $\text{Fructose-1,6-Bisphosphate} \rightleftharpoons \text{Glyceraldehyde 3-Phosphate} + \text{Dihydroxyacetone Phosphate}$
  • Products: Glyceraldehyde 3-phosphate (Gliceraldehído 3 fosfato) and Dihydroxyacetone phosphate (DHAP).
  • Interconversion via Triose Phosphate Isomerase: Dihydroxyacetone phosphate is rapidly and reversibly isomerized into a second molecule of Glyceraldehyde 3-phosphate by triose phosphate isomerase, allowing both triose halves of glucose to proceed through the remainder of the pathway simultaneously.

Phase 2: Energy Generation Phase (Generación de Energía)

• Overall Goal of Phase 2:

  • To oxidize and dephosphorylate the two triose phosphate units, capturing the released free energy in the form of high-energy phosphate bonds ($\text{ATP}$) and reducing equivalents ($\text{NADH}$).
  • Because each molecule of glucose yields $2$ units of triose phosphate, all chemical and energetic yields in Phase 2 occur in duplicate (multiplied by a factor of $2$ per original glucose molecule).
  • Total gross yields for this phase: $4\,\text{ATP}$ and $2\,\text{NADH}$.

• Step 5 (Pathway Step 6): Oxidation of Glyceraldehyde 3-Phosphate:

  • Substrate: Glyceraldehyde 3-phosphate (Gliceraldehído 3 fosfato).
  • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase (Deshidrogenasa).
  • Reaction Type: Concurrent oxidation and phosphorylation using inorganic phosphate ($\text{P}_i$) rather than ATP:         $\text{Glyceraldehyde 3-Phosphate} + \text{NAD}^+ + \text{P}_i \rightleftharpoons \text{1,3-Bisphosphoglycerate} + \text{NADH} + \text{H}^+$
  • Reducing Power Produced: Generates $1\,\text{NADH}$ per triose ($2\,\text{NADH}$ produced per original glucose molecule).
  • Product: 1,3-Bisphosphoglycerate (1,3 Bifosfoglicerato).
  • Significance: Produces an acyl phosphate with a high standard free energy of hydrolysis ($\Delta G^{\circ\prime} \approx -49.3\,\text{kJ}\,\text{mol}^{-1}$), conserving the oxidative energy released from the aldehyde group.

• Step 6 (Pathway Step 7): First Substrate-Level Phosphorylation:

  • Substrate: 1,3-Bisphosphoglycerate (1,3 Bifosfoglicerato).
  • Enzyme: Phosphoglycerate kinase (Fosfoglicerato quinasa).
  • Reaction Type: Reversible phosphoryl group transfer from the high-energy carboxyl group of 1,3-BPG to ADP:         $\text{1,3-Bisphosphoglycerate} + \text{ADP} \rightleftharpoons \text{3-Phosphoglycerate} + \text{ATP}$
  • Energy Generated: Generates $1\,\text{ATP}$ per triose ($2\,\text{ATP}$ produced per original glucose molecule).
  • Product: 3-Phosphoglycerate / Glycerate 3-phosphate (*Glicerato 3