Part 1

Lactic Acid Fermentation

Lactic acid fermentation occurs under anaerobic conditions, such as those found in certain bacteria and in working muscle tissues. This process involves the reduction of pyruvate to lactate, during which NADH is oxidized to NAD+. The enzyme responsible for this reaction is lactate dehydrogenase, which facilitates the transfer of hydrogen.

Alcoholic Fermentation

Alcoholic fermentation is characterized by the release of carbon dioxide (CO2) through the action of the enzyme pyruvate decarboxylase. This process generates acetaldehyde as a product, and it requires thiamine pyrophosphate as a coenzyme. Subsequently, acetaldehyde is reduced to ethanol, with the electrons for this reduction derived from NADH, leading to the production of NAD+. Yeast cells commonly perform this fermentation, where the CO2 released causes bread dough to rise by metabolizing glucose found in the flour. Additionally, this metabolic process is responsible for the generation of alcoholic beverages and also serves as a source of biofuel.

Thiamine Pyrophosphate Function in Pyruvate Decarboxylase

Thiamine pyrophosphate (TPP) functions as an electron sink that stabilizes the carbanion intermediate during the reaction catalyzed by pyruvate decarboxylase. The aldehyde group carried by TPP plays a critical role in this stabilization and is essential for the overall reaction to proceed, making TPP (derived from vitamin B1) vital in the energy conversion processes.

Cofactor List

Enzyme Cofactors

A detailed table (TABLE 6.2) lists various enzyme cofactors along with their respective enzymes, grouped into coenzymes:

  • Thiamine pyrophosphate (TPP): Enzymes include pyruvate decarboxylase and pyruvate dehydrogenase.

  • Flavin adenine dinucleotide (FAD): Functions in several enzymes.

  • Nicotinamide adenine dinucleotide (NAD+): Serves in many oxidation and reduction reactions.

  • Pyridoxal phosphate (PLP): Engaged in amino acid transformations.

Carrier Molecules in Activated Form

  • ATP: It carries phosphoryl groups.

  • NADH and NADPH: Carries electrons.

  • FADH2: Another electron carrier.

  • Other important groups carried include carbonic anhydrase, acetyl CoA, biotin, and various metal complexes required as cofactors.

Entry of Other Monosaccharides into Glycolysis

Monosaccharides such as fructose, galactose, and mannose can enter glycolysis through distinct pathways. These sugars are common in human diets, with sucrose containing fructose and lactose containing galactose. Multiple enzymatic reactions convert these monosaccharides into glycolytic intermediates. Additionally, sugars may be carried as part of nucleotide sugars.

Fructose Metabolism

The metabolism of fructose involves a specific pathway comprising four steps. It serves as a precursor for lipid synthesis and fits into the reaction class known as lyases.

Regulation of Glycolysis

The regulation of glycolysis is primarily centered around phosphofructokinase (PFK), which is considered a key regulatory enzyme within the glycolytic pathway.

Reactions of Glycolysis

Energy Changes and Enzymes

A table (TABLE 16.1) highlights various glycolytic reactions, the free energy (AG) changes at each step, and the associated enzymes:

  • Step 1: Glucose + ATP ⇌ Glucose 6-Phosphate + ADP + H⁺; Enzyme: Hexokinase; AG = -33.5 kcal mol⁻¹ / -140 kJ mol⁻¹.

  • Step 2: Glucose 6-Phosphate ⇌ Fructose 6-Phosphate; Enzyme: Phosphoglucose isomerase; AG = +2.7 kcal mol⁻¹ / +11.3 kJ mol⁻¹.

  • Step 3: Fructose 6-Phosphate + ATP ⇌ Fructose 1,6-Bisphosphate + ADP + H⁺; Enzyme: Phosphofructokinase; AG = -22.2 kcal mol⁻¹ / -93 kJ mol⁻¹.

  • Step 4: Fructose 1,6-Bisphosphate ⇌ Dihydroxyacetone Phosphate + Glyceraldehyde 3-Phosphate; Enzyme: Aldolase; AG = +23.8 kcal mol⁻¹ / +99 kJ mol⁻¹.

  • Subsequent steps involve further transformations of 3-Phosphoglycerate and pyruvate, with detailed energy changes and reaction types documented for each enzymatic step.

Regulation of Glycolysis in Muscle

Phosphofructokinase (PFK) is crucial for the regulation of glycolysis, with ATP levels acting as a control point. PFK acts as an allosteric regulator, where higher ATP concentrations inhibit PFK activity due to its dual ATP binding sites—one for catalytic function and the other for regulation. When ATP levels increase, glycolysis slows down, thus operating through feedback inhibition, as ATP is also a product of glycolysis. Conversely, AMP stimulates PFK, competing with ATP for the regulatory binding site, thus ensuring glycolysis continues when energy levels are low. Consequently, PFK activity is modulated by the ATP/AMP ratio.

Hexokinase Regulation

Hexokinase is negatively inhibited by its product, Glucose 6-Phosphate (Glc-6-P). If PFK inhibition occurs, the accumulation of Fructose 6-Phosphate (Frc-6-P) results in the build-up of Glc-6-P. The equilibrium constant for the phosphoglucose isomerase reaction is close to zero, which results in significant inhibition of hexokinase, thereby preventing glucose phosphorylation. This mechanism prevents glucose from entering glycolysis, allowing it to remain in the bloodstream for uptake by other tissues.

Regulation of Glycolysis in Liver

In the liver, glycolysis plays a critical role in maintaining blood glucose levels. PFK is activated when glucose is abundant; the regulatory molecule fructose-2,6-Bisphosphate (Frc-2,6-BP) is produced under high glucose conditions from Frc-6-P through the action of PFK2—a separate enzyme not directly involved in glycolysis. Frc-2,6-BP serves as an allosteric activator of PFK, enhancing its affinity for Frc-6-P and blocking inhibition by ATP. This regulatory mechanism ensures that when glucose levels are high, glycolysis is promoted.

Isozymes and Tissue-Specific Regulation

Glycolysis regulation varies across different tissues through the expression of various isozymes. The liver expresses an isozyme of hexokinase called glucokinase, which has a much higher Km for glucose (50x higher). Therefore, in low glucose conditions, glucokinase does not initiate glycolysis within the liver cells, allowing glucose to be prioritized for other tissues, such as muscles and the brain. Additionally, the liver has a different glucose transporter that requires high glucose concentrations for glucose entry into cells, unlike other tissues.

Pyruvate Kinase Regulation

In both muscle and liver contexts, pyruvate kinase regulation is significant. In the muscle, pyruvate kinase is inhibited by ATP and activated by Fructose 1,6-Bisphosphate. In the liver, pyruvate kinase is also regulated by protein phosphorylation, a mechanism activated during low glucose conditions which results in its inactivation, thereby slowing glycolysis to prioritize the use of glucose in other tissues. This regulation is facilitated by cAMP-dependent protein kinase (PKA).

Insulin Release Mechanism

High glucose concentrations stimulate insulin release. Insulin is a peptide hormone composed of 51 amino acids, structured by two polypeptide chains linked by disulfide bonds. Under elevated glucose levels, glucose enters pancreatic cells through ligand-gated and facilitated transporters, ultimately leading to the secretion of insulin, which is critical for glucose homeostasis in the body.