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Lactate Fermentation
Lactate produced in muscle cells is transported via blood to the liver.
In the liver, lactate is converted to pyruvate through a reduction process.
Pyruvate undergoes cellular respiration, leading to the production of carbon dioxide (CO₂) and water (H₂O).
Mammals cannot convert pyruvate to ethanol and CO₂ due to the absence of the enzyme pyruvate decarboxylase.
The enzyme lactate dehydrogenase is responsible for the reduction of pyruvate to lactate, and this reaction is reversible.
Alcoholic Fermentation
Alcoholic fermentation is utilized by various organisms, including yeast and bacteria, to regenerate NAD⁺ under anaerobic conditions.
This fermentation process results in the production of ethanol and CO₂ from pyruvate.
Mechanism of Alcoholic Fermentation
Under anaerobic conditions, yeast convert pyruvate into ethanol and CO₂, oxidizing NADH back to NAD⁺.
Step 1: Decarboxylation of pyruvate by the enzyme pyruvate decarboxylase to produce acetaldehyde and CO₂.
Step 2: Dehydrogenation by alcohol dehydrogenase to yield ethanol, during which NADH is oxidized to NAD⁺.
It is noteworthy that human cells do not produce the enzyme pyruvate decarboxylase.
Yoghurt Production
Lactic acid bacteria play a crucial role in many milk-based fermented products, such as yoghurt.
Yoghurt results from the fermentation of milk and is believed to confer health benefits by containing active cultures of lactic acid bacteria.
It is plausible that these bacteria can help inhibit pathogenic strains like E. coli and Salmonella typhimurium.
Yoghurt-Making Process
Conditioning:
Lowering milk's water content through evaporation.
Heating milk at temperatures of 86 – 96°C for a duration of 30 – 60 minutes to denature enzymes and eliminate competing microbes.
Adding Lactic Acid Bacterial Culture:
Cool milk to 45°C and incorporate Streptococcus thermophilus and Lactobacillus bulgaricus.
Lactate Fermentation:
The bacterial culture ferments the milk sugars into lactic acid, occurring at 45°C until the desired acidity level is achieved within 3 – 5 hours.
Finishing:
Flavourings, fruits, or preservatives are added, and the final product is stored at 4°C until consumed.
Cheese-Making Process
Curd Formation:
Milk is pasteurized to eliminate bacteria and is then fermented with starter lactic acid bacterial culture.
Rennet, an enzyme, is added to facilitate curd formation.
Curd Concentration:
Whey, the liquid residual from curd formation, is removed.
Fresh cheeses do not require concentration, while soft cheeses are ripened using fungi or molds, and hard cheeses undergo a cooking process.
Ripening:
Curd is pressed into molds, salted, and left to ripen for periods ranging from weeks to years.
Beer Production
Beer is created through the alcoholic fermentation of barley and hops (a flower part) by yeast.
Generally, starch from barley is converted into glucose, which is then fermented into ethanol and CO₂ by yeast.
The final product is aged and packaged for distribution and consumption.
Beer-Making Process
Malt Preparation (Barley Grain):
Barley is soaked in water to facilitate germination, which produces amylases (converting starch to sugar) and proteases (converting proteins to amino acids).
Mashing (Mixture of Malt and Hot Water):
The temperature is raised to 75°C to deactivate enzymes and solubilize starch.
Boiling with Hops:
This step concentrates and sterilizes the mixture.
Alcoholic Fermentation:
Brewer’s yeast Saccharomyces carlsbergensis is introduced, fermenting at temperatures between 3.3 – 14°C for 8-14 days.
Aging:
The product is kept at 0°C for several weeks to months.
Finishing:
Involves filtering, pasteurization, carbonation, clarification, and bottling.
Bread-Making Overview
Bread is produced via the alcoholic fermentation of sugar (glucose) into ethanol and CO₂ by the yeast Saccharomyces cerevisiae.
Ingredients (flour, sugar, milk, yeast, etc.) are mixed and allowed to incubate at 26 – 28°C for 1-2 hours.
The CO₂ generated causes the bread to rise and develop a porous texture.
Critical parameters in bread-making are the proportion of ingredients and the incubation temperature.
Summary of Fermentation
The primary aim of fermentation is to replenish NAD⁺.
Energy derived from fermentation processes includes lactate and alcoholic fermentation.
Applications of fermentation techniques are evident in the production of yoghurt, cheese, beer, and bread.
Pyruvate Metabolism
As a metabolic intermediate, pyruvate can undergo several fates based on oxygen availability:
Under anaerobic conditions:
Fermentation to ethanol and CO₂ in yeast.
Conversion to lactate in muscle cells and some microorganisms.
Under aerobic conditions:
Conversion to acetyl-CoA, followed by entry into the citric acid cycle (TCA cycle, Krebs Cycle).
Overview of TCA Cycle
The tricarboxylic acid cycle (TCA cycle), or Krebs cycle (also known as citric acid cycle), occurs in the mitochondrial matrix and is crucial for aerobic metabolism.
It features two stages:
Conversion of Pyruvate to Acetyl CoA:
Oxidation of Acetyl CoA through the TCA Cycle:
The TCA cycle acts as a hub for the catabolism of carbohydrates, lipids, and amino acids and is also amphobolic, meaning it serves both catabolic and anabolic functions.
Stage 1: Pyruvate to Acetyl CoA
Pyruvate is transported to the mitochondrial matrix through a transporter called pyruvate translocase.
Pyruvate is converted into acetyl-CoA by the pyruvate dehydrogenase complex (PDC) in a process that produces NADH and releases CO₂.
The reaction is irreversible.
The PDC comprises:
E1: Pyruvate dehydrogenase (which catalyzes the conversion of pyruvate to acetyl, releasing CO₂).
E2: Dihydrolipoyl transacetylase (attaches CoA to acetyl).
E3: Dihydrolipoyl dehydrogenase (reduces NAD⁺ to NADH).
Keys to the regulation of PDC involve biochemical factors such as NAD⁺ and CoA-SH serving as substrates and NADH and acetyl-CoA acting as inhibitors.
Stage 2: Oxidation of Acetyl CoA
Acetyl-CoA joins oxaloacetate to form citrate, initiating a cyclic series of reactions known as the TCA cycle.
During this cycle, energy is derived in the form of:
1 ATP molecule
3 NADH molecules
1 FADH₂ molecule
2 CO₂ molecules
As each glucose molecule yields two acetyl-CoA, this process effectively doubles, with each set of reactions occurring twice for each glucose molecule.
Cellular Respiration: Electron Transport Chain and Oxidative Phosphorylation
Overview of Aerobic Respiration:
Comprises glycolysis, pyruvate oxidation (transition reaction to form acetyl-CoA), the TCA cycle, and the electron transport chain (ETC).
Energy Generation:
During complete aerobic respiration, each glucose molecule is oxidized to produce CO₂, water, and energy.
The equations detailing this process, including energy yields via glycolysis and oxidative phosphorylation, reflect the catabolism of glucose into the energy currency ATP.
Mechanism of ATP Production via the Electron Transport Chain
The electron transport chain is integral to aerobic respiration and is located in the inner mitochondrial membrane where it utilizes transported electrons from NADH and FADH₂.
As electrons pass through the chain via a series of protein complexes, energy is harnessed to create a proton gradient across the mitochondrial membrane.
The energy stored in this gradient drives ATP synthesis as protons flow back into the mitochondrial matrix through ATP synthase, allowing ADP and inorganic phosphate (Pi) to form ATP.
Summary of ATP Yield
The total yield of ATP from a single glucose molecule oxidatively phosphorylated can substantiate net gains between 36-38 ATP via glycolysis, TCA, and oxidative phosphorylation.
Efficiency of complete oxidation of glucose approximately stands at 32-39%.
Glycogen Metabolism
Glycogen serves as a branched polymer of glucose, vital for energy storage in organisms.
Synthesis and breakdown occur mainly in the liver and muscle, regulated through enzyme action and hormonal influences (insulin and glucagon).
Gluconeogenesis functions to synthesize glucose from non-carbohydrate sources during periods of scarcity.
Enzymatic Regulation of Key Metabolic Pathways
Metabolic pathways including glycolysis, gluconeogenesis, and glycogen metabolism require stringent regulatory mechanisms to maintain homeostasis and energy balance within biological systems.
Glycogen Synthesis and Regulation
Glycogenin initiates glycogen synthesis by catalyzing the addition of glucose molecules to itself, forming a primer.
Glycogen synthase and branching enzymes further extend this primer into a complex structure ready for energy mobilization.
Glycogen metabolism is tightly controlled by allosteric effects and hormonal signals that ensure appropriate responses to cellular energy demands.
Glycogen Breakdown Mechanism
Glycogen is degraded into glucose-1-phosphate by glycogen phosphorylase.
The involvement of debranching enzymes ensures that the full glycogen stores can be mobilized effectively.
Liver and muscle tissue exhibit distinct roles in glycogen utilization, impacting overall glucose availability in the bloodstream.
Gluconeogenesis
Gluconeogenesis is a critical metabolic pathway allowing glucose synthesis from non-carbohydrate precursors.
This pathway operates mainly in the liver and is essential under conditions of fasting or intensive exercise.
Notably, gluconeogenesis employs distinct yet reciprocal enzymatic mechanisms compared to glycolysis, indicating a sophisticated regulation to prevent futile cycling between these two pathways.
Conclusion
Comprehensive understanding of metabolic pathways is essential, encapsulating the interrelated roles of glycolysis, TCA cycle, electron transport chain, glycogen metabolism, and gluconeogenesis in maintaining energy homeostasis and metabolic flexibility.