(Integrates with Chapters 19 and \(20 .\) ) Acetate produced in ethanol metabolism can be transformed into acetyl-CoA by the acetyl thiokinase reaction: $$\text { Acetate }+\mathrm{ATP}+\mathrm{CoASH} \longrightarrow \text { acetyl-CoA }+\mathrm{AMP}+\mathrm{PP}_{\mathrm{i}}$$ Acetyl-CoA then can enter the citric acid cycle and undergo oxidation to \(2 \mathrm{CO}_{2}\). How many ATP equivalents can be generated in a liver cell from the oxidation of one molecule of ethanol to \(2 \mathrm{CO}_{2}\) by this route, assuming oxidative phosphorylation is part of the process? (Assume all reactions prior to acetyl-CoA entering the citric acid cycle occur outside the mitochondrion.) Per carbon atom, which is a better metabolic fuel, ethanol or glucose? That is, how many ATP equivalents per carbon atom are generated by combustion of glucose versus ethanol to \(\mathrm{CO}_{2}\) ?

Step by step solution

01

Determine ATP generated from the oxidation of ethanol

The oxidation of one molecule of ethanol yield two molecules of acetate. Each acetate can be transformed into acetyl-CoA, which when enters the citric acid cycle produces 12 ATP. Two acetyl-CoA molecules are thus expected, hence, the total ATPs from the ethanol oxidation is \(2 * 12 = 24 ATP\). Note that the initial conversion of acetate to acetyl-CoA consumes one molecule of ATP for each acetate. Therefore, the Net ATP produced from ethanol is 22.

02

Determine ATP generated from the oxidation of glucose

One molecule of glucose (C6H12O6) gets oxidized to produce six molecules of \(CO_2\) which generates \(6 * 4 = 24 ATP\) theoretically. Since the initial steps of glycolysis consume 2 ATP, net ATP produced from the glucose is 22.

03

Compare the metabolic efficiency of glucose and ethanol

To compare the efficiency per carbon atom of both substances, we divide the number of ATPs generated by the number of carbon atoms. For glucose, it's \(\frac{22 ATP}{6 C} = 3.67 ATP/C\), for ethanol, it's \(\frac{22 ATP}{2 C} = 11 ATP/C\). Hence, ethanol is a better metabolic fuel per carbon atom compared to glucose.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Acetyl-CoA Formation

Understanding acetyl-CoA formation is critical when examining ethanol metabolism. Let's start with an essential process that occurs after ethanol consumption. The body converts ethanol into acetate. This acetate then combines with ATP (adenosine triphosphate), the primary energy currency of the cell, and CoASH (coenzyme A) to form acetyl-CoA, a pivotal molecule in metabolism.

In simple terms, the reaction is like exchanging currency; acetate trades with ATP and CoASH to 'purchase' a new form that's more usable for the body, which in this case is acetyl-CoA. This process consumes ATP, indicating an investment into the conversion that's needed before acetyl-CoA can enter the next metabolic stage. It's worthwhile to note that this step is akin to setting the stage before the main act of energy production begins.

Citric Acid Cycle

Once acetyl-CoA is formed, it's time for it to enter the 'metabolic stage show' known as the citric acid cycle, or Krebs cycle. Imagine this cycle as a complex machine in a factory. Acetyl-CoA is the raw material that goes in, and through a series of enzyme-controlled steps, it gets dismantled. The dismantling process releases carbon dioxide, but more importantly for energy production, it generates high-energy carriers like NADH and FADH₂.

The citric acid cycle itself doesn't produce much ATP directly. However, it sets up the energy-rich molecules that are vital for the next phase, where the real power generation happens. Think of the citric acid cycle as a setup for the grand finale of energy production.

Oxidative Phosphorylation

Powering the Cellular Grid

With the stage set by the citric acid cycle, the performance can reach its climax through oxidative phosphorylation. This is the main power-generating step in the cell's energy production process. It's here that NADH and FADH₂, the energetic molecules produced earlier, work to power the production of ATP.

The electron transport chain within the mitochondria takes the electrons from NADH and FADH₂ and passes them along a series of 'electron accepting' complexes. As electrons flow through, they power the pumping of protons across the mitochondrial membrane, creating an energy store known as a proton gradient. This gradient is like a dam holding back water. The potential energy stored in this proton gradient is then converted into ATP as protons flow back through an enzyme called ATP synthase.

ATP Generation

In the grand scheme of cellular energy production, ATP generation is the final, most crucial step. The process involves the enzyme ATP synthase within the mitochondrial membrane, which acts similarly to a turbine in a hydroelectric power plant, converting flow energy into usable power — in our case, capturing the energy from the proton gradient into forming ATP.

ATP is the main source of energy for many cellular processes, from muscle contraction to nerve impulse propagation. Our bodies generate and use approximately our body weight in ATP every day, though only a small amount is present in the body at any moment. This incredible turnover underlines the efficiency and importance of ATP generation in our daily functioning.

Metabolic Fuel Efficiency

Assessing the Energy Yield

When considering the metabolic fuel efficiency of substances like glucose and ethanol, we're basically asking which gives us more bang for our buck. Measuring in 'ATP equivalents', we can compare how many ATP molecules can be generated from the breakdown of these fuels.

As demonstrated in the exercise provided, ethanol appears to be a better metabolic fuel per carbon atom because it produces more ATP per carbon when fully oxidized compared to glucose. It's a bit like comparing two types of fuel for cars; ethanol here would give you more miles per gallon (energy per carbon atom) than glucose if we assume that all conditions in the body are perfect for this comparison to hold true.