Problem 3 - Biochemistry: Concepts and Connections

Question

Calculate the number of ATPs generated by the complete metabolic oxidation of tripalmitin (tripalmitoylglycerol). Hydrolysis of the triacylglycerol occurs at the cell surface. Consider the energy yield from catabolism of glycerol, as well as from the fatty acids. Calculate the ATP yield per carbon atom oxidized, and compare it with the energy yield from glucose.

Step-by-Step Solution

Answer

Tripalmitin yields 338 ATPs and about 6.63 ATPs per carbon atom, which is higher than the yield from glucose.

Step 1: Analyze the Structure of Tripalmitin

Tripalmitin, also known as tripalmitoylglycerol, contains one glycerol molecule and three palmitate (C16:0) fatty acid chains. Each palmitate fatty acid chain has 16 carbon atoms.

Step 2: Calculate ATP from Glycerol

Glycerol is converted to glyceraldehyde 3-phosphate and enters glycolysis, ultimately producing approximately 20 ATPs, considering the glycolytic pathway and oxidative phosphorylation processes that follow.

Step 3: Beta-Oxidation of Palmitate Fatty Acids

Each C16 fatty acid undergoes beta-oxidation, which includes 7 cycles breaking down the chain. Each cycle yields 1 FADH2 (1.5 ATPs), 1 NADH (2.5 ATPs), and produces an acetyl-CoA (10 ATPs from its oxidation in the TCA cycle). Therefore, with 7 rounds of beta-oxidation and 8 acetyl-CoAs per palmitate, we calculate the total ATP output for one palmitate as 106 ATPs.

Step 4: Total ATP from Tripalmitin

Combining the ATP from 3 palmitate chains (3 x 106 ATP) and 20 ATP from glycerol, the total ATP yield from one molecule of tripalmitin is 338 ATPs.

Step 5: Calculate the ATP Yield Per Carbon Atom

Tripalmitin has a total of 51 carbon atoms (16 from each of the three palmitate chains plus 3 from glycerol). To find the ATP yield per carbon, divide the total ATPs (338) by the total number of carbons (51), resulting in approximately 6.63 ATPs per carbon atom.

Step 6: Compare ATP Yield to Glucose

Glucose (C6H12O6) yields approximately 30-32 ATPs per molecule, which translates to about 5-5.33 ATPs per carbon atom, meaning the ATP yield per carbon for tripalmitin (6.63) is higher than glucose.

Key Concepts

Beta-Oxidation

Beta-oxidation is a critical metabolic process by which fatty acids are broken down in mitochondria to produce energy. Specifically, during beta-oxidation, each fatty acid is sequentially cleaved into two-carbon units, creating acetyl-CoA, which then enters the citric acid cycle (also known as the Kreb's cycle) for further ATP production. For a saturated fatty acid like palmitate (16 carbons long), beta-oxidation occurs through a series of cycles, each of which generates one FADH2 and one NADH, alongside an acetyl-CoA. Each FADH2 contributes about 1.5 ATPs during electron transport chain processes, while each NADH contributes approximately 2.5 ATPs. Thus, the complete breakdown of a 16-carbon palmitate molecule through beta-oxidation leads to the production of 7 FADH2, 7 NADH, and subsequently, 8 acetyl-CoA. This ultimately results in significant ATP production as acetyl-CoA enters the TCA cycle, where each acetyl-CoA can yield around 10 ATPs. Collectively, a single palmitate fatty acid can contribute about 106 ATPs to cellular metabolism.

Tripalmitin Structure

Tripalmitin, also known as triacylglycerol or triglyceride, is a common form of fat storage in animals. Its structure is composed of a glycerol backbone esterified with three palmitate fatty acids. Each palmitate consists of 16 carbon atoms, making tripalmitin a molecule densely packed with energy. In biochemical processes, tripalmitin is first hydrolyzed into its components: one glycerol and three palmitate fatty acids. This hydrolysis step is an essential part of the digestion and mobilization of stored fats. Glycerol proceeds into the glycolysis pathway after conversion to glyceraldehyde 3-phosphate, while the palmitate undergoes beta-oxidation, a process that produces substantial amounts of ATP. The dense carbon structure in each of the palmitate chains is what allows triglycerides like tripalmitin to store more energy per molecule compared to carbohydrates such as glucose.

Energy Yield Comparison

The energy yield per carbon atom is a significant metric to understand the efficiency of different macronutrients in ATP production. In the context of tripalmitin, its metabolic oxidation generates a total of 338 ATPs from its 51 carbon atoms, yielding approximately 6.63 ATP per carbon atom. This is higher compared to glucose, a primary energy source, which produces about 30-32 ATP from its 6 carbon atoms, translating to roughly 5-5.33 ATPs per carbon. This difference highlights the efficiency of fats as an energy reservoir. The carbon-rich structure of fatty acids allows for a higher energy yield compared to carbohydrates, making them a dense form of energy storage in living organisms. This also explains why energy extraction from fats is a crucial process during periods of low carbohydrate availability, such as fasting or low-carbohydrate diets. Understanding this comparison helps in elucidating why metabolic pathways prioritize different macronutrients under varying physiological conditions.