Question

A researcher has prepared a solution that contains all the enzymes and cofactors necessary for fatty acid biosynthesis from added acetyl-CoA and malonyl-CoA. a. She then adds \(\left[2-{ }^{2} \mathrm{H}\right]\) acetyl-CoA (labeled with deuterium, the heavy isotope of hydrogen) and an excess of unlabeled malonyl- CoA as substrates. How many deuterium atoms incorporate into every molecule of palmitate? What are their locations? Explain. b. In a separate experiment, the researcher adds unlabeled acetyl-CoA and \(\left[2-{ }^{2} \mathrm{H}\right]\) malonyl-CoA as substrates. How many deuterium atoms incorporate into every molecule of palmitate? What are their locations? Explain.

Short answer

a) 1 deuterium at the terminal methyl. b) 14 deuteriums at internal methylene groups.

Step-by-step solution

Understand the Problem Background

Fatty acid biosynthesis involves the addition of acetyl-CoA and malonyl-CoA building blocks to form long-chain fatty acids like palmitate (C16). Each cycle extends the growing chain by two carbons, incorporating one acetyl unit and one malonyl unit, with the malonyl unit providing two carbons after decarboxylation.

Determine Deuterium Incorporation from Labeled Acetyl-CoA

In part (a), the labeled acetyl-CoA has deuterium, specifically at the 2-position. The very first two carbons in palmitate come from acetyl-CoA, meaning the deuterium remains on the terminal methyl of palmitate. Since only one acetyl group is incorporated at the beginning, there is only one deuterium atom incorporated.

Determine Deuterium Incorporation in a Second Experiment

In part (b), when malonyl-CoA is labeled, every malonyl unit provides two carbon atoms to the chain after decarboxylation. The labeled 2-position of malonyl-CoA, after decarboxylation, becomes the new methyl group of the growing chain, thus, effectively isolating the deuterium. When building palmitate with 7 rounds of extension, 14 labeled malonyl units (one for each pair of carbons besides the initial acetyl group) will each ultimately contribute one deuterium to the chain.

Locations of Deuterium in Palmitate

For part (a), the deuterium is at the terminal methyl group of palmitate because it started with labeled acetyl-CoA. For part (b), because the deuterium is part of the labeled malonyl-CoA, each unit contributes deuterium at various internal methylene (CH2) groups on the fatty acid.

Key concepts

Deuterium Labeling

Deuterium labeling is a method used to track specific atoms within a molecule during chemical reactions. Deuterium is a stable isotope of hydrogen, often used because its presence does not alter the chemical properties of the molecule significantly. This technique is essential in metabolic studies as it helps researchers understand how a molecule is transformed throughout a reaction.

In this exercise, deuterium labeling allows us to identify where hydrogen atoms from acetyl-CoA and malonyl-CoA integrate into the structure of the synthesized palmitate. By using deuterium-labeled substrates, scientists can pinpoint the exact position of these atoms within the resultant molecule, providing insight into biological processes such as fatty acid biosynthesis.

• The labeled position on acetyl-CoA is crucial as it ultimately influences where deuterium appears in the palmitate.
• Tracking deuterium allows researchers to verify theoretical models of these biochemical pathways.
• This technique supports understanding of how enzymes and cofactors interact during fatty acid formation.

Acetyl-CoA

Acetyl-CoA is a central molecule in metabolism, acting as a building block in fatty acid biosynthesis. Comprised of an acetyl group linked to coenzyme A, it plays a pivotal role by providing two carbon atoms for incorporation into growing fatty acid chains.

In fatty acid biosynthesis, acetyl-CoA initiates the process by contributing its acetyl group to start the fatty acid chain. This is crucial in synthesizing molecules like palmitate, a common 16-carbon saturated fatty acid. Once acetyl-CoA donates its acetyl group, other molecules like malonyl-CoA further elongate the chain.

• The acetyl group from acetyl-CoA becomes the terminal carbon in palmitate.
• This process showcases the integral role of acetyl-CoA in energy storage and utilization.
• Acetyl-CoA acts as a common currency of carbon in metabolic pathways, underscoring its importance.

Malonyl-CoA

Malonyl-CoA is another crucial molecule in fatty acid biosynthesis, formed by the carboxylation of acetyl-CoA. It acts as a donor of two-carbon units (once decarboxylated) for the elongation of fatty acid chains.

During the synthesis of palmitate, malonyl-CoA is continually added to extend the growing fatty acid chain, each time contributing two carbon atoms. The enzyme acetyl-CoA carboxylase plays a vital role in converting acetyl-CoA to malonyl-CoA, which is then used iteratively to build longer fatty acids.

• Each malonyl-CoA molecule adds two carbons to the chain, a process repeated multiple times to synthesize palmitate.
• Malonyl-CoA serves as a key regulatory point in fatty acid metabolism.
• This molecule is central to the regulation and rate of fatty acid biosynthesis.

Palmitate Synthesis

Palmitate synthesis is the process of forming palmitate, a 16-carbon saturated fatty acid, through a cycle of reactions involving acetyl-CoA and malonyl-CoA. This synthesis occurs in the cytoplasm of cells and is primarily carried out by a complex of enzymes known collectively as fatty acid synthase.

Palmitate is formed through a series of repeated cycles, each consisting of four key reactions: condensation, reduction, dehydration, and a second reduction. Starting with an acetyl group from acetyl-CoA, a seven-step cycle of additions involving malonyl-CoA leads to the elongated chain until palmitate is formed.

• Palmitate synthesis is a fundamental process in lipid metabolism, important for creating energy reserves.
• The process involves multiple steps, each carefully regulated by enzymes and cofactors.
• Understanding this synthesis helps in comprehending broader metabolic dynamics within organisms.

As highlighted in the exercise, through deuterium labeling, students learn how the different positions of labeled and unlabeled substrates affect the final structure of palmitate, offering insights into enzyme function and molecular transformations in biosynthesis.