In this lesson, you’ll learn what acetyl coenzyme A is by discovering its structure and how it’s synthesized or formed by the cell. You will further uncover a bit about the function of coenzyme A.
Acetyl Coenzyme A
Understanding the structure of acetyl coenzyme A (acetyl CoA) is a bit like understanding the structure of Frankenstein’s monster.
Part of acetyl CoA – coenzyme A (CoA) – is quite the monster itself, which is why scientists often use its abbreviation (CoA) when writing out structures that involve CoA (and there are many).Like Frankenstein’s monster, which was composed of different corpse parts, coenzyme A (CoA) is composed of different pieces of various molecules and chemical groups. Once we make sense of CoA, understanding the acetyl part of acetyl CoA is relatively simple.
The monster coenzyme A (CoA) consists of three parts:
- The body: vitamin B5
- The head: a form of ADP
- The tail: beta-mercaptoethylamine
The main body of the beast is composed of vitamin B5- pantothenate; you eat vitamins so you may be able to remember the name by thinking panto then ‘ate,’ or pantothenic acid.The head of the beast is a form of adenosine diphosphate, a nucleotide. Just as your head has different parts such as the eyes, nose, and mouth, a nucleotide is composed of different groups: a base (adenine – like what you find in DNA), a sugar (ribose), and phosphate (PO4).Now that you’re familiar with coenzyme A’s head and body, it’s time to talk about the tail: beta-mercaptoethylamine.
It’s quite the mouthful, but the important thing to remember about the tail is the sulfhydryl group (SH-). The sulfhydryl group is how other groups and molecules attach themselves to CoA.Coenzyme A is exactly what the name implies, a coenzyme. It helps enzymes function and acts as sort of a hanger for other molecules. You can hang all kinds of molecules onto the tail of CoA as well as add groups to make fatty acids.
Structure of Acetyl CoA
The head (nucleotide), the body (vitamin B5), and the tail (beta-merceptoehtylamine) make up the whole beast: CoA. Now we’ll talk about the acetyl part.
An acetyl group is simply a carbon with three hydrogens bonded to another carbon that is bonded to another group and double bonded to an oxygen (CH3-CO-). It sounds more complicated than it looks.When an acetyl group is added to the tail of CoA, the whole molecule becomes known as acetyl coenzyme A (acetyl CoA). Therefore, acetyl CoA’s structure is simply CoA with an acetyl group attached to the tail (or the sulfhydryl group).
Synthesis of Acetyl CoA
One important molecule used by the cell is pyruvate which is formed when sugar (glucose) is broken down. Pyruvate looks like an acetyl group combined with carbon dioxide (CO2).Pyruvate can be cannibalized by the cell to make up Frankenstein’s monster, or rather the cellular beast acetyl CoA. The cell uses a scalpel – or, ahem, the enzyme pyruvate dehydrogenase – to cleave off CO2 and add the acetyl group to the carrier molecule. The acetyl group is then transferred from the carrier molecule onto a CoA that’s hanging around the cell after being made by a different process we won’t get into here.Some amino acids can be used to synthesize acetyl CoA as well. Alanine, serine, glycine, cystine, and threonine all form acetyl CoA through the pyruvate pathway.
Leucine, phenylalanine, tyrosine, lysine, and tryptophane use a separate pathway to form acetyl CoA.
Formation of acetyl CoA
While acetyl CoA can be synthesized via pyruvate or amino acids, it can also be formed by the breakdown of acyl-CoA. What is acyl CoA? Remember the tail of CoA can act as a hanger.
An acyl group hangs on to the end of CoA like the acetyl group does. As such, acyl CoA is CoA with an acyl group hanging onto its tail.Inside the mitochondrial matrix, acyl-CoA undergoes a process known as beta-oxidation. Beta-oxidation is good for the cell because it creates acetyl CoA, which is used in the citric acid cycle and gives electrons and hydrogen to the mitochondrial electron transport chain (ETC). The electrons and hydrogen are then used by the ETC to generate energy.
Acetyl CoA consists of an acetyl group attached to the sulfhydryl (SH) tail of CoA. CoA consists of a nucleotide head (adenine + ribose + phosphate), a vitamin B5 body, and a beta-mercaptoethylamine tail which contains a sulfhydryl group.
The sulfhydryl group is where many different molecular groups attach, including acetyl (CH3-CO-) and acyl groups (RCO-). Acetyl CoA can be synthesized from pyruvate. Many amino acids can also be used to synthesize acetyl CoA.
Beta-oxidation, which occurs inside the mitochondrial matrix, is the process by which acetyl CoA is formed from acyl-CoA. The by-products of acetyl CoA formation are used to fuel the mitochondrial electron transport chain, and acetyl CoA itself is utilized by the cell in many different ways.