Review of Metabolism - Online Tutor, Practice Problems & Exam Prep

Metabolism encompasses catabolic and anabolic pathways for macromolecule digestion, particularly carbohydrates and proteins. Carbohydrates are broken down into glucose, leading to pyruvate, which can convert to Acetyl CoA and enter the citric acid cycle for ATP production. Proteins yield amino acids that undergo transamination or oxidative deamination, linking to pyruvate or Acetyl CoA. The urea cycle, involving aspartate and fumarate, connects protein metabolism to the citric acid cycle, highlighting the interdependence of these pathways in energy production and waste elimination.

Review of Metabolism Concept 1

In this video, we're going to take a look at metabolism as a whole when it comes to the digestion of macromolecules. Overall, metabolism is seen as catabolic, where we degrade larger molecules, and anabolic, where we synthesize larger molecules. We will say here that these different macromolecules have metabolic pathways that are interconnected by common metabolites. We've talked about the metabolism in relation to lipids versus carbohydrates before. Now, we're going to add an additional column when talking about proteins.

Before we begin, our key here talks about the different pathways. Catabolic represents our black arrows, anabolic represents our green arrows, and then here our purple arrows represent the transportation across our mitochondrial membrane. You might notice that in this image, we don't have any green arrows. That's because in this part, we're going to really focus on the interconnections between carbohydrates and proteins because there are metabolites that overlap between them heavily. For lipids, we're going to ignore them because lipids don't overlap with proteins here. And because we've talked about lipids in the past, we're going to gray out a vast majority of what connects to lipids. So, if you look here, a lot of the green arrows that we could have had are in sections that are grayed out.

Right now, we're mainly concerned with carbohydrates and proteins. Here, if we're looking at our carbohydrates, remember we have the exterior of the cell, we have our cell membrane, we have our cytosol, our mitochondrial membranes, and then our mitochondrial matrix. Regarding carbohydrates, we have the digestion of it to form glucose. And then through glycolysis, we create pyruvate. Pyruvate is where we're going to start caring about the digestion of carbohydrates. Pyruvate itself will cross the mitochondrial membrane, so we have this purple arrow. From pyruvate, pyruvate can do one of two things. It can feed into Acetyl CoA, which in turn feeds into the citric acid cycle. Here we have oxaloacetate as a metabolic intermediary in terms of the citric acid cycle. We could also have pyruvate just going straight into the citric acid cycle. From there, it enters the electron transport chain (ETC), then oxidative phosphorylation for the creation of ATP.

Now, how does this connect to proteins? With proteins, we have the digestion of proteins as they cross the cell membrane, and we have our amino acids. These amino acids can do one of two things. They can undergo transamination or they can undergo oxidative deamination. With transamination, they help to make alpha keto acids. This alpha keto acid itself can cross the mitochondrial membrane, and from there, the alpha keto acid has different routes it can take. It can change into pyruvate itself, thus linking it to carbohydrates because they share that metabolite in common, or it can go into Acetyl CoA which then also feeds into the citric acid cycle, then the ETC in oxidative phosphorylation to the generation of ATP. Meanwhile, we have oxidative deamination. Here we're skipping some key parts. Remember that the ammonium ion does not directly feed into the urea cycle. We're skipping the carbamoyl phosphate molecule that is actually prepared.

We're making our chart as simple as possible and not cramming it full of all the intermediates and all the enzymes that are involved. We've talked about these in greater detail in earlier videos. We're not going to waste time talking about it here. Remember the urea cycle? We're going to say that it's composed of reactions 1, 2, 3, and 4. Only reaction 1 happens within the mitochondrial matrix. Reactions 2, 3, and 4 happen outside of the mitochondria in the cytosol. We know that in reaction 4, we're going to create our ornithine molecule again, which is regenerated to start the process again. But more importantly, we create urea as our waste material in the cytosol that's eventually excreted out in our urine.

What also connects proteins and carbohydrates has to do with the connection between the urea cycle and the citric acid cycle. The metabolites that they share in common are your aspartate amino acid here and then we have fumarate here. Remember, when it comes to reaction 2 of the urea cycle, we have our aspartate molecule coming in from the outside. Well, that's coming from the citric acid cycle. Technically, the step 2 happens outside the mitochondria. So remember, it's reaction 1 that's happening inside the mitochondrial matrix when it comes to the urea cycle, and reactions 2, 3, and 4 are happening in the cytosol. Next, we're going to say that fumarate, which is created in reaction 3 of the urea cycle, the cleavage step, can feed into the citric acid cycle. These two metabolites are what connect the urea cycle of proteins to the citric acid cycle that we have here. Again, the citric acid cycle can also connect to proteins via a longer pathway, but it can also come from carbohydrates itself.

So just keep in mind, we have these different types of macromolecules, lipids, carbohydrates, and proteins; they are connected to one another because they share certain key metabolites with one another. So just keep a big idea, big picture when it comes to how these macromolecules are digested, the different types of structures that are produced, how they traverse across membranes, and they undergo catabolic or anabolic processes in order to generate other molecules or to create ATP.

Review of Metabolism Example 1

Here it says, "Which of the following statements is true? Lipid and Protein Metabolism Provide the Only Catabolic Pathways to Acetyl CoA Formation." That statement is false. We know that carbohydrates, when digested, can also provide another catabolic pathway to Acetyl CoA formation. So, this statement is false.

Here, a transaminase is the class of enzymes that catalyzes transamination reactions. The name gives it away. This is what this enzyme is intended to do. Remember, in a transamination reaction, we are exchanging, in a reversible reaction, the amino group of an amino acid with the keto group of an alpha-keto acid. So, this statement is true.

When glycogen stores are at max levels, fatty acids are degraded for energy. No. If you have a surplus of carbohydrates or sugars, we break that down first to gain energy.

Alanine and alpha-ketoglutarate reacting to form pyruvate and glutamate represents an oxidative deamination. Remember, in an oxidative deamination, we have glutamate undergoing oxidation to create alpha-ketoglutarate. We use NAD positive in order to do this. This is saying the opposite. So here it's saying that we're using alpha-ketoglutarate to make glutamate when it should be reversed. That is what oxidative deamination is.

So here, the only statement that is true would have to be option B.

Which of the following statements is incorrect?

• A. Ammonia molecules are converted to carbamoyl phosphate before entering the Urea Cycle. 
• B. The production of urea requires the reaction between water and a glutamate molecule. 
• C. The Urea Cycle excretes urea as a waste material by using NH₄⁺ and aspartate as the sources of nitrogen. 
• D. It requires 4 total ATP molecules to produce one urea molecule.

In the synthetic pathway of serine, 3-phosphoglycerate must first be converted into 3-phosphohydroxypyruvate.

Identify the type of reaction represented by this conversion. 

• A. Condensation
• B. Reduction
• C. Transamination 
• D. Oxidation

Which of the following statements is incorrect?

• A. The products of a transamination reaction are a new amino acid and an α-keto acid. 
• B. The ammonium ion produced in the liver must immediately be excreted as urea because of its toxicity. 
• C. Glutamate converting to α-ketoglutarate and an ammonium ion represent an oxidative deamination reaction. 
• D. Ammonia molecules directly enter the Urea Cycle to serve as the only nitrogen source.  

Determine whether each of the following is involved in Glycolysis (A), β-Oxidation (B), Transamination/ Oxidative Deamination (C), or Urea Cycle (D).

I.  _____ Using alanine transaminase on alanine and α-ketoglutarate to form pyruvate and glutamate.

II. _____ Phosphofructokinase or fructose-1,6-bisphosphate.

III. _____ The continuous regeneration of Ornithine. 

IV. _____ Cleavage of a 2-carbon acetyl group. 

Here it says, "Determine whether each of the following is involved in glycolysis with A, beta oxidation with B, transamination/oxidative deamination with C, or the urea cycle with D." So, using alanine transaminase on alanine and alpha-ketoglutarate to form pyruvate and glutamate. Transaminase. Transamination. The enzyme kind of gives away which process we are talking about in terms of the statement. So, this would have to be option C. Phosphofructokinase or fructose 1,6-bisphosphate? OOS represents a carbohydrate, a sugar. Here, if we're looking at glycolysis, we're talking about how we can break our glucose down to help us make pyruvate. So, these things here have to be in some relation connected to glycolysis. The continuous regeneration of ornithine. Ornithine serves as the carrier molecule within the urea cycle, so this would have to be option D. Then finally, cleavage of a 2-carbon acetyl group. Here, this would have to be the process of beta oxidation. Although, we didn't talk about the pathways for lipids, we did talk about it in earlier sections. So, remember, when it comes to beta oxidation, it is in essence the cleavage of 2-carbon acetyl groups. So here, this would have to be option B. This is how we describe each one of the statements in relation to glycolysis, beta oxidation, transamination/oxidative deamination, as well as the urea cycle.

Here’s what students ask on this topic:

What is the difference between catabolic and anabolic pathways in metabolism?

Catabolic pathways involve the breakdown of larger molecules into smaller ones, releasing energy in the process. For example, the digestion of carbohydrates into glucose and further into pyruvate is a catabolic process. Anabolic pathways, on the other hand, involve the synthesis of larger molecules from smaller ones, which requires energy. An example is the synthesis of proteins from amino acids. Both pathways are interconnected and essential for maintaining the body's energy balance and building cellular structures.

How are carbohydrates metabolized to produce ATP?

Carbohydrates are metabolized starting with their digestion into glucose. Glucose undergoes glycolysis in the cytosol, producing pyruvate. Pyruvate then enters the mitochondria and is converted into Acetyl CoA. Acetyl CoA enters the citric acid cycle (Krebs cycle), where it is oxidized to produce NADH and FADH₂. These electron carriers then donate electrons to the electron transport chain (ETC) in the mitochondrial membrane, leading to oxidative phosphorylation and the production of ATP.

What role do amino acids play in metabolism?

Amino acids, derived from protein digestion, play a crucial role in metabolism. They can undergo transamination to form alpha-keto acids, which can be converted into pyruvate or Acetyl CoA, linking protein metabolism to carbohydrate metabolism. Alternatively, amino acids can undergo oxidative deamination, producing ammonium ions that enter the urea cycle for waste elimination. The urea cycle also connects to the citric acid cycle through shared metabolites like aspartate and fumarate, highlighting the interdependence of these metabolic pathways.

How is the urea cycle connected to the citric acid cycle?

The urea cycle and the citric acid cycle are connected through shared metabolites. Aspartate, an amino acid, is involved in the urea cycle and is derived from the citric acid cycle. Fumarate, produced in the urea cycle, can feed into the citric acid cycle. This interconnection allows for the efficient use of metabolites and energy production, as well as the elimination of waste products like urea.

What is the significance of pyruvate in metabolism?

Pyruvate is a key metabolite in metabolism, serving as a critical junction between carbohydrate and protein metabolism. It is produced from glucose through glycolysis and can be converted into Acetyl CoA, which enters the citric acid cycle for ATP production. Pyruvate can also be formed from amino acids through transamination, linking protein metabolism to carbohydrate metabolism. Its versatility makes it essential for energy production and the synthesis of various biomolecules.