Practice - Pentose Phosphate Path: Study with Video Lessons, Practice Problems & Examples

The pentose phosphate pathway involves transketolases and transaldolases, converting ribulose 5-phosphate into ribose 5-phosphate and other sugars. Key products include erythrose 4-phosphate, which is vital for aromatic amino acid synthesis, and ribose for nucleotides. The enzyme 6-phosphogluconate dehydrogenase plays a crucial role in this pathway. Notably, the decarboxylation of 6-phosphogluconate releases carbon dioxide, primarily from carbon 1 of glucose, highlighting the pathway's significance in metabolic processes.

Practice - Pentose Phosphate Path

If you haven't answered question 36 yet, pause the video now. Transketolases transfer two carbons to other sugar molecules and you'll remember that during the course of the pentose phosphate pathway, starting with ribulose 5-phosphate and producing xylulose 5-phosphate and ribose 5-phosphate. With those two 5-carbon sugars, the ribose and the xylulose, a transketolase will transfer 2 carbons from the xylulose to the ribose making sedoheptulose 7-phosphate and, G3P or glyceraldehyde 3-phosphate. After that, transaldolase takes 3 carbons from your sedoheptulose 7-phosphate and transfers it to the glyceraldehyde 3-phosphate leaving you with, erythrose 4-phosphate, a 4-carbon sugar, and fructose 6-phosphate, our 6-carbon sugar that can enter the glycolytic pathway. Which of the following statements about the pentose phosphate pathway is correct? The answer is that it provides precursors for amino acid biosynthesis and you might remember that that molecule that we literally just talked about, which is erythrose 4-phosphate. That can be used to produce your aromatic amino acids. Additionally, the ribose produced, from the isomerization of ribulose. So ribose 5-phosphate can be used to produce histidine as well as nucleotides. Now, which of the following acts in the pentose phosphate pathway? Glycogen phosphorylase is going to be an enzyme that breaks down glycogen. Aldolase is a glycolytic enzyme. Phosphofructokinase also glycolytic enzyme. I guess aldolase could also be a gluconeogenic enzyme. And pyruvate kinase is also a glycolytic enzyme. The answer is C, 6-phosphogluconate dehydrogenase, which you might remember is the enzyme that carries out the reaction, that converts 6-phosphogluconate to ribulose 5-phosphate. So, that is the third step of the pentose phosphate pathway and of course after this, ribulose 5-phosphate can be, isomerized into ribose 5-phosphate. That molecule we were just talking about before. Last question. Glucose labeled with carbon 14 in all of its carbons is added to a crude liver extract rich in the enzymes of the pentose phosphate pathway. The most rapid production of labeled carbon dioxide will occur from glucose 1. Now, the answer might not be immediately clear. So we need to think about the second, I'm sorry. The third reaction that we were just talking about where 6-phosphogluconate is converted to ribose 5-phosphate because in the process of that reaction, we decarboxylate 6-phosphogluconate. Here, this is the one carbon, right. This is the one carbon from glucose. Right. And so this is attached to the rest of the molecule. I'm just going to write R. This is actually going to decarboxylate and leave in the course of the reaction as CO₂ and then you're left with ribose 5-phosphate. And that carbon, that's C1 or carbon 1 from glucose which is why that's going to be the carbon with the most rapid production of CO₂ because earliest on in the pathway that's going to be the carbon that decarboxylates. So regardless of where the molecule goes after that, that carbon 1 is going to be coming off first from all of the molecules entering the pentose phosphate pathway in this particular scenario. Alright. That's all the questions we have. So good luck studying for your exams and please feel free to ask me any questions in the comment section of these videos. Good luck!

Here’s what students ask on this topic:

What is the role of transketolase in the pentose phosphate pathway?

Transketolase is an enzyme that plays a crucial role in the non-oxidative phase of the pentose phosphate pathway. It transfers two-carbon units from ketose donors to aldose acceptors. Specifically, it converts xylulose 5-phosphate and ribose 5-phosphate into glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate. This reaction is essential for the interconversion of sugars, allowing the pathway to produce intermediates that can enter glycolysis or be used for nucleotide and amino acid biosynthesis.

Which enzyme is responsible for the decarboxylation of 6-phosphogluconate in the pentose phosphate pathway?

The enzyme responsible for the decarboxylation of 6-phosphogluconate in the pentose phosphate pathway is 6-phosphogluconate dehydrogenase. This enzyme catalyzes the conversion of 6-phosphogluconate to ribulose 5-phosphate, releasing carbon dioxide (CO₂) in the process. This reaction is a key step in the oxidative phase of the pathway and is crucial for the production of NADPH, which is used in various biosynthetic processes and for maintaining cellular redox balance.

How does the pentose phosphate pathway contribute to nucleotide biosynthesis?

The pentose phosphate pathway contributes to nucleotide biosynthesis by producing ribose 5-phosphate, a crucial precursor for the synthesis of nucleotides. Ribose 5-phosphate is generated from ribulose 5-phosphate through an isomerization reaction. This ribose sugar forms the backbone of nucleotides, which are essential for DNA and RNA synthesis. Additionally, the pathway provides NADPH, which is necessary for reductive biosynthetic reactions, including the synthesis of nucleotides.

What is the significance of erythrose 4-phosphate in the pentose phosphate pathway?

Erythrose 4-phosphate is a significant intermediate in the pentose phosphate pathway. It is produced through the action of transaldolase, which transfers a three-carbon unit from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate. Erythrose 4-phosphate is a four-carbon sugar that serves as a precursor for the synthesis of aromatic amino acids, such as phenylalanine, tyrosine, and tryptophan. These amino acids are essential for protein synthesis and various metabolic functions.

Why is carbon 1 of glucose the first to be released as CO₂ in the pentose phosphate pathway?

Carbon 1 of glucose is the first to be released as CO₂ in the pentose phosphate pathway because it is involved in the decarboxylation reaction catalyzed by 6-phosphogluconate dehydrogenase. During this reaction, 6-phosphogluconate is converted to ribulose 5-phosphate, and carbon 1 is released as CO₂. This decarboxylation step occurs early in the oxidative phase of the pathway, making carbon 1 the first carbon to be removed from glucose as CO₂.