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!
Review 2: Biosignaling, Glycolysis, Gluconeogenesis, & PP-Pathway
Practice - Pentose Phosphate Path