Question

(a) Identify 6 -phosphglucono- \(\delta\) -lactone.______________ (b) Which reactions produce NADPH?_______________ (c) Identify ribulose 5 -phosphate.______________ (d) What reaction generates \(\mathrm{CO}_{2}\) ?___________ (e) Identify 6 -phosphogluconate.____________ (f) Which reaction is catalyzed by phosphopentose isomerase?______________ (g) Identify ribose 5 -phosphate_____________ (h) Which reaction is catalyzed by lactonase?______________ (i) Identify glucose 6 -phosphate.______________ (j) Which reaction is catalyzed by 6 -phosphogluconate dehydrogenase?_______________ (k) Which reaction is catalyzed by glucose 6 -phosphate dehydrogenase?______________ (EQUATION CAN'T COPY)

Short answer

(a) First product of oxidative phase; (b) In reactions 2 and 4; (c) Following 6-phosphogluconate dehydrogenase; (d) In step with ribulose 5-P production; (e) After lactonase action; (f) Converts ribulose 5-P to ribose 5-P; (g) After phosphopentose isomerase action; (h) Hydrolyzes 6-phosphoglucono-δ-lactone; (i) Starting molecule; (j) Converts 6-phosphogluconate; (k) Converts glucose 6-P.

Step-by-step solution

Understand the Pathway Context

This exercise involves identifying various compounds and reactions within the Pentose Phosphate Pathway (PPP). Understanding where each molecule fits in the PPP will help solve the questions.

Identify 6-Phosphoglucono-δ-lactone

6-Phosphoglucono-δ-lactone is the first product formed in the oxidative phase of the PPP, specifically from the oxidation of glucose 6-phosphate by glucose 6-phosphate dehydrogenase.

Reactions Producing NADPH

NADPH is produced in the oxidative phase of the PPP: once during the oxidation of glucose 6-phosphate to 6-phosphoglucono-δ-lactone and again during the conversion of 6-phosphogluconate to ribulose 5-phosphate by 6-phosphogluconate dehydrogenase.

Identify Ribulose 5-Phosphate

Ribulose 5-phosphate is produced in the oxidative phase of the PPP after 6-phosphogluconate is converted by 6-phosphogluconate dehydrogenase, with the release of CO₂.

Generation of CO₂

The reaction catalyzed by 6-phosphogluconate dehydrogenase converts 6-phosphogluconate to ribulose 5-phosphate, generating CO₂ as a byproduct.

Identify 6-Phosphogluconate

6-Phosphogluconate is produced from 6-phosphoglucono-δ-lactone by the action of lactonase in the oxidative phase of the PPP.

Reaction Catalyzed by Phosphopentose Isomerase

Phosphopentose isomerase catalyzes the conversion of ribulose 5-phosphate to ribose 5-phosphate in the non-oxidative phase of the PPP.

Identify Ribose 5-Phosphate

Ribose 5-phosphate is derived from ribulose 5-phosphate by the action of phosphopentose isomerase and is an important sugar for nucleotide synthesis.

Reaction Catalyzed by Lactonase

Lactonase catalyzes the hydrolysis of 6-phosphoglucono-δ-lactone to form 6-phosphogluconate early in the PPP.

Identify Glucose 6-Phosphate

Glucose 6-phosphate is the starting substrate of the PPP, synthesized from glucose by the action of hexokinase in glycolysis.

Reaction Catalyzed by 6-Phosphogluconate Dehydrogenase

6-Phosphogluconate dehydrogenase converts 6-phosphogluconate into ribulose 5-phosphate, releasing CO₂ and producing NADPH.

Reaction Catalyzed by Glucose 6-Phosphate Dehydrogenase

Glucose 6-phosphate dehydrogenase catalyzes the reaction of glucose 6-phosphate to 6-phosphoglucono-δ-lactone, producing NADPH.

Key concepts

NADPH production

The production of NADPH is a crucial aspect of the Pentose Phosphate Pathway (PPP). NADPH plays a vital role in anabolic reactions, such as fatty acid and nucleotide synthesis, as well as in maintaining the redox balance within cells by combating oxidative stress.
NADPH is primarily generated in two reactions during the oxidative phase of the PPP:

• Firstly, the oxidation of glucose 6-phosphate is catalyzed by the enzyme glucose 6-phosphate dehydrogenase, leading to the formation of 6-phosphoglucono-δ-lactone and producing the first molecule of NADPH.
• Secondly, the oxidation of 6-phosphogluconate to ribulose 5-phosphate is catalyzed by the enzyme 6-phosphogluconate dehydrogenase, resulting in the production of a second NADPH molecule.

These reactions are essential for cells that require reducing power, such as red blood cells, where NADPH helps to combat the effects of oxidative damage. Without the efficient production of NADPH, cells would struggle to perform essential functions related to growth and repair.

6-phosphoglucono-δ-lactone

6-Phosphoglucono-δ-lactone is a critical intermediate in the oxidative phase of the Pentose Phosphate Pathway. It is the direct product formed from the enzymatic reaction catalyzed by glucose 6-phosphate dehydrogenase.
This reaction involves the oxidation of glucose 6-phosphate and importantly, leads to the generation of one molecule of NADPH.
The formation of 6-phosphoglucono-δ-lactone is a necessary step that connects glucose metabolism to various biosynthetic processes. As an intermediate, this compound is quickly converted by lactonase into 6-phosphogluconate, continuing the progression through the oxidative phase of the PPP.
The presence of 6-phosphoglucono-δ-lactone thus signifies an important control point in the flux through the pathway, as each step directly contributes to the cellular balance of NADPH and ribose sugars needed for nucleic acid synthesis.

ribulose 5-phosphate

Ribulose 5-phosphate is an essential product of the oxidative phase in the Pentose Phosphate Pathway. Its formation is catalyzed by the enzyme 6-phosphogluconate dehydrogenase, which also leads to the release of carbon dioxide (CO₂) as a byproduct. This phase of the PPP is where the second molecule of NADPH is generated.
Ribulose 5-phosphate holds a significant role, as it serves as a precursor for the non-oxidative reactions within the pathway.

• It can be converted into ribose 5-phosphate through the action of phosphopentose isomerase, which is crucial for nucleotide creation and thus, DNA and RNA synthesis.
• Alternatively, it can be converted into other sugar phosphates that enter various metabolic pathways, demonstrating its versatility in cellular metabolism.

This dual potential highlights ribulose 5-phosphate's importance in cellular physiology, balancing between energy coinvestment and anabolic growth processes.

6-phosphogluconate

6-Phosphogluconate is an intermediate in the oxidative phase of the Pentose Phosphate Pathway. It is generated through the action of the enzyme lactonase, which hydrolyzes 6-phosphoglucono-δ-lactone to form 6-phosphogluconate.
This stage of the pathway prepares the molecule for the subsequent enzyme-catalyzed transformation, which is key to the pathway's main outputs.
Upon its formation, 6-phosphogluconate undergoes oxidative decarboxylation, a reaction catalyzed by 6-phosphogluconate dehydrogenase. This transformation leads to the production of ribulose 5-phosphate, the emission of carbon dioxide, and the formation of a molecule of NADPH.
The efficient conversion of 6-phosphogluconate to downstream products is vital for maintaining a steady supply of NADPH and other substrate molecules that participate in various biosynthetic routes, highlighting the compound's significance as a metabolic hub.