Practice - Pentose Phosphate Path: Videos & Practice Problems

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.

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.

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.

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.

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.