Question

Which of the following is the correct sequence of glycolysis? (a) \(\mathrm{G} 6-\mathrm{P} \rightarrow \mathrm{PEP} \rightarrow 3-\mathrm{PGAL} \rightarrow 3-\mathrm{PGA}\) (b) \(\mathrm{G} 6-\mathrm{P} \rightarrow 3-\mathrm{PGAL} \rightarrow 3-\mathrm{PGA} \rightarrow \mathrm{PEP}\) (c) \(\mathrm{G} 6-\mathrm{P} \rightarrow \mathrm{PEP} \rightarrow 3-\mathrm{PGA} \rightarrow 3-\mathrm{PGAL}\) (d) \(\mathrm{G} 6-\mathrm{P} \rightarrow 3-\mathrm{PGA} \rightarrow 3-\mathrm{PGAL} \rightarrow \mathrm{PEP}\)

Short answer

The correct sequence of glycolysis is G 6-P -> 3-PGAL -> 3-PGA -> PEP, so the answer is (b).

Step-by-step solution

Understanding the abbreviations

The abbreviations used in the problem stand for: G 6-P - Glucose 6-phosphate, PEP - Phosphoenolpyruvate, 3-PGAL - Glyceraldehyde 3-phosphate, 3-PGA - 3-Phosphoglycerate.

Identifying the correct sequence

The pathway of glycolysis goes as follows: Glucose is first converted to Glucose 6-phosphate (G 6-P). This is then converted to Glyceraldehyde 3-phosphate (3-PGAL). Next, 3-PGAL is converted to 3-Phosphoglycerate (3-PGA), which finally gets converted to Phosphoenolpyruvate (PEP).

Matching the correct sequence with options

By comparing the sequence of glycolysis we identified in Step 2, to the options given, we find that sequence (b) G 6-P -> 3-PGAL -> 3-PGA -> PEP is the correct sequence of events in glycolysis.

Key concepts

Glucose 6-phosphate

Glucose 6-phosphate (G6P) is the first key metabolite in the glycolysis sequence. Glycolysis is a fundamental pathway that converts glucose, the primary source of cellular energy, into pyruvate. This process begins with the phosphorylation of glucose, which involves adding a phosphate group to the glucose molecule, thereby transforming it into G6P.

The significance of this step lies in its regulatory function. By phosphorylating glucose, cells effectively 'trap' the molecule within the cell. G6P cannot easily cross the cell membrane, so it remains within the cell for further metabolism. Furthermore, this reaction is catalyzed by the enzyme hexokinase, or glucokinase in liver cells, and is an important regulatory point in the metabolic pathway.

The transition from glucose to G6P is the first step toward the ultimate breakdown of glucose into pyruvate, which will then be utilized for energy production in the form of adenosine triphosphate (ATP).

Glyceraldehyde 3-phosphate

Glyceraldehyde 3-phosphate (G3P) is a central intermediate in the glycolytic pathway. Following several steps after the formation of Glucose 6-phosphate, the molecule is eventually split into two three-carbon compounds. One of these is G3P, which plays a dual role in the next phases of glycolysis.

In a series of reactions, two molecules of G3P are oxidized and phosphorylated, eventually leading to the production of ATP—the energy currency of the cell. It's important to note that for each initial glucose molecule, two G3P molecules will enter the subsequent steps of glycolysis. This means that each of the later steps occurs twice per original molecule of glucose, effectively doubling the potential yield of ATP and other metabolites like NADH.

The conversion of G3P to the next molecule, 3-Phosphoglycerate, is accompanied by the reduction of NAD+ to NADH, which is another crucial factor in the cellular respiration process and further energy extraction in the mitochondria.

3-Phosphoglycerate

The compound 3-Phosphoglycerate (3-PGA) emerges as part of the glycolytic pathway's payoff phase. After Glyceraldehyde 3-phosphate undergoes oxidation and phosphorylation, 3-PGA is generated. This molecule is pivotal on multiple fronts: as a metabolite in glycolysis and as a precursor for several biosynthetic pathways.

As G3P is processed, a high-energy phosphate group is transferred to ADP, forming ATP in the process. Simultaneously, the 3-carbon molecule is converted to 3-PGA. Subsequent steps will shift the phosphate group from the third carbon to the second carbon, transforming 3-PGA into 2-Phosphoglycerate.

3-PGA's formation marks the phase of glycolysis where ATP is produced. Considering that glycolysis processes two molecules of G3P for every glucose molecule, a total of two 3-PGA molecules will result in the production of ATP further along the pathway, making it a vital step in energy extraction.

Phosphoenolpyruvate

Phosphoenolpyruvate (PEP) is one of the final products in the glycolysis pathway, formed from the enolization of 2-phosphoglycerate through the enzyme enolase. PEP is a high-energy compound. Its subsequent hydrolysis releases a substantial amount of energy, which is harnessed to convert ADP to ATP in one of the most dramatic energy-yielding steps of glycolysis.

The conversion of PEP to pyruvate, catalyzed by the enzyme pyruvate kinase, is not only crucial for the production of ATP but also marks the end of glycolysis. Pyruvate can then proceed to enter the mitochondrial matrix for further processing within the Krebs cycle (if oxygen is available), or it can undergo lactic acid fermentation in the absence of oxygen.

Understanding the role of PEP in glycolysis is crucial as it links glycolysis to the citric acid cycle and the anaerobic fermentation processes, ultimately connecting glucose metabolism to the wider scope of cellular respiration and energy production.