Key Enzymes in Glycolysis, Krebs Cycle, and Photosynthesis

Enzymes are essential in regulating the biochemical pathways involved in glycolysis, the Krebs cycle (also called the citric acid cycle), and photosynthesis. These processes are central to energy production, storage, and usage in cells. Below are the key enzymes involved in each of these pathways:

1. Key Enzymes in Glycolysis

Glycolysis is the breakdown of glucose (6 carbon molecules) into pyruvate (3 carbon molecules), producing a small amount of energy in the form of ATP and NADH. It occurs in the cytoplasm.

Key Enzymes in Glycolysis:

Hexokinase
- Catalyzes the phosphorylation of glucose to form glucose-6-phosphate, the first step of glycolysis. This traps glucose inside the cell.
Phosphoglucose isomerase
- Converts glucose-6-phosphate to fructose-6-phosphate by isomerization.
Phosphofructokinase (PFK)
- A rate-limiting enzyme in glycolysis, PFK catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. It is regulated by ATP (inhibitor) and AMP (activator), indicating the cell’s energy status.
Aldolase
- Splits fructose-1,6-bisphosphate into two 3-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.
Glyceraldehyde-3-phosphate dehydrogenase
- Catalyzes the oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, coupled with the reduction of NAD+ to NADH.
Phosphoglycerate kinase
- Catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, producing ATP by substrate-level phosphorylation.
Phosphoglycerate mutase
- Converts 3-phosphoglycerate to 2-phosphoglycerate.
Enolase
- Catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate (PEP), releasing water.
Pyruvate kinase
- Catalyzes the final step in glycolysis, converting PEP to pyruvate and producing ATP.

2. Key Enzymes in the Krebs Cycle (Citric Acid Cycle)

The Krebs cycle takes place in the mitochondrial matrix and is essential for the complete oxidation of acetyl-CoA to generate ATP, NADH, FADH2, and CO2.

Key Enzymes in the Krebs Cycle:

Citrate synthase
- Catalyzes the reaction between acetyl-CoA and oxaloacetate to form citrate, the first step of the Krebs cycle.
Aconitase
- Catalyzes the isomerization of citrate into isocitrate, via the intermediate cis-aconitate.
Isocitrate dehydrogenase
- Catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate, producing NADH and CO2.
Alpha-ketoglutarate dehydrogenase
- Catalyzes the conversion of alpha-ketoglutarate to succinyl-CoA, producing NADH and releasing CO2.
Succinyl-CoA synthetase
- Converts succinyl-CoA to succinate, producing GTP (or ATP) through substrate-level phosphorylation.
Succinate dehydrogenase
- Catalyzes the oxidation of succinate to fumarate, producing FADH2.
Fumarase
- Catalyzes the hydration of fumarate to malate.
Malate dehydrogenase
- Catalyzes the oxidation of malate to oxaloacetate, producing NADH.

3. Key Enzymes in Photosynthesis

Photosynthesis occurs in chloroplasts of plant cells and is responsible for converting light energy into chemical energy stored in glucose. The process consists of two main phases: the light reactions and the Calvin cycle (dark reactions).

Key Enzymes in Photosynthesis:

A. Light Reactions (occur in the thylakoid membrane of the chloroplast)

Photosystem II (PSII)
- The primary enzyme complex in the light reactions that absorbs light energy to split water into oxygen, protons, and electrons.
Plastoquinone
- Transports electrons from photosystem II to the cytochrome b6f complex.
Cytochrome b6f complex
- Facilitates the transfer of electrons from plastoquinone to plastocyanin, contributing to the proton gradient necessary for ATP synthesis.
Photosystem I (PSI)
- Absorbs light to further excite electrons, which are passed to ferredoxin and ultimately used to reduce NADP+ to NADPH.
ATP synthase
- Uses the proton gradient created by the light reactions to generate ATP from ADP and inorganic phosphate (Pi) via chemiosmosis.

B. Calvin Cycle (occur in the stroma of the chloroplast)

Rubisco (Ribulose bisphosphate carboxylase/oxygenase)
- Catalyzes the first major step of the Calvin cycle: the fixation of carbon dioxide to ribulose bisphosphate (RuBP), forming 2 molecules of 3-phosphoglycerate (3-PGA).
Phosphoglycerate kinase
- Converts 3-PGA to 1,3-bisphosphoglycerate using ATP, a key step in the reduction phase.
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)
- Reduces 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P) using NADPH.
Transketolase
- Transfers carbon units between sugar molecules, helping to regenerate RuBP and balance the cycle’s intermediates.
Ribulose-5-phosphate isomerase
- Converts ribulose-5-phosphate to ribulose bisphosphate, completing the cycle and enabling the continuation of carbon fixation.

Summary Table of Key Enzymes

Pathway	Key Enzyme	Function
Glycolysis	Hexokinase	Phosphorylates glucose to glucose-6-phosphate
	Phosphofructokinase (PFK)	Catalyzes rate-limiting step: fructose-6-phosphate to fructose-1,6-bisphosphate
	Pyruvate kinase	Converts PEP to pyruvate, generating ATP
Krebs Cycle	Citrate synthase	Forms citrate from acetyl-CoA and oxaloacetate
	Isocitrate dehydrogenase	Converts isocitrate to alpha-ketoglutarate, producing NADH
	Succinate dehydrogenase	Oxidizes succinate to fumarate, producing FADH2
Photosynthesis (Light Reaction)	Photosystem II (PSII)	Splits water and excites electrons
	ATP synthase	Synthesizes ATP using proton gradient
Photosynthesis (Calvin Cycle)	Rubisco	Fixes CO2 to ribulose bisphosphate (RuBP)
	Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)	Reduces 1,3-bisphosphoglycerate to G3P

Conclusion

Enzymes in glycolysis, the Krebs cycle, and photosynthesis are vital for energy production, storage, and conversion in cells. They control key steps in the breakdown of glucose, the generation of ATP through cellular respiration, and the synthesis of organic molecules using light energy. Their regulation and action ensure that these pathways run efficiently and in response to the cell’s energy needs.