Metabolic cycles – Urea, pentose phosphate, beta-oxidation, Measles, Folate

1. The urea cycle (ornithine cycle)

Function: Converts toxic ammonia (NH₃) into urea, which is then eliminated from the body.
Where it occurs: In the liver.
Main stages:
1. Ammonia + co → → carbamoyl phosphate.
2. Carbamoyl Phosphate + ornithine → Citrulline.
3. Citrulline + aspartate → argininosuccinate.
4. Arginineosuccinate → arginine + fumarate.
5. Arginine → urea + ornithine (the cycle closes).
Meaning: Protecting the body from toxic ammonia produced by protein breakdown.

2. The pentose phosphate pathway (hexose monophosphate shunt)

Function:
- Generation of NADPH (necessary for the synthesis of fatty acids and antioxidant protection).
- Ribose-5-phosphate synthesis (required for nucleotide synthesis).
Where it occurs: In the cytoplasm of cells.
Main stages:
1. Oxidative phase: glucose-6-phosphate → ribulose-5-phosphate + NADPH.
2. Non-oxidative phase: regeneration of intermediate products.
Meaning: Supporting biosynthesis and antioxidant protection.

3. Measles cycle (glucose-lactate cycle)

Function: Conversion of lactate produced in the muscles during anaerobic exercise into glucose in the liver.
Where it occurs: Between the muscles and the liver.
Main stages:
1. In the muscles: glucose → lactate + ATP.
2. Lactate is transported to the liver.
3. In the liver: lactate → glucose (via gluconeogenesis).
4. Glucose is returned to the muscles.
Meaning: Blood glucose maintenance and muscle recovery.

4. The cycle of beta-oxidation of fatty acids

Function: Breakdown of fatty acids to acetyl-CoA for use in the Krebs cycle.
Where it occurs: In the mitochondria.
Main stages:
1. Activation of fatty acid to acyl-CoA.
2. Oxidation to form acetyl-CoA, NADH, and FADH₂.
Value: Energy source during prolonged exercise or starvation.

5. Folate cycle (single-carbon group exchange)

Function: Transfer of one-carbon groups (methyl, formyl, etc.) for the synthesis of nucleotides and amino acidsAmino acids are the fundamental building blocks of proteins and play a key role in biological processes. There are a total of 22 standard amino acids used for protein synthesis in living organisms..
Where it occurs: In the cytoplasm of cells.
Main stages:
1. Conversion of serine to glycineGlycine is an amino acid that regulates metabolic processes in the central nervous system. It belongs to neurotransmitters (participates in the transmission of nerve impulses). Glycine is a key component of collagen, which gives structure to bones, muscles, connective tissues, and skin. It also participates in the transmission of nerve impulses, increasing the efficiency of information transfer between neurons. with the release of a single-carbon group.
2. Using folate to transfer groups.
Meaning: Supports the synthesis of DNA, RNA, and amino acids.

Conclusion

These cycles, like the Krebs cycle, are important metabolic pathways that support energy metabolism, biomolecule synthesis, and detoxification. They are closely related to each other and support the body’s homeostasis.