1. Egg Digestion and Release of Amino Acids
When you eat an egg, protein digestion begins in the stomach. The acidic environment, due to hydrochloric acid (HCl), denatures the proteins, breaking down their three-dimensional structure. Pepsin, a proteolytic enzyme present in the stomach, acts on these denatured proteins, breaking them down into smaller fragments called peptides. These peptides then pass into the small intestine, where they are exposed to pancreatic enzymes such as trypsin and chymotrypsin, which further break them down into individual amino acids. Among these amino acids, tyrosine is one of the end products that will be absorbed by the intestinal cells.
2. Transport of Tyrosine to the Liver
After absorption by the cells of the small intestine, tyrosine is transported through the bloodstream, reaching the liver via the hepatic portal vein. In the liver, tyrosine can be used in several metabolic pathways, but a significant portion is released back into the bloodstream to be distributed to the tissues that require this amino acid, including the brain, where it will be used for the synthesis of neurotransmitters.
3. Tyrosine Uptake by Neurons in the Brain
In the brain, tyrosine is taken up by dopaminergic neurons, which are responsible for the synthesis of dopamine. These neurons are located mainly in regions such as the substantia nigra and the ventral tegmentum. Tyrosine crosses the blood-brain barrier using specific transporters that regulate its entry into nerve cells.
4. Conversion of Tyrosine to L-DOPA
Within dopaminergic neurons, tyrosine is converted to L-DOPA by the action of the enzyme tyrosine hydroxylase. This step is crucial because tyrosine hydroxylase adds a hydroxyl group to tyrosine, producing L-DOPA, a direct precursor of dopamine. This enzyme is considered the rate-limiting step in the catecholamine synthesis pathway, meaning it is the most regulated and critical step for dopamine production.
5. Conversion of L-DOPA to Dopamine
The L-DOPA generated by tyrosine hydroxylase is then rapidly converted to dopamine by the enzyme aromatic L-amino acid decarboxylase (also known as dopa decarboxylase). This enzyme removes a carboxyl group from L-DOPA, converting it to dopamine. Dopamine is then ready to be used as a neurotransmitter in the central nervous system.
6. Storage and Release of Dopamine
Synthesized dopamine is stored in synaptic vesicles at the terminals of dopaminergic neurons. When a nerve impulse arrives at the neuron terminal, these vesicles fuse with the cell membrane and release dopamine into the synaptic cleft, the space between the presynaptic neuron and the postsynaptic neuron. Dopamine then binds to dopamine receptors on postsynaptic neurons, where it exerts its effects, regulating functions such as movement, pleasure, motivation, and attention.
7. Dopamine Reuptake and Degradation
After dopamine has served its purpose, it is reuptaken by dopamine transporters into the presynaptic neuron, where it can be recycled and stored for future release. Alternatively, dopamine can be degraded by enzymes such as monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT), which break down dopamine into inactive metabolites, ending its action.
This detailed process describes how tyrosine from an egg is converted into dopamine, an essential neurotransmitter in the human brain.
