TAVISHI
There is virtually no debate over the best amino acid. It is, of course, tyrosine. Tyrosine, with its aromatic benzene ring, speaks to me like no other amino acid: within me, it lights a fire of unimaginable passion.
Allow me to set the scene: Germany, 1846. Justus Von Liebeg was tinkering away in his lab (or so I like to imagine) when he came across the amino acid tyrosine in a block of cheese. He found the amino acid in the protein casein. Thus, the word tyrosine is derived from the Ancient Greek turos, meaning cheese.
Tyrosine is generally considered a polar amino acid. There is a little dispute on whether or not we can consider the entire amino acid polar because only a little part of the molecule makes it polar, truly. Typically, in biochemistry, any molecule with a hydroxyl group can be considered polar. However, some chemists argue that as a whole, the molecule is nonpolar, because of the ring. But, naturally, we can disregard anything chemists say because they are just always wrong.
Below is the structure of tyrosine.
Tyrosine is one of three amino acids with a benzene ring, accompanied only by phenylalanine and tryptophan. Tyrosine's beauty extends far past its structure, but further onto its use in biology. Take, for example, tyrosine kinase receptors. Tyrosine kinase receptors, in their inactive forms, are two separated monomers. Each monomer has an alpha helix structure in the membrane, and a ligand binding site. Extending into the cytoplasm is a tail of tyrosines. When a ligand binds to the receptors, the two receptors monomers dimerize, and the kinase is activated.
Specifically, the alpha helices in the membrane do not join; instead, it is only the kinase domains on each monomer that join. Dimerization either occurs via a conformational change or just the two kinase domains being brought close enough together that they are functional. The tyrosines are then phosphorylated, causing signal transduction within the cell. Phosphotyrosines are often sites to which other proteins bind to, resulting in conformational changes.
The proteins binding to phosphotyrosines have either SH2 domains or PTB domains. SH2 domains are used in the ubiquitylation of RTKs, specifically in the c-CBL protein. (Ubiquitylation is the marking of something by ubiquitin for destruction!) If an RTK is faulty, we want to get rid of it, or massive cell problems might occur
These receptor tyrosine kinases are receptors for insulin, epidermal growth factors, and more.
Tyrosine's functions are also present in my worst enemy: plants. Call me the Onceler, because plants are, indeed, my opps. I do not care if the Lorax emerges from a tree to yell at me, I will take it, because I hate plants. (This is mostly a joke, don't come for me, Ms. Briggs...)
During photosynthesis, when photosystem II is reduced, tyrosine residues act as electron donors for the chlorophyll. Tyrosine loses the oxygen in its phenolic hydroxyl group to reduce the chlorophyll, but that is okay. (Phenolic indicates a hydroxyl attached to a benzene group!) Afterwards, the hole in tyrosine's heart, ripped so rudely from it by those disgusting photosystems, is once again filled through the graciousness of the manganese clusters in photosystem II.
Let us zoom out from niche biochemistry now, and rather, focus in on the role of tyrosine in the body.
Tyrosine is the precursor of several neurotransmitters and hormones.
Take, for example, our thyroid hormones, triiodothyronine and thyroxine.
Triiodothyronine, or T3, can either be synthesized directly from tyrosine or from T4 (thyroxine). Direct synthesis is less common, and begins with the iodination of tyrosine to form monoiodotyrosine and diiodotyrosine. Iodine enters the thyroid follicular cell via a sodium-iodide symporter, and then joins the tyrosine either at the third or fifth carbon on the phenol ring. If it is only on the third, monoiodotyrosine is formed, and if it is on both, diiodotyrosine is formed.
The adding of the iodine to the tyrosines is catalyzed by thyroid peroxidase, and as a result, hydrogen peroxides are reduced. Thyroid peroxidase also catalyzes the joining of one monoiodotyrosine and one diiodotyrosine to form T3. T4, or thyroxine, is two diiodotyrosine. If thyroxine needs to be converted to T3, deiodinases do just the trick.
Tyrosines also are precursors to key neurotransmitters, including dopamine, epinephrine, and norepinephrine. These neurotransmitters are all collectively called catecholamines.
To form dopamine, tyrosine first, gains a hydroxyl on its third carbon as a result of the enzyme tyrosine hydroxylase, forming L-DOPA. L-DOPA is later converted to dopamine by the removal of the carboxyl group, using the enzyme DOPA decarboxylase.
L-DOPA is often used as a medication to treat disorders with low dopamine levels, including Parkinson's. This is because L-DOPA can cross the blood-brain barrier, unlike dopamine, and as a result, can effectively synthesize dopamine.
Now that I have sung the praises of tyrosine, I hope you too favor this amino acid above them all.
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