Proteins are constructed from smaller units called amino acids, which are linked together in long chains. The specific sequence of these amino acids is dictated by a genetic script found within our DNA. This script is read in three-letter “words” known as codons. Each codon corresponds to a particular amino acid, instructing the cellular machinery which one to add next to the growing protein chain.
The Genetic Code for Tyrosine
The instructions for synthesizing proteins are transcribed from DNA into a messenger molecule called mRNA. Within this mRNA sequence, the two codons that specify the amino acid tyrosine are UAU and UAC. The “U” in these codons stands for uracil, a base that replaces thymine (T) in RNA. The original DNA code on the gene’s coding strand from which these mRNA codons are derived would be TAT and TAC.
This feature, where two different codons result in the same amino acid, is an example of the genetic code’s redundancy. Most amino acids are encoded by more than one codon, which provides a buffer against certain genetic mutations. If a mutation changes the third base of a tyrosine codon from U to C, or vice-versa, the same amino acid is still incorporated into the protein, preventing a potential alteration in its function.
Mechanism of Translation
The process of converting genetic information from an mRNA sequence into a protein is called translation, which occurs within the ribosome. The ribosome moves along the mRNA strand, reading each three-letter codon sequentially. This process requires transfer RNA (tRNA) molecules to match each codon with its corresponding amino acid.
Each tRNA molecule has two primary features: an anticodon and a binding site for a specific amino acid. The anticodon is a three-base sequence complementary to an mRNA codon. For the tyrosine codons UAU and UAC, the corresponding tRNA anticodons are AUA and AUG. A tRNA with the AUA anticodon binds to the UAU codon, while a tRNA with the AUG anticodon recognizes the UAC codon.
When the ribosome encounters a tyrosine codon on the mRNA, the tRNA with the matching anticodon is recruited. This tRNA delivers a tyrosine amino acid to the ribosome, which then forms a peptide bond, linking it to the growing polypeptide chain. After delivering its amino acid, the tRNA is released, and the ribosome moves to the next codon to repeat the process.
Consequences of Codon Mutations
Changes in the DNA sequence, known as mutations, can alter mRNA codons and potentially affect the resulting protein. A point mutation, a change in a single DNA base, can lead to several outcomes within a gene. The specific consequence depends on how the mutation alters the codon that is read during translation.
One possible outcome is a silent mutation. If a mutation changes the DNA sequence so that the mRNA codon UAU becomes UAC, the genetic information has changed. However, because both UAU and UAC code for tyrosine, the correct amino acid is still added to the protein chain, rendering the mutation “silent” at the protein level.
A missense mutation happens when a point mutation alters the codon to one that specifies a different amino acid. For example, if the first base in the UAU codon is changed to a C, the new codon becomes CAU, which codes for the amino acid histidine. Swapping tyrosine for histidine can alter the protein’s three-dimensional structure, stability, or function, as their chemical properties are different.
A nonsense mutation occurs when a codon for an amino acid is changed into a “stop” codon. The three stop codons—UAA, UAG, and UGA—do not code for any amino acid; instead, they signal the ribosome to terminate protein synthesis. If the tyrosine codon UAC mutates to UAG, for instance, the ribosome will halt translation prematurely, resulting in a shortened, and typically non-functional, protein fragment.
Functional Role of the Tyrosine Amino Acid
The significance of accurately encoding tyrosine lies in the amino acid’s specific biochemical functions. Its structure includes a phenol side chain, characterized by an aromatic ring attached to a hydroxyl (-OH) group. This structure allows tyrosine to participate in forming hydrogen bonds, which helps stabilize the complex three-dimensional shapes of proteins. The aromatic ring also contributes to hydrophobic interactions important for protein folding.
Beyond its structural contributions, tyrosine plays a role in cellular communication. The hydroxyl group on its side chain is a primary target for a process called phosphorylation. Enzymes known as tyrosine kinases attach a phosphate group to this site, acting as a molecular switch. This addition of a phosphate can activate or deactivate a protein, propagating signals that control cellular activities like growth and division.
The regulation of these signaling pathways is precise. The specific addition of phosphate groups to tyrosine residues by tyrosine kinases is a main way cells respond to their environment. Errors in this signaling process, sometimes caused by mutations affecting tyrosine codons, can lead to uncontrolled cell growth and are implicated in various diseases.