Question

RECALL Are amino acids other than the usual 20 amino acids found in proteins? If so, how are such amino acids incorporated into proteins? Give an example of such an amino acid and a protein in which it occurs.

Short answer

Yes, amino acids other than the usual 20 are found in proteins. For example, selenocysteine is incorporated using the UGA codon and found in Glutathione peroxidase.

Step-by-step solution

Understanding Unusual Amino Acids

Recall that while proteins are typically made from 20 standard amino acids, there are other amino acids that can be found in proteins. These are sometimes called 'non-standard' or 'uncommon' amino acids.

Incorporation Mechanism

Non-standard amino acids can be incorporated into proteins through specific post-translational modifications of standard amino acids or through the insertion of a special amino acid during translation. This often involves specialized tRNA and unique codon assignments.

Example of Non-Standard Amino Acid

Selenocysteine is a notable example of a non-standard amino acid. It is similar to cysteine but contains selenium instead of sulfur. Selenocysteine is incorporated into proteins via the UGA codon, which is usually a stop codon, but in this context, it codes for selenocysteine due to the presence of a specific sequence called the SECIS element in the mRNA.

Example of Protein Containing Selenocysteine

An example of a protein containing selenocysteine is Glutathione peroxidase. This enzyme is crucial for protecting the organism from oxidative damage by reducing lipid hydroperoxides to their corresponding alcohols and free hydrogen peroxide to water.

Key concepts

Post-translational Modifications

Proteins often undergo changes after they are formed. These changes are called post-translational modifications (PTMs). They help proteins become fully functional by refining their structure and function. Common PTMs include:

• Phosphorylation - adding phosphate groups
• Glycosylation - adding sugar molecules
• Acetylation - adding acetyl groups

PTMs are vital for regulating proteins. These modifications affect protein stability, activity, and cellular location.
Understanding PTMs is crucial in biochemistry and medicine.

Selenocysteine

Selenocysteine is a unique amino acid, sometimes called the 21st amino acid. It looks like cysteine but has selenium instead of sulfur. Selenium gives selenocysteine special reactive properties. This amino acid is essential in many enzymes, playing a critical role in redox reactions. Unlike the standard amino acids, selenocysteine is directly inserted into proteins during translation. This process needs special signals in the mRNA to work properly. Recognizing selenocysteine's importance helps us understand more about protein function and enzyme activity.

Specialized tRNA

Specialized tRNA plays a key role in including non-standard amino acids like selenocysteine in proteins. tRNA molecules help decode mRNA messages and build proteins. For selenocysteine, a unique tRNA known as tRNA^[Sec] is required. This tRNA recognizes the UGA codon in mRNA, which usually signals stop. The unique tRNA binds to this codon when other specific sequences are present. This allows selenocysteine to be placed into a growing protein chain. Having specialized tRNA is necessary for accurate protein production.

SECIS Element

A SECIS element (Selenocysteine Insertion Sequence) is a special sequence in mRNA. It is crucial for placing selenocysteine into proteins. The SECIS element is found in the 3' untranslated region (UTR) of the mRNA. This unique sequence works with specialized factors to identify UGA codons in the coding region. By doing so, the UGA is recoded to incorporate selenocysteine instead of stopping translation. Understanding SECIS elements helps explain how organisms can naturally include non-standard amino acids in proteins.

Glutathione Peroxidase

Glutathione peroxidase is an essential enzyme that contains selenocysteine. It helps protect cells from damage caused by free radicals and oxidative stress. Specifically, it reduces lipid hydroperoxides to alcohols and hydrogen peroxide to water. This function is crucial for maintaining cellular health and preventing diseases caused by oxidative stress. Selenocysteine in glutathione peroxidase is vital for its catalytic activity. Learning about this enzyme showcases the significant roles that non-standard amino acids can play in biological systems.