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Chapter 3: Problem 6

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

Expert verified
Yes, non-standard amino acids such as selenocysteine are found in proteins. Selenocysteine is incorporated via recoding a UGA stop codon and is found in glutathione peroxidase.

Step by step solution

01

Understand the Question

The question is asking whether there are amino acids other than the 20 usual amino acids found in proteins and how these are incorporated into proteins. Additionally, an example of such an amino acid and a protein that contains it needs to be provided.
02

Identify Unusual Amino Acids

Yes, there are amino acids other than the usual 20 that are found in proteins. These are known as non-standard or non-canonical amino acids.
03

Explain How Non-Standard Amino Acids Are Incorporated

Non-standard amino acids can be incorporated into proteins through specific and unique mechanisms. Some of these mechanisms include post-translational modifications or being directly encoded by genes with special codons.
04

Provide an Example of a Non-Standard Amino Acid

An example of such an amino acid is selenocysteine. It is incorporated into proteins through the recoding of a UGA stop codon to code for selenocysteine.
05

Identify a Protein Containing the Non-Standard Amino Acid

Selenocysteine is found in several selenoproteins such as glutathione peroxidase, which plays an important role in protecting the cell from oxidative damage.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

non-canonical amino acids
Proteins are primarily built from 20 standard amino acids, but did you know that there are others involved too? These are known as non-canonical amino acids. They differ from the standard ones in their unique chemical properties and roles. These amino acids are not directly coded for by the universal genetic code, making them quite special.
Non-canonical amino acids can occur naturally or can be synthetically produced in labs, used for research and medical purposes.
  • Natural Non-Canonical Amino Acids: Occur in nature through unique biological processes.
  • Synthetic Non-Canonical Amino Acids: Man-made in labs for experimentation and therapy.
Understanding these amino acids helps us grasp the versatility and complexity of protein functions in the cell.
post-translational modifications
Post-translational modifications (PTMs) are a way to introduce non-standard amino acids into proteins.
These modifications occur after a protein has been synthesized in the cell. Through PTMs, proteins can gain new functionalities or improve their stability and performance.
  • Phosphorylation: Addition of a phosphate group to amino acids like serine, threonine, or tyrosine.
  • Methylation: Addition of a methyl group, often impacting histones and gene regulation.
  • Glycosylation: Attachment of sugar molecules, crucial for cell signaling and protein folding.
PTMs provide proteins with enhanced capabilities and are essential for many biological processes.
selenocysteine incorporation
Selenocysteine is a unique amino acid and often called the 21st amino acid. Unlike typical amino acids, it is incorporated into proteins through a special mechanism. The intriguing part is that a UGA stop codon can be recoded to incorporate selenocysteine.
This process requires a special selenocysteine insertion sequence (SECIS) and other factors to recode the UGA stop codon.
This recoding ensures that selenocysteine is included during protein synthesis, adding distinctive properties to the protein.
glutathione peroxidase
Glutathione peroxidase is a key enzyme in the cell that uses selenocysteine for its function. It plays a critical role in protecting the cell from oxidative damage by reducing harmful peroxides.
Here’s how it works:
  • Reduces hydrogen peroxide and organic hydroperoxides into water and alcohols.
  • Uses glutathione as a substrate, which is oxidized in the process.
  • Ensures cellular protection by maintaining the redox balance.
By incorporating selenocysteine, glutathione peroxidase becomes highly effective at maintaining cell health under oxidative stress conditions.

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Most popular questions from this chapter

Most proteins contain more than 100 amino acid residues. If you decided to synthesize a " 100 -mer," with 20 different amino acids available for each position, how many different molecules could you make?

An organic chemist is generally happy with \(95 \%\) yields. If you synthesized a polypeptide and realized a \(95 \%\) yield with each amino acid residue added, what would be your overall yield after adding 10 residues (to the first amino acid)? After adding 50 residues? After 100 residues? Would these low yields be biochemically "satisfactory"? How are low yields avoided, biochemically?

Identify the nonpolar amino acids and the acidic amino acids in the following peptide: Glu-Thr-Val-Asp-Ile-Ser-Ala

If you were to have a mythical amino acid based on glutamic acid, but one in which the hydrogen that is attached to the \(\gamma\) -carbon were replaced by another amino group, what would be the predominant form of this amino acid at \(\mathrm{pH} 4,7,\) and \(10,\) if the \(\mathrm{p} K_{\mathrm{a}}\) value were 10 for the unique amino group?

Suggest a reason why amino acids are usually more soluble at \(\mathrm{pH}\) extremes than they are at neutral pH. (Note that this does not mean that they are insoluble at neutral pH.)
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