why methionine is starting amino acids
Methionine is considered a starting amino acid because it acts as the initiator for protein synthesis in many organisms, including humans. It is the first amino acid incorporated into the growing peptide chain during translation in the ribosome.
One reason for this is that methionine has a special codon, AUG, which serves as the start codon for protein synthesis. The start codon signals the ribosome to begin translation and allows it to recognize the correct reading frame for mRNA.
Additionally, methionine is involved in various important cellular processes, such as protein synthesis, metabolism, and detoxification. It plays a crucial role in the initiation of translation and is necessary for proper protein folding and function.
Methionine is also an essential amino acid, meaning it cannot be synthesized by the body and needs to be obtained from dietary sources. Therefore, ensuring that methionine is available at the beginning of protein synthesis helps to ensure the proper assembly of proteins in the cell.
Methionine is often referred to as the "starting" amino acid because it plays a critical role in initiating protein synthesis in cells. Here are the step-by-step reasons why methionine is considered the starting amino acid:
1. Start codon recognition: Protein synthesis begins with the recognition of a specific start codon (usually AUG) on the messenger RNA (mRNA) molecule by the ribosome, the cellular machinery responsible for translating the mRNA into a protein.
2. Methionine as the initiator tRNA: The start codon signals the recruitment of a specific initiator transfer RNA (tRNA) molecule, known as tRNAiMet or initiator methionine tRNA. This tRNA carries the amino acid methionine, which will be the first amino acid incorporated into the growing polypeptide chain.
3. Formation of the initiation complex: The tRNAiMet with methionine binds to the start codon on the mRNA, forming an initiation complex. This complex also involves various initiation factors and the small ribosomal subunit.
4. Assembly of the ribosome: The large ribosomal subunit then joins the small subunit, completing the assembly of the ribosome. This results in the formation of an active ribosome ready for protein synthesis.
5. Peptide bond formation: Once the initiation complex is formed, the ribosome "reads" the mRNA sequence, aligning it with the appropriate complementary tRNA molecules carrying the corresponding amino acids. The ribosome catalyzes the formation of a peptide bond between the methionine amino acid on the initiator tRNA and the next amino acid in the sequence.
6. Elongation of the polypeptide chain: The ribosome continues to move along the mRNA, synthesizing the polypeptide chain by adding amino acids one by one in a specific order dictated by the mRNA sequence.
In summary, methionine is considered the starting amino acid because it serves as the first amino acid in the synthesis of proteins. It is specifically selected and incorporated into the growing polypeptide chain during the initiation stage of protein synthesis.
Methionine is often called a "starting amino acid" because it serves as the initiating amino acid during protein synthesis. When a cell is building a protein, the process begins with the synthesis of the mRNA transcript, which contains the genetic information for the particular protein. This mRNA is then transported to the ribosomes, the cellular structures responsible for protein synthesis.
The ribosomes "read" the genetic information on the mRNA and guide the assembly of the protein by linking together individual amino acids. However, before the ribosome can start assembling the protein, it needs a specific signal to know where exactly to begin. This signal is called a start codon, which is a specific sequence of three nucleotides in the mRNA.
In most cases, the start codon is the AUG codon, which codes for the amino acid methionine. Methionine serves as the first amino acid incorporated into the growing protein chain during translation. Once the methionine is in place, the ribosome continues reading the mRNA transcript and adds additional amino acids to build the protein.
So, methionine is referred to as a starting amino acid because it is the first amino acid incorporated into the protein during translation, as dictated by the start codon in the mRNA.