Question

The Direction of Protein Synthesis In 1961, Howard Dintzis established that protein synthesis on ribosomes begins at the amino terminus and proceeds toward the carboxyl terminus. He used immature red blood cells that were still synthesizing hemoglobin. He added radioactively labeled leucine (chosen because it occurs frequently in both the \(a\) and \(\beta\) subunits) for various lengths of time, rapidly isolated only the full-length (completed) \(a\) subunits, and then determined where in the peptide the labeled amino acids were located. After the labeled leucine and extract had been incubated together for one hour, the protein was labeled uniformly along its length. However, after much shorter incubation times, the labeled amino acids were clustered at one end. At which end, amino or carboxyl terminus, did Dintzis find the labeled residues after the short exposure to labeled leucine?

Short answer

The labeled residues were found at the amino terminus after short exposure.

Step-by-step solution

Understanding Protein Synthesis

Protein synthesis occurs in a specific direction on the ribosome. The synthesis begins at the amino (N) terminus and continues to the carboxyl (C) terminus as new amino acids are added.

Experiment Design Analysis

Howard Dintzis used red blood cells actively synthesizing hemoglobin to study the direction of protein synthesis. He introduced radioactively labeled leucine into the system, which was incorporated into newly synthesized proteins.

Interpreting Short Incubation Results

After a short incubation with the labeled leucine, the newly synthesized amino acids showed clusters of labeled residues at one end of the hemoglobin chain. This indicates the start of the protein synthesis.

Determining the Direction of Synthesis

In Dintzis's experiment, if the labeled residues clustered at the same end, it would represent the recently synthesized portion of the chain. Given that synthesis starts at the amino terminus, clusters at the amino terminus suggest recent synthesis.

Conclusion

Based on Dintzis’s results, the labeled residues after short exposure were found at the amino terminus, confirming that protein synthesis proceeds from the amino to the carboxyl terminus.

Key concepts

Ribosome Function

Ribosomes are essential molecular machines in cells responsible for synthesizing proteins. They serve as the sites where genetic information, in the form of messenger RNA (mRNA), is translated into proteins. This process is critical for cellular function and overall organismal health.

The ribosome is composed of two subunits: the small and the large subunits. These two parts come together to form a complete ribosome when synthesizing proteins. The small subunit binds to the mRNA, while the large subunit facilitates the binding of transfer RNA (tRNA) and the formation of peptide bonds.

The ribosomes read the mRNA in segments of three bases known as codons. Each codon corresponds to a specific amino acid, directing the tRNA to deliver the correct building block for the protein chain being assembled. This process continues until a stop codon is encountered, signaling the ribosome to release the completed protein.

• The small subunit reads mRNA.
• The large subunit forms peptide bonds.
• Translation ends at stop codons.

Directionality of Protein Synthesis

Protein synthesis is a highly directional process that starts at the amino (N) terminus of a protein and proceeds towards the carboxyl (C) terminus. This directionality is a fundamental aspect of protein biosynthesis and ensures proper assembly of the protein sequence.

In Howard Dintzis's classic experiment, it was demonstrated that protein synthesis initiates at the amino terminus. By using radioactively labeled leucine, Dintzis could track where new amino acids were incorporated during hemoglobin synthesis in red blood cells. Following brief periods of incubation, the experimental results showed clusters of labeled leucine at the amino terminus, confirming that this is where synthesis starts.

This directed flow of synthesis is not arbitrary. The ribosome moves along the mRNA chain in a 5' to 3' direction, ensuring that proteins elongate correctly as the ribosome reads the codons.

• Synthesis starts at the amino terminus.
• Experiments confirmed the directional flow.
• Ribosomes move in a 5' to 3' direction along mRNA.

Experiment Design in Biochemistry

Designing experiments in biochemistry requires careful selection of materials, well-thought-out procedures, and controls to ensure reliable and valid results. In the context of protein synthesis, Dintzis's experiment serves as a model for how to effectively design and execute a biochemical study.

First, it is crucial to select a suitable biological system. Dintzis chose immature red blood cells because they were still actively synthesizing proteins. This choice allowed him to observe protein synthesis in real-time. Choosing the right biochemical markers is another important element. In this experiment, radioactively labeled leucine was the marker of choice because it appears frequently in hemoglobin, making it easier to monitor incorporation.

Another key aspect is the timing and conditions of the experiment. Short incubations helped identify which end of the protein became labeled first. This strategic approach enables researchers to pinpoint specific stages in a biochemical process. Finally, interpreting the data correctly is vital. By correlating the labeled cluster's location with known synthesis directionality, reliable conclusions can be drawn.

• Choose an active biological system.
• Select appropriate biochemical markers.
• Use strategic timing to observe processes.
• Ensure accurate data interpretation.