BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 27. PROTEIN SYNTHESIS

QUESTIONS AND PROBLEMS

1. The formation of Ile-tRNA proceeds via an enzyme-bound intermediate, Ile-AMP. Would you expect 32P-labeled ATP to be formed from 32PPi upon incubating each of the following sets of components in the presence of the specific activating enzyme?

a) ATP and 32PPi.

b) tRNA, ATP, and 32PPi.

c) Isoleucine, ATP, and 32PPi.

2. Ribosomes were isolated from Bacteria grown on "heavy" (containing 13C and 15N) and "light" (12C and 14N) media. These 70S ribosomes were added to an in vitro system actively synthesizing Proteins. After a few hours, a sample was removed from the mixture and analyzed by density gradient centrifugation. How many 70S ribosome bands would you expect to observe in the density gradient?

3. How many energy-rich phosphate bonds are expended in the synthesis of a protein consisting of 200 residues, starting from ready-made Amino Acids?

4. There are two principal Mechanisms for the elongation of biological molecules (Fig. 27.26). The 1st mechanism involves the Cleavage of the activating group (marked with a cross in the figure) from the growing chain. In the 2nd mechanism, the activating group is cleaved from the monomer being added to the growing chain. Indicate whether the following biosynthetic reactions proceed via the 1st or the 2nd mechanism.

Class="center">Fig. 27.26. Two mechanisms of elongation

a) Glycogen synthesis.

b) Fatty acid synthesis.

c) C5 → C10 → C15 in Cholesterol synthesis.

d) DNA Synthesis.

e) RNA Synthesis.

f) Protein Synthesis.

5. Mutations that result in the generation of termination codons are called nonsense mutations. These mutations can be suppressed by altered tRNAs. For example, a mutant tRNA reads the UGA codon as a Tryptophan codon. What is the most likely base substitution that occurred in this mutant tRNA?

6. Design an affinity labeling reagent for the tRNA-binding sites on the ribosome. How would you synthesize such a reagent?

7. The mRNA transcript of a T7 phage Gene contains the following base sequence:

Predict the consequence of mutating the G indicated by the arrow to an A.

8. What do error correction in PROTEIN SYNTHESIS AND DNA have in common?



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