CHAPTER SUMMARY
4.1 PROTEINS ARE LINEAR POLYMERS OF AMINO ACIDS THAT FORM THREE-DIMENSIONAL STRUCTURES WITH SPECIFIC FUNCTIONS.
- An amino acid consists of an alpha carbon connected by covalent bonds to an amino group, a carboxyl group, a hydrogen atom, and a side chain or R group.
- There are 20 common amino acids that differ in their side chains.
- Amino acids are connected by peptide bonds to form proteins.
- The primary structure of a protein is its amino acid sequence. The primary structure determines how a protein folds, which in turn determines how it functions.
- The secondary structure of a protein results from the interactions of nearby amino acids. Examples include the alpha helix and beta sheet.
- The tertiary structure of a protein is its three-dimensional shape, which results from long-range interactions of amino acid side chains.
- Some proteins are made up of several polypeptide subunits; this group of subunits is the protein’s quaternary structure.
- Chaperones help some proteins fold properly.
4.2 TRANSLATION IS THE PROCESS BY WHICH THE SEQUENCE OF BASES IN MESSENGER RNA SPECIFIES THE ORDER OF SUCCESSIVE AMINO ACIDS IN A NEWLY SYNTHESIZED PROTEIN.
- Translation requires many cellular components, including ribosomes, tRNAs, and proteins.
- mRNAs have three possible reading frames.
- tRNAs have an anticodon that base pairs with the codon in the mRNA and carries a specific amino acid.
- Aminoacyl tRNA synthetases attach specific amino acids to tRNAs.
- The genetic code defines the relationship between the three-letter codons of nucleic acids and their corresponding amino acids. It was deciphered using synthetic RNA molecules.
- The genetic code is redundant in that many amino acids are specified by more than one codon.
- Translation consists of three steps: initiation, elongation, and termination.
4.3 PROTEINS EVOLVE BY COMBINING FUNCTIONAL UNITS AND THROUGH MUTATION AND SELECTION.
- Protein families are groups of proteins that are structurally and functionally related.
- There are far fewer protein families than the total number of possible proteins because the probability that a random sequence of amino acids will fold properly to carry out a specific function is very small.
- A region of a protein that folds in a particular way and that carries out a specific function is called a folding domain.
- Proteins evolve by combining different folding domains.
- Changes in the amino acid sequence of a protein occur by mutation and selection.