1.7 Proteins
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Amino Acid | 氨基酸 | The monomer of proteins, consisting of a central carbon attached to a hydrogen, a carboxyl group, an amine group, and an R group. |
| Polypeptide | 多肽 | A linear chain of many amino acids linked together by peptide bonds. |
| Peptide Bond | 肽键 | The covalent bond formed between the carboxyl group of one amino acid and the amine group of another via dehydration synthesis. |
| R Group (Side Chain) | R基团 (侧链) | The variable part of an amino acid that determines its unique chemical properties (hydrophobic, hydrophilic, or ionic). |
| Denaturation | 变性 | A process in which a protein loses its native 3D shape due to the disruption of weak chemical bonds, resulting in a loss of function. |
| Disulfide Bridge | 二硫键 | A strong covalent bond formed between the sulfur atoms of two cysteine amino acids, stabilizing the tertiary structure of a protein. |
1. The Building Blocks: Amino Acids
Proteins are incredibly diverse molecules that perform most of the work inside cells (acting as enzymes, structural supports, antibodies, and transporters). Despite their vast diversity, all proteins are linear chains (polymers) built from the same set of 20 amino acid monomers.
Every amino acid has the same fundamental core structure: a central carbon atom covalently bound to:
- A hydrogen atom (\(\ce{-H}\))
- A carboxyl group (\(\ce{-COOH}\))
- An amine group (\(\ce{-NH2}\))
- A variable R group (also called a side chain)
AP Exam Exclusion Statement: Amino Acid Structures
Memorizing the specific molecular structures of the 20 different amino acids is beyond the scope of the AP Exam. You only need to know how their R groups are categorized based on their chemical properties!
The Importance of the R Group
The R group is what makes each of the 20 amino acids unique. The R group can be categorized by three possible chemical properties:
- Hydrophobic / Nonpolar: R groups consisting mostly of hydrocarbons. They tend to fold into the interior of a protein, away from water.
- Hydrophilic / Polar: R groups containing oxygen or nitrogen. They tend to position themselves on the exterior of the protein to interact with water.
- Ionic (Charged): R groups that act as acids (negatively charged) or bases (positively charged).
The specific interactions of these R groups determine the ultimate 3D structure and function of that region of the protein.
2. Forming the Polypeptide Chain
Proteins comprise linear chains of amino acids connected by the formation of strong covalent bonds called peptide bonds.
Through a dehydration synthesis reaction, a peptide bond forms between the carboxyl group (\(\ce{-COOH}\)) of one amino acid and the amine group (\(\ce{-NH2}\)) of the next amino acid. A water molecule (\(\ce{H2O}\)) is removed, resulting in a growing peptide chain (a polypeptide).
3. The Four Levels of Protein Structure
A functional protein is not just a loose string of amino acids; it is intricately folded into a highly specific 3D shape. All four levels of a protein's structure determine its ultimate biological function.
- Primary Structure (\(1^\circ\)): The specific, linear sequence of amino acids in a polypeptide. This sequence is directly determined by DNA and ultimately dictates how the entire protein will fold.
- Secondary Structure (\(2^\circ\)): Local folding of the polypeptide chain. This is driven entirely by hydrogen bonding between atoms of the polypeptide backbone (NOT the R groups). The two most common shapes are coiled \(\alpha\)-helices (alpha-helices) and folded \(\beta\)-pleated sheets (beta-pleated sheets).
- Tertiary Structure (\(3^\circ\)): The overall three-dimensional shape of a single polypeptide. This complex folding results from interactions between the R groups of the amino acids. These interactions include hydrogen bonds, hydrophobic interactions, ionic bonds, and strong covalent disulfide bridges (formed between sulfur-containing amino acids).
- Quaternary Structure (\(4^\circ\)): This level arises only when a functional protein consists of multiple polypeptide chains interacting together (e.g., Hemoglobin is made of four separate polypeptide subunits).
4. Structure Dictates Function: Denaturation
Because the function of a protein relies entirely on its specific 3D shape, any change to that shape can destroy its ability to work. When a protein unravels and loses its native shape due to changes in its physical or chemical environment (like high heat, extreme pH, or certain chemicals), the process is called denaturation.
Real-World Application: Perming Hair
Hair is primarily made of a protein called keratin, which relies heavily on strong covalent disulfide bridges to maintain its shape (tertiary structure).
When a person gets a "perm" (烫头发), a stylist applies a chemical that specifically breaks these disulfide bridges. The hair is then wrapped around curlers into a new shape. Finally, a second chemical (a neutralizer) is applied to rebuild the disulfide bridges in this new position.
By chemically altering the interactions of the R groups, the 3D structure of the protein is permanently changed, altering the phenotype of the hair from straight to curly!
Another classic example is Sickle Cell Anemia. A genetic mutation causes just one amino acid in the primary sequence of hemoglobin to be changed from hydrophilic to hydrophobic. This single alteration changes the tertiary/quaternary folding of the protein, causing the red blood cells to deform into a sickle shape, severely impacting their function.
Quiz
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