Proteins: The Workhorses of the Cell
Proteins are the most abundant macromolecules because they perform a vast array of functions, acting as the "tools" of the cell.
- Proteins have very specific jobs, like specialized tools in a kitchen.
- The building blocks of proteins are amino acids.
- Proteins are made of amino acids, similar to how a brick wall is made of bricks and mortar.
- Breaking down a protein yields amino acids.
Enzymes: Biological Catalysts
Many proteins function as enzymes.
- Enzymes facilitate almost all chemical reactions within a cell (99.99%).
- Each enzyme typically performs a single, specific task, analogous to a can opener.
Signaling Proteins: Long-Lasting Messengers
Signaling proteins transmit sustained messages throughout the body.
- These proteins circulate systemically, continuously informing cells of their functions.
Protein Shape and Function
The shape of a protein dictates its function.
- Fibrous proteins (e.g., keratin) are fiber-like.
- Globular proteins (common enzymes) have specific shapes suited to their roles.
Changes in pH, temperature, or chemical exposure can denature proteins, causing them to lose their shape and function.
- Denaturation involves unfolding or misfolding, altering the protein's form.
- Denaturation is analogous to a can opener being damaged to the point where it cannot open cans.
- The body maintains specific temperatures and pH levels to prevent protein denaturation.
- When proteins, especially enzymes, denature, cellular processes cease.
Amino Acid Structure
Amino acids, the building blocks of proteins, have a consistent structure:
- A central carbon atom
- An amino group
- A carboxyl group
- A hydrogen atom
- An "R" side chain: "R"
- The R group determines the unique characteristics of each amino acid (charge, acidity, etc.).
Polypeptides and Peptide Bonds
A polypeptide is a chain of amino acids.
- A polypeptide is not yet a protein; it must fold into a specific shape to function.
- Analogy: a bicycle in parts is not a bicycle until assembled.
- Peptide bonds link amino acids in a polypeptide chain.
- A polypeptide chain is a chain made of multiple peptide bonds.
Levels of Protein Structure
Primary Structure
The primary structure is the sequence of amino acids.
- It involves the specific order of amino acids connected by peptide bonds.
Secondary Structure
Secondary structure arises from hydrogen bonds between amino acids.
- This leads to shapes like alpha-helices and beta-pleated sheets.
- These shapes begin to condense the polypeptide chain.
Tertiary Structure
The tertiary structure is the overall three-dimensional shape of the folded polypeptide chain.
- It results from interactions between the R groups of amino acids.
- Some proteins are functional at this level.
Quaternary Structure
The quaternary structure involves multiple tertiary structures combining to form a single protein complex.
- Analogy: A hammer consists of a metal head and a wooden handle, each subunit contributing to the overall function.