Proteins
Proteins are the most complex macromolecules in the cell, composed of linear polymers called polypeptides containing amino acids connected by peptide bonds.
Each amino acid contains a central carbon atom attached to four substituents:
A carboxyl group
An amino group
A hydrogen atom
An R group (side chain)
Proteins perform a variety of functions in organisms:
Enzymatic catalysis: They speed up chemical reactions.
Structure: Provide support and shape to cells and tissues.
Transport: Move molecules across cell membranes or throughout the body.
Defense: Form antibodies to protect against pathogens.
Signaling: Act as hormones and cell signaling molecules.
The structure of proteins can be categorized into four levels:
Primary structure: The unique sequence of amino acids in a polypeptide chain.
Secondary structure: Local folding of the polypeptide chain into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
Tertiary structure: The overall three-dimensional shape of a single polypeptide chain, influenced by interactions among side chains.
Quaternary structure: The assembly of multiple polypeptide chains into a functional protein complex.
Proteins can denature, losing