Proteins may also serve as enzymes, hormones, antibodies, neurotransmitters, or oxygen carriers (like hemoglobin).
Proteins are built of chains of amino acids.
- All amino acids contain Carbon, Nitrogen, Oxygen, and Hydrogen.
Some also contain sulfur.- Each amino acid contains an amino group, –NH2
— — and a carboxylic acid group, –COOH , bonded to the same carbon.- Different amino acids have different side chains (symbolized as –R)
attached to the same central carbon.- Twenty amino acids commonly occur in proteins.
- Eight amino acids are considered essential for adult human nutrition
because we cannot make them ourselves; we need them in our diet.- Most animal proteins, but few plant proteins, have all 8 essential amino acids, and are thus considered nutritionally complete.
- Primary structure is a sequence of amino acids
connected by peptide bonds.
- Secondary structures are shapes held together by weak hydrogen bonds:
- Alpha helix (most common) has 3 amino acids per 360o turn.
- Beta pleats form sheets with zig-zag folds.
- Tertiary structure is the overall 3-dimensional shape, determined by disulfide bridges
or by electrical forces between positive and negative charges.
Many proteins are folded into compact, globular shapes. - Quarternary structure (in some proteins only):
Some proteins cannot function as enzymes or oxygen carriers
unless several tertiary structures are combined. - Conjugated proteins contain a sugar or lipid component.