Two main groups

The enormous variety of proteins falls into two groups:

1. Structural proteins – building materials of the organism. Examples: actin and myosin of muscle and other contractile systems; collagen in connective tissue and ligaments; keratin in skin, hair, hooves, horns and feathers.
2. Regulatory (functional) proteins – regulate the processes of the organism: enzymes (control chemical reactions), antibodies (immunity), hormones, and other substances that let the organism respond to changing internal and external conditions.