Proteins play a wide variety of functional roles, including:
• enzymes as catalysts, such as ribonuclease, that accelerate specific chemical reactions up to 10 billion times faster than they would spontaneously occur; one cell may contain some thousand different types of enzymes at one time
• structural materials, including keratin (the protein found in hair and nails) and collagen (the protein found in connective tissue)
• specific binding, such as antibodies that bind specifically to foreign substances to identify them to the body's immune system
• specific carriers, including membrane transport proteins that move substances across cell membranes, and blood proteins, such as hemoglobin, that carry oxygen, iron, and other substances through the body
• contraction, such as actin and myosin fibers that interact in muscle tissue
• signaling, including hormones such as insulin that regulate sugar levels in blood
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