 Proteins can be informally divided into three main classes , which <unk> with typical tertiary structures : globular proteins , fibrous proteins , and membrane proteins . Almost all globular proteins are soluble and many are enzymes . <unk> proteins are often structural , such as <unk> , the major component of <unk> tissue , or keratin , the protein component of hair and nails . Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane . 
