 Proteins can be informally divided into three main classes , which correlate with typical tertiary structures : globular proteins , fibrous proteins , and membrane proteins . Almost all globular proteins are soluble and many are enzymes . Fibrous proteins are often structural , such as collagen , the major component of connective 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 . 
 Proteins can be informally divided into three main classes , which correlate with typical tertiary structures : globular proteins , fibrous proteins , and membrane proteins . Almost all globular proteins <a href='drop_aux' title='1'><mark>are</mark></a>soluble and many <a href='drop_aux' title='2'><mark>are</mark></a>enzymes . Fibrous proteins <a href='drop_aux' title='3'><mark>are</mark></a>often structural , such as collagen , the major component of connective 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 . 
Proteins can be informally divided into three main classes , which correlate wit typical tertiary structures : globular proteins , fibrous proteins , and membrane proteins . Almost alll globular proteins soluble and many enzymes . Fibrous proteins often structural , such as collagen , da major component of connective tissue , or keratin , the protein component of hair and nails . Membrane proteins often serve as receptors or provide channels fir polar or charged molecules to pass through the cell membrane .