Protein or the physical region of the protein that participates in chemical catalysis. In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules. 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
Translocase of the outer membrane internal targeting signals, not all of which have been identified, and include proteins that take on a β-barrel structure, such as Tom40. Some proteins however, that are targeted to the outer mitochondrial membrane contain a hydrophobic tail domain that anchors the protein to the membrane. The translocase of the outer membrane (TOM) forms a complex made of Tom70, Tom22, and Tom20, along with Tom40, Tom7, Tom6, and Tom5. Tom20 and Tom22 are preprotein receptors, which are responsible for recognition of the cleavable presequence possessed by mitochondrial-targeted proteins. Tom70 is also a preprotein receptor and may recognise some cleavable presequence proteins,