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Globular proteins are somewhat water-soluble (forming colloids in water), unlike the fibrous or membrane proteins. [1] There are multiple fold classes of globular proteins, since there are many different architectures that can fold into a roughly spherical shape. The term globin can refer more specifically to proteins including the globin fold. [2]
They are one of the common types of protein along with soluble globular proteins, fibrous proteins, and disordered proteins. [5] They are targets of over 50% of all modern medicinal drugs. [ 6 ] It is estimated that 20–30% of all genes in most genomes encode membrane proteins.
A fibrous protein's peptide sequence often has limited residues with repeats; these can form unusual secondary structures, such as a collagen helix. The structures often feature cross-links between chains (e.g., cys-cys disulfide bonds between keratin chains). Fibrous proteins tend not to denature as easily as globular proteins.
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.
Collagen is one of the long, fibrous structural proteins whose functions are quite different from those of globular proteins, such as enzymes. Tough bundles of collagen called collagen fibers are a major component of the extracellular matrix that supports most tissues and gives cells structure from the outside, but collagen is also found inside ...
Membrane receptor proteins relay signals between the cell's internal and external environments. Transport proteins move molecules and ions across the membrane. They can be categorized according to the Transporter Classification database. Membrane enzymes may have many activities, such as oxidoreductase, transferase or hydrolase. [3]
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils.It is found in essentially all eukaryotic cells, where it may be present at a concentration of over 100 μM; its mass is roughly 42 kDa, with a diameter of 4 to 7 nm.
A structural domain is a compact, globular sub-structure with more interactions within it than with the rest of the protein. [68] Therefore, a structural domain can be determined by two visual characteristics: its compactness and its extent of isolation. [69]