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Two tyrosines separated by a single amino acid, typically valine or another tyrosine, form a short intra-molecular diphenylether crosslink. [11] This can be crosslinked further by the enzyme extensin peroxidase [12] [13] [14] to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.
Hydroxyproline is a major component of the protein collagen, [3] comprising roughly 13.5% of mammalian collagen. Hydroxyproline and proline play key roles for collagen stability. [4] They permit the sharp twisting of the collagen helix. [5]
Additionally, structural proteins (1-5%) are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP), arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence.
In 1954, Ramachandran & Kartha (13, 14) advanced a structure for the collagen triple helix on the basis of fiber diffraction data. It consists of a triple helix made of the repetitious amino acid sequence glycine-X-Y, where X and Y are frequently proline or hydroxyproline. [2] [3] Collagen folded into a triple helix is known as tropocollagen.
O-GalNAc sugars are common on membrane glycoproteins, where they help increase rigidity of the region close to the membrane so that the protein extends away from the surface. [6] For example, the low-density lipoprotein receptor (LDL) is projected from the cell surface by a region rigidified by O-glycans.
Glycoproteins are proteins which contain oligosaccharide (sugar) chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification .
HRG is a glycoprotein of 70-75kDa present at a relatively high concentration in the plasma of vertebrates.The primary structure of human HRG is predicted to be a 507 amino acid multidomain polypeptide consisting of two cystatin-like regions at the N-terminus, a histidine-rich region (HRR) flanked by proline-rich regions (PRR), and a C-terminal domain. [10]
O-linked glycans attached to the hydroxyl oxygen of serine, threonine, tyrosine, hydroxylysine, or hydroxyproline side-chains, or to oxygens on lipids such as ceramide. Phosphoglycans linked through the phosphate of a phosphoserine. C-linked glycans, a rare form of glycosylation where a sugar is added to a carbon on a tryptophan side-chain.