Materials for Biomedical Engineering. Mohamed N. Rahaman

Materials for Biomedical Engineering - Mohamed N. Rahaman


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      In nonaqueous environments, such as the lipid bilayer of the cell membrane, the arrangement of the polar and nonpolar groups is opposite to that in aqueous environments. The nonpolar groups predominantly gather on the outside of the protein chain where they can interact with the nonpolar (hydrophobic) side chains of membrane fatty acids whereas the polar groups arrange themselves anywhere they can contact water. For transmembrane proteins, for example, which project through both sides of the membrane, the polar groups are found at each point where the polypeptide chain emerges from the membrane.

      Fibrous Proteins

      Fibrous proteins are abundant in mammalian tissues, particularly in the extracellular matrix, a complex structural network of proteins and other substances surrounding and supporting cells. Extracellular matrix proteins are secreted by cells into their surroundings where they often assemble into sheets or fibers. The most abundant of these proteins is a family called collagen. A major component of skin and bone, collagen is the most abundant protein in mammals, comprising approximately 25% of the total dry protein mass.

Schematic illustration of (a) single alpha -chain composed of the amino acid sequence Gly-X-Y where Gly is the three letter symbol for glycine that occurs at every third position in the sequence, and X and Y are the one-letter symbol of any amino acid. (b) Illustration of triple-helix structure of collagen formed by winding of three alpha -chains around each other.
Amino acid Number of residues per 1000 residues
Glycine 334
Proline 122
Hydroxyproline 96
Acidic or polar (Asn; Glu; Asn) 124
Basic or polar (Lys; Arg; His) 91
Others 233
Schematic illustration of the structure of the modified amino acid residues hydroxyproline and hydroxylysine formed from proline and lysine residues by enzymatic reaction.

      Interchain hydrogen bonding between the carbonyl oxygen (C=O) and amine hydrogen (N–H) stabilizes the triple‐helix structure. Proline also plays a role in stabilizing the structure because it has a limited ability to change its conformation due to the ring structure of its side chain. As the triple‐helix already has a structure with the minimum energy due to the closely wound hydrogen‐bonded chains and the regular pattern of the amino acid residues, changes to its conformation to provide an entropic contribution to its free energy are not required. Thus, collagen molecules maintain a stable triple‐helix structure, unlike globular proteins that can undergo conformational changes.

      2.6.4 Quaternary Structure