Fibrous tertiary structures

Proteins typically have either globular or fibrous tertiary structures. These tertiary structures do not occur randomly. Under the proper environmental conditions the tertiary structure of a protein occurs in one particular way—a way that is characteristic of that particular protein and one that is often highly important to its function. 

Collagen, the principal fibrous protein in mammalian tissue, has a tertiary structure made up of twisted a-helices. Three polypeptide chains, each of which is a left-handed helix, are twisted into a right-handed super helix to form an extremely strong tertiary structure. It has remarkable tensile strength, which makes it important in the structure of bones, tendons, teeth, and cartilage. 

Protein Architecture—Tertiary Structure of Fibrous Proteins... 

Tertiary structure is the complete three-dimensional structure of a polypeptide chain. There are two general classes of proteins based on tertiary structure fibrous and globular. 

The previous chapter described the types of secondary and tertiary structures that are the bricks-and-mortar of protein architecture. By arranging these fundamental structural elements in different combinations, widely diverse proteins can be constructed that are capable of various specialized functions. This chapter examines the relationship between structure and function for the clinically important globular hemeproteins. Fibrous structural proteins are discussed in Chapter 4. 

Also, increase in water temperature favors -conformation. Inasmuch as the conformation of CP probably determines the tertiary structure of MM, slight changes in CP conformation introduced by cellular or environmental effects may alter MM conformation. This in turn is reflected in the mineral form and structural pattern of the inorganic phases. Perhaps, nacreous layers in molluscs represent an almost ideal situation where MM and CP are aligned in a symmetrical way. In fish otoliths, the fibrous organic matrix is a mixture of helices and 0-pleated sheets. It is tentatively concluded that the morphology of shell structures is a macroscopic expression of the molecular interactions between MM and CP which are controlled in part by cellular activities and in part by the environment. 

Proteins may be fibrous or globular. The structure and polarity of the particular amino acid R groups and their sequence affect the solubility properties and tertiary structure of proteins. Quaternary structure refers to the aggregation of similar protein subunits. 

Review Secs. 17.13 and 17.14. Secondary structures of importance are hydrogen bonds and the geometry of the peptide bond. The tertiary structures of fibrous sproteins are dominated by p-sheets (the presence of small amino acids with nonpolar side chains) and, in many cases, significant amounts of cysteine (the amino acid where R=CH2SH) cross-linked by disulfide bonds. 

Coils such as those found in alpha-keratin are not the only structural motifs present in fibrous proteins. Silk, for example, is largely composed of fibrous proteins whose structures resemble interleaved sheets, see also Quaternary Structure Secondary Structure Tertiary Structure. 

Side chains of the amino acids participate in tertiary (3°) structure, that is, they stabilize the overall conformation of the protein molecule. The forces which hold tertiary structure together include covalent (disulfide bridges) and noncovalent (hydrogen bonding, salt bridge, hydrophobic) interactions. Shapes of tertiary structure subunits can be globular or fibrous. 

Most fibrous proteins, such as silk, collagen, and the a-keratins, are almost completely insoluble in water. (Our skin would do us very little good if it dissolved in the rain.) The majority of cellular proteins, however, are soluble in the cell cytoplasm. Soluble proteins are usually globular proteins. Globular proteins have three-dimensional structures called the tertiary structure of the protein, which are distinct from their secondary structure. The pol)cpeptide chain with its regions of secondary structure, a-helix and fj-pleated sheet, further folds on itself to achieve the tertiary structure. 

Solubility is one property that can be used to classify the proteins that result from the various levels of structure. For example, fibrous proteins are not soluble in water. Many familiar components of tissues are composed of fibrous proteins, including keratin (the protein present in hair), collagen (a structural protein found in tendons and cartilage), myosin (a protein found in most muscle tissue), and fibrin (the protein that allows blood to clot and form scabs). Conversely, globular proteins are soluble in water. For example, albumins are water-soluble proteins that provide a familiar example of what happens when a protein loses its secondary and tertiary structure, a process called denaturation. When an egg is cooked, the egg white changes from translucent to white this color change is indicative of the change in structure that has taken place in the albumin proteins. 

Proteins can also be dangerous or unhealthy. For many who suffer from allergies to agents like pollen, it is proteins on the surface of the pollen that cause an immune response that triggers the allergic reaction. More seriously, many natural toxins are proteins. Snake venom is one example of a naturally occurring protein-based toxin, see also Active Site Amino Acid Denaturation Enzymes Fibrous Proteins Globular Proteins Neurotransmitters Peptide Bond Protein Solubility Protein Synthesis Protein Translation RNA Synthesis Secondary Structure Tertiary Structure Transmembrane Proteins Venom. 

Fibrous proteins Globular proteins Alpha-amino acids Side chains Dipeptide Peptide linkage Polypeptide Primary structure Secondary structure Alpha-helix Pleated sheet Tertiary structure Disulfide linkage Denaturation Enzymes... 

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