Permanent Waving Is Biochemical Engineering

When hair is exposed to moist heat, it can be stretched. At the molecular level, the a helices in the a-keratin of hair are stretched out until they arrive at the fully extended 3 conformation. On cooling they spontaneously revert to the a-helical conformation. The characteristic "stretchability" of a-keratins, and their numerous disulfide cross-linkages, are the basis of permanent waving. The hair to be waved or curled is first bent around a form of appropriate shape. A solution of a reducing agent, usually a compound containing a thiol or sulfhydryl group (—SH), is then applied with heat. The reducing agent cleaves the cross-linkages by reducing each disulfide bond to form two Cys residues. The moist heat breaks hydrogen bonds and causes the a-helical structure of the polypeptide chains to uncoil. After a time the reducing solution is removed, and an oxidizing agent is added to establish new disulfide bonds between pairs of Cys residues of adjacent polypeptide chains, but not the same pairs as before the treatment. After the hair is washed and cooled, the polypeptide chains revert to their a-helical conformation. The hair fibers now curl in the desired fashion because the new disulfide cross-linkages exert some torsion or twist on the bundles of a-helical coils in the hair fibers. A permanent wave is not truly permanent, because the hair grows; in the new hair replacing the old, the a-keratin has the natural, nonwavy pattern of disulfide bonds.

Disulfide Bond PermDisulfide Bonds Collagen

(a)The a chain of collagen has a repeating secondary structure unique to this protein. The repeating tripeptide sequence Gly-X-Pro or Gly-X-4-Hyp adopts a left-handed helical structure with three residues per turn. The repeating sequence used to generate this model is Gly-Pro-4-Hyp. (b) Space-filling model of the same a chain. (c)Three of these helices (shown here in gray, blue, and purple) wrap around one another with a right-handed twist. (d) The three-stranded collagen superhelix shown from one end, in a ball-and-stick representation. Gly residues are shown in red. Glycine, because of its small size, is required at the tight junction where the three chains are in contact. The balls in this illustration do not represent the van der Waals radii of the individual atoms. The center of the three-stranded superhelix is not hollow, as it appears here, but is very tightly packed.

from collagen; it has little nutritional value as a protein, because collagen is extremely low in many amino acids that are essential in the human diet. The unusual amino acid content of collagen is related to structural constraints unique to the collagen helix. The amino acid sequence in collagen is generally a repeating tripeptide unit, Gly-X-Y, where X is often Pro, and Y is often 4-Hyp. Only Gly residues can be accommodated at the very tight junctions between the individual a chains (Fig. 4-12d); The Pro and 4-Hyp residues permit the sharp twisting of the collagen helix. The amino acid sequence and the supertwisted quaternary structure of collagen allow a very close packing of its three polypep-tides. 4-Hydroxyproline has a special role in the structure of collagen—and in human history (Box 4-3).

The tight wrapping of the a chains in the collagen triple helix provides tensile strength greater than that

Collagen Striations

Heads of collagen Cross-striations molecules 640 Â (64 nm)

Heads of collagen Cross-striations molecules 640 Â (64 nm)

Collagen Striations

Section of collagen molecule

FIGURE 4-13 Structure of collagen fibrils. Collagen (Mr 300,000) is a rod-shaped molecule, about 3,000 A long and only 15 A thick. Its three helically intertwined a chains may have different sequences, but each has about 1,000 amino acid residues. Collagen fibrils are made up of collagen molecules aligned in a staggered fashion and cross-linked for strength. The specific alignment and degree of cross-linking vary with the tissue and produce characteristic cross-striations in an electron micrograph. In the example shown here, alignment of the head groups of every fourth molecule produces striations 640 A apart.

Section of collagen molecule

FIGURE 4-13 Structure of collagen fibrils. Collagen (Mr 300,000) is a rod-shaped molecule, about 3,000 A long and only 15 A thick. Its three helically intertwined a chains may have different sequences, but each has about 1,000 amino acid residues. Collagen fibrils are made up of collagen molecules aligned in a staggered fashion and cross-linked for strength. The specific alignment and degree of cross-linking vary with the tissue and produce characteristic cross-striations in an electron micrograph. In the example shown here, alignment of the head groups of every fourth molecule produces striations 640 A apart.

of a steel wire of equal cross section. Collagen fibrils (Fig. 4-13) are supramolecular assemblies consisting of triple-helical collagen molecules (sometimes referred to as tropocollagen molecules) associated in a variety of ways to provide different degrees of tensile strength. The a chains of collagen molecules and the collagen molecules of fibrils are cross-linked by unusual types of co-valent bonds involving Lys, HyLys (5-hydroxylysine; see Fig. 3-8a), or His residues that are present at a few of the X and Y positions in collagens. These links create uncommon amino acid residues such as dehydrohy-droxylysinonorleucine. The increasingly rigid and brittle character of aging connective tissue results from accumulated covalent cross-links in collagen fibrils.

Collagen Gly Pro Hyp

(a)The a chain of collagen has a repeating secondary structure unique to this protein. The repeating tripeptide sequence Gly-X-Pro or Gly-X-4-Hyp adopts a left-handed helical structure with three residues per turn. The repeating sequence used to generate this model is Gly-Pro-4-Hyp. (b) Space-filling model of the same a chain. (c)Three of these helices (shown here in gray, blue, and purple) wrap around one another with a right-handed twist. (d) The three-stranded collagen superhelix shown from one end, in a ball-and-stick representation. Gly residues are shown in red. Glycine, because of its small size, is required at the tight junction where the three chains are in contact. The balls in this illustration do not represent the van der Waals radii of the individual atoms. The center of the three-stranded superhelix is not hollow, as it appears here, but is very tightly packed.

XCH—CH2—CH2—CH2—CH=N—CH2—CH—CH2—CH2—CH

Polypeptide Lys residue HyLys Polypeptide chain minus e-amino residue chain group (norleucine)

Dehydrohydroxylysinonorleucine

Fibroin Structure And Function
FIGURE 4-14 Structure of silk. The fibers used to make silk cloth or a spider web are made up of the protein fibroin. (a) Fibroin consists of layers of antiparallel ß sheets rich in Ala (purple) and Gly (yellow) residues. The small side chains interdigitate and allow close packing
Fibroin Antiparallel

of each layered sheet, as shown in this side view. (b) Strands of fibroin (blue) emerge from the spinnerets of a spider in this colorized electron micrograph.

TA typical mammal has more than 30 structural variants of collagen, particular to certain tissues and each somewhat different in sequence and function. Some human genetic defects in collagen structure illustrate the close relationship between amino acid sequence and three-dimensional structure in this protein. Osteogenesis imperfecta is characterized by abnormal bone formation in babies; Ehlers-Danlos syndrome is characterized by loose joints. Both conditions can be lethal, and both result from the substitution of an amino acid residue with a larger R group (such as Cys or Ser) for a single Gly residue in each a chain (a different Gly residue in each disorder). These single-residue substitutions have a catastrophic effect on collagen function because they disrupt the Gly-X-Y repeat that gives collagen its unique helical structure. Given its role in the collagen triple helix (Fig. 4-12d), Gly cannot be replaced by another amino acid residue without substantial deleterious effects on collagen structure. ■

Silk Fibroin Fibroin, the protein of silk, is produced by insects and spiders. Its polypeptide chains are predominantly in the 3 conformation. Fibroin is rich in Ala and Gly residues, permitting a close packing of 3 sheets and an interlocking arrangement of R groups (Fig. 4-14). The overall structure is stabilized by extensive hydrogen bonding between all peptide linkages in the polypeptides of each 3 sheet and by the optimization of van der Waals interactions between sheets. Silk does not stretch, because the 3 conformation is already highly extended (Fig. 4-7; see also Fig. 4-15). However, the structure is flexible because the sheets are held together by numerous weak interactions rather than by covalent bonds such as the disulfide bonds in a-keratins.

of each layered sheet, as shown in this side view. (b) Strands of fibroin (blue) emerge from the spinnerets of a spider in this colorized electron micrograph.

Lose 10 Pounds Naturally

Lose 10 Pounds Naturally

Studies show obesity may soon overtake tobacco as the leading cause of death in America. Are you ready to drop those extra pounds you've been carrying around? Awesome. Let's start off with a couple positive don't. You don't need to jump on a diet craze and you don't need to start exercising for hours each day.

Get My Free Ebook


Responses

  • aira
    How is permanent waving a biochemical engineering?
    3 years ago

Post a comment