BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 9. CONNECTIVE TISSUE PROTEINS: COLLAGEN, ELASTIN, AND PROTEOGLYCANS

9.6. The small size of glycine makes it an indispensable structural component

We can now ask: why is every third position in the Amino Acid Sequence of tropocollagen occupied by Glycine? The interior of the triple-helical fiber is very crowded (Fig. 9.10). In fact, no amino acid residue other than glycine could fit inside the triple helix. Since there are three amino acid residues per turn of the helix, every third residue in each of the chains must inevitably be glycine. The two residues on either side of glycine are located on the outside of the triple helix, where the bulky rings of Proline and hydroxyproline residues can easily be accommodated.

Class="center">Fig. 9.10. Cross section through a model of Collagen. Each chain is hydrogen-bonded to the other two chains (dots indicate Hydrogen Bonds). The α-carbon atom of the glycine residue in each chain is designated by the letter C. Every third residue in the Amino acid sequences must be glycine, because there is no room for a larger residue near the axis (center) of the helix. Note that the pyrrolidine rings are located on the outside. The different chains of the triple helix are shown in different colors.

Of all Amino Acids, glycine might seem to be the least important, since its side chain is merely a single hydrogen atom. However, in the Organization of Cell/13.html">Protein Structure, such simplicity can be an asset. Glycine plays a crucial role precisely because it is small and, taking up very little space, does not prevent polypeptide chains from packing closely together. We have already encountered this in Myoglobin and Hemoglobin, where glycine is invariantly located at position B6, facilitating the close approach of helices B and E. In Chymotrypsin, a large aromatic group can fit into the substrate-binding pocket only because the two residues lining the cavity are glycines. In the Evolution of the electron carrier cytochrome c, glycine has proven to be the most highly conserved amino acid residue. In collagen, we again find that glycine plays an especially vital role, expanding the range of possible Conformations during polypeptide chain folding.

9.7. The stability of the collagen helix depends on cooperative interactions

If a solution of tropocollagen is heated, its physical properties change at a characteristic Temperature (Fig. 9.11). For instance, the viscosity of the solution drops sharply, indicating a loss of the fibrous structure. Judging by The change in optical rotation, the helical STRUCTURE OF THE individual chains disappears. This implies that thermal motion overcomes the forces that stabilize the triple helix, resulting in a disrupted structure—gelatin, which has a random coil configuration. This structural transition occurs abruptly at a specific temperature, analogous to the melting of a crystal. The term "melting," borrowed by biochemistry, has come to be used when the loss of a highly organized structure occurs within a narrow temperature range. The tropocollagen helix is characterized by a high degree of order along its length. The sharpness of the structural transition with increasing temperature indicates that the stabilization of the triple helix is due to cooperative interactions. In other words, the helical structure is maintained by The formation of many mutually reinforcing bonds, each of which is relatively weak on its own. The formation of each of these stabilizing bonds depends heavily on whether neighboring bonds are formed simultaneously. A zipper serves as an example of such a cooperative structure. In subsequent chapters, we will examine several other highly cooperative macromolecular structures: DNA, Viruses, and cell membranes.

Fig. 9.11. Melting curve of a collagen molecule

The temperature at which the helical structure is half-lost is called the melting temperature (T_m). The T_m of tropocollagen serves as a measure of the stability of its triple helix. For intact collagen fibrils, a comparable measure is the shrinkage temperature, T_s. Collagens from different species of organisms vary in their melting temperatures. It turns out that the T_m and T_s of collagens are related to the body temperature of the animal (Table 9.2). Smelt collagens are characterized by the lowest T_m, whereas in warm-blooded animals, T_m reaches the highest values. Differences in thermal stability correlate with the imino acid content (proline and hydroxyproline) in collagen. The higher the imino acid content, the more stable the Collagen helix. The imino acid content in collagen increased during evolution from cold-blooded to warm-blooded animals.

Table 9.2. Dependence of collagen thermal stability on imino acid content

Thus, the Stability of the individual tropocollagen chain helix depends on proline and hydroxyproline, which act as locks. Furthermore, the triple helix is stabilized by interchain Hydrogen bonds and Van der Waals interactions between residues on different chains. The presence of glycine at every third position in The amino acid sequence is a steric requirement for the existence of the superhelix.

Determination of the melting temperature of chemically synthesized polypeptide models of collagen has shed light on the Biological Significance of proline hydroxylation. Thus, it was found that the T_m of poly(Pro-Pro-Gly) is 24°C, whereas the T_m of poly(Pro-Hyp-Gly) is 58°C; this indicates that hydroxylation significantly increases the stability of the triple helix. This Conclusion is supported by studies of unhydroxylated collagen obtained by incubating tendon Cells with α,α'-bipyridyl, a compound that chelates iron and therefore inhibits prolyl hydroxylase. The unhydroxylated collagen synthesized by these cells does not form a triple helix at 37°C, but rapidly folds into a helix at temperatures below 24°C.