A SUBUNIT MODEL FOR THE TROPOCOLLAGEN MACROMOLECULE

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Abstract
A subunit model is presented for the triple-stranded tropocollagen macromolecule, based on two kinds of poly-peptide subunits having lengths in the ratio 75. The three constituent strands of molecular weight ca. 100,000 are postulated to terminate in register at a length equal to the "beat period" of the two subunit lengths. Each of the two [alpha]1 strands is described as a repeating sequence of five identical subunits [sigma]1; the single [alpha]2 strand, as a repeating sequence of seven identical subunits [sigma]2. The assignment of five and seven subunits to the two kinds of strands is consistent with various experimental data but rests primarily on an analysis of the band pattern in SLS crystallites of the calfskin TC macromolecule observed in the EM after phosphotungstic-acid staining. It is found that both the intensities and positions of the bands can be fitted satisfactorily to a 5:7 subunit model of the type proposed. When normalized to the observed length of the TC macromolecule, the model indicates that the subunit lengths l1 and l2, the period D in the native-type fibril, and the over-all molecular length L are all integral multiples of a quantity lo = 86 [plus or minus] 1 A spanning 30 [plus or minus] 1 residues per strand. It yields l1 = 71o, 12 = 510, D = 810, L = 351o. The quantity lo equals one complete, right-handed turn of the tree strands around a common axis in the Rich-Crick model for the collagen triple helix.

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