Protein‐decorated micelle structure of sodium‐dodecyl‐sulfate–protein complexes as determined by neutron scattering

Abstract

The structure of the complex between sodium dodecyl sulfate (SDS) and a deuterated bifunctional enzyme, N-5'-phosphoribosylanthranilate isomerase/indole-3-glycerol-phosphate synthase (Mr 49 484), has been studied in dilute solution by small-angle neutron scattering. The complex nearly acquired its final size, as shown by molecularsieve chromatography, at the chosen SDS concentration of 1.6 mM, which is slightly below the critical micelle concentration of 1.8 mM (at the ionic strength of 0.1 M). The 452 amino-acid residues of the bifunctional enzyme were combined with 216 detergent molecules. The complex was found to be composed of three protein-decorated SDS micelles of unequal size, connected by short flexible polypeptide segments. The largest of the three micelles was the middle one. The SDS-protein complex contained the dodecyl hydrocarbon moieties in three globular cores. Each core was surrounded by a hydrophilic shell, formed by the hydrophilic and amphiphilic stretches of the polypeptide chain, and by the sulfate head groups of the detergent. The average thickness of these shells was 0.7-0.8 nm.The three-micelle complex was cleaved with trypsin at a single site, possibly in a micelle-connecting segment, into a single-micelle fragment at the carboxyl-terminal which comprised 73 SDS molecules and 163 amino-acid residues, and a dual-micelle fragment. One of the micelles within this larger fragment contained 42 SDS molecules and about 90 amino-acid residues; the other micelle contained 101 SDS molecules and about 190 amino-acid residues. The individual micelle sizes seemed to be determined by the amino-acid sequence.The anionic detergent sodium dodecyl sulfate (SDS) effects solubilization of, for instance, membrane, ribosomal and viral proteins. SDS is widely used for size separations of these proteins as well as of water-soluble proteins or protein subunits, by gel electrophoresis [l -51 and by molecular-sieve chromatography [6 -121. Knowledge of the structure of SDSprotein complexes can be useful for understanding and for further developing these important methods. SDS binds to water-soluble proteins approximately in proportion to the polypeptide length. On average, the saturated complexes contain one SDS molecule per two amino-acid residues at moderate ionic strength [13].The structure of SDS-protein complexes has not yet been determined in detail. Different and partly contradictory structural models have been proposed, mainly for the SDS complexes with water-soluble proteins. Five models (a -e) are briefly described below. b) The 'rodlike-particle model' was proposed on the basis of hydrodynamic measurements [14]. Upon binding of SDS, the polypeptide chain forms a 'rodlike particle', about 3.6 nm in diameter and 0.074 nmlamino-acid residue in length. A reinterpretation has been made: the particle was described as 'short rigid rodlike segment, with intervening regions possessing some flexibility' [13].c) The 'necklace model', which contradicts the 'rodlike particle' model (b) above, was derived mainly from...

Structural change of jack bean urease induced by addition surfactants studied with synchrotron‐radiation small‐angle X‐ray scattering

“…So about 72 SDS molecules (21 kDd288 Da) would bind to one urease subunit (72/840 = 0.086 SDS molecule/aminoacid residue). However, in the case of SDSPAGE the amount of bound SDS/g protein is known to be about 1.4 g. The recent report using neutron scattering shows that the bound SDS molecules/amino-acid residue are in the range 0.44-0.53 [24]. Comparing with those values it suggests that a relatively small amount of SDS binding was enough to induce the dissociation of urease into its subunits.…”

Photocontrol of Protein Folding: The Interaction of Photosensitive Surfactants with Bovine Serum Albumin

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Phage P22 tailspike protein: Removal of head‐binding domain unmasks effects of folding mutations on native‐state thermal stability

“…When temperature scans were done in the presence of SDS, the protein remained soluble. The CD amplitude increased upon denaturation, when the native P-sheet secondary structure was lost and replaced by the helical secondary structure proteins assumed in association with SDS micelles (Mattice et al, 1976;Ibel et al, 1990). Thermal unfolding transitions so determined for TSPAN wild-type and two mutants are depicted in Figure 3C.…”