A 'give it a go' breast-feeding culture and early cessation among low-income mothers.

OBJECTIVE: to examine cultural expectations and experiences of breast feeding amongst first time mothers from low-income areas, in order to improve understanding of why many cease breast feeding in the early days of their babies' lives. DESIGN: qualitative interviews were carried out with 16 women, who expressed an intention to breast feed, at 37 weeks in their pregnancy and again at 3-9 weeks postnatally. SETTING: women were interviewed in their own homes in low-income areas of North Tyneside, north-east England. FINDINGS: decisions about breast-feeding cessation were usually made within the first few days as women negotiated the pathways of informal cultures of feeding babies and the availability and quality of formal care. A 'give it a go' breast-feeding culture is identified, where women who intended to breast feed had a strong expectation of difficulties and even failure. Expertise and confidence with bottle feeding were more widespread among family and friends. The many influences on the mothers' decision-making were interconnected and contingent upon each other: if one aspect of breast feeding 'goes wrong', other reasons were often brought into play and the underlying pessimism that was felt antenatally was borne out. CONCLUSIONS: positive experiences of formal support could make a crucial difference in the early days of breast feeding. However non-breast-feeding cultures permeated and found expression in negative discourses. Support needs to take account of the cultural contexts in which mothers make decisions and the fact that breast feeding is affected by a multitude of factors simultaneously. Access to advice at the right time is a key issue for some low-income women.

Quantitative analysis of the stabilization by substrate of Staphylococcus aureus PC1 beta-lactamase.

BACKGROUND: The stabilization of enzymes in the presence of substrates has been recognized for a long time. Quantitative information regarding this phenomenon is, however, rather scarce since the enzyme destroys the potential stabilizing agent during the course of the experiments. In this work, enzyme unfolding was followed by monitoring the progressive decrease of the rate of substrate utilization by the Staphylococcus aureus PC1 beta-lactamase, at temperatures above the melting point of the enzyme. RESULTS: Enzyme inactivation was directly followed by spectrophotometric measurements. In the presence of substrate concentrations above the K(m) values, significant stabilization was observed with all tested compounds. A combination of unfolding kinetic measurements and enzymatic studies, both under steady-state and non-steady-state regimes, allowed most of the parameters characteristic of the two concurrent phenomena (i.e. substrate hydrolysis and enzyme denaturation) to be evaluated. In addition, molecular modelling studies show a good correlation between the extent of stabilization, and the magnitude of the energies of interaction with the enzyme. CONCLUSIONS: Our analysis indicates that the enzyme is substantially stabilized towards heat-induced denaturation, independently of the relative proportions of non-covalent Henri-Michaelis complex (ES) and acyl-enzyme adduct (ES*). Thus, for those substrates with which the two catalytic intermediates are expected to be significantly populated, both species (ES and ES*) appear to be similarly stabilized. This analysis contributes a new quantitative approach to the problem.

A basic residue at position 36p of the propeptide is not essential for the correct folding and subsequent autocatalytic activation of prochymosin.

Position 36p in the propeptides of gastric aspartic proteinases is generally occupied by lysine or arginine. This has led to the conclusion that a basic residue at this position, which interacts with the active-site aspartates, is essential for folding and activation of the zymogen. Lamb prochymosin has been shown by cDNA cloning to possess glutamic acid at 36p. To investigate the effect of this natural mutation which appears to contradict the proposed role of this residue, calf and lamb prochymosins and their two reciprocal mutants, K36pE and E36pK, respectively, were expressed in Escherichia coli, refolded in vitro, and autoactivated at pH 2 and 4.7. All four zymogens could be activated to active chymosin and, at both pH values, the two proteins with Glu36p showed higher activation rates than the two Lys36p forms. Glu36p was also demonstrated in natural prochymosin isolated from the fourth stomach of lamb, as well as being encoded in the genomes of sheep, goat and mouflon, which belong to the subfamily Caprinae. A conserved basic residue at position 36p of prochymosin is thus not obligatory for its folding or autocatalytic activation. The apparently contradictory results for porcine pepsinogen A [Richter, C., Tanaka, T., Koseki, T. & Yada, R.Y. (1999) Eur. J. Biochem. 261, 746-752] can be reconciled with those for prochymosin. Lys/Arg36p is involved in stabilizing the propeptide-enzyme interaction, along with residues nearer the N-terminus of the propeptide, the sequence of which varies between species. The relative contribution of residue 36p to stability differs between pepsinogen and prochymosin, being larger in the former.

Overview of protein folding.

This overview discusses aspects of protein folding including aggregation, folding pathways, disulfide bonds, stabilization of the native functional state, and approaches for achieving an acceptable rate of protein folding starting with a denatured polypeptide.

The residual pro-part of cathepsin C fulfills the criteria required for an intramolecular chaperone in folding and stabilizing the human proenzyme.

The 13.5 kDa N-terminal part of the propeptide remains associated with mature cathepsin C after proteolytic activation and excision of the activation peptide. This residual pro-part, isolated from the recombinant enzyme, folds spontaneously and rapidly to a stable, compact monomer with secondary structure and stable tertiary interactions. Folding and unfolding kinetics of the residual pro-part with intact disulfides are complex, and accumulation of transient intermediates is observed. The cleaved form of the pro-part isolated from natural human cathepsin C also folds, suggesting that the intact form comprises two folding domains. The linkages of the two disulfide bridges have been established as 30-118 and 54-136 for the native enzyme. The native disulfide bonds can be re-formed from the fully reduced and denatured state by oxidative refolding, resulting in a domain that is spectroscopically indistinguishable from the original refolded residual pro-part. Both disulfides are solvent-exposed and can be reduced in the absence of denaturant. The reduced form retains most or all of the native tertiary structure and is only approximately 2 kcal.mol(-1) less stable than the oxidized form. It folds fast relative to the rate of biosynthesis, to the same conformation as the oxidized form. Folding and disulfide formation are sequential. These results indicate that the proenzyme folds sequentially in vivo and that the residual pro-part constitutes a rapidly and independently folding domain that stabilizes the mature enzyme. It thus fulfills the criteria required of an intramolecular chaperone. It may also be involved in stabilizing the tetrameric structure of the mature enzyme.

Location of the binding site for chloride ion activation of cathepsin C.

Cathepsin C, a tetrameric lysosomal dipeptidyl-peptide hydrolase, is activated by chloride ion. The activation is shown here to be specific and pH-dependent, dissociation constants for chloride being lower at low pH. Bound chloride decreases the Km for the hydrolysis of chromophore labelled substrates without any significant change in Vmax, confirming its involvement in substrate binding. Determination of the kinetic parameters of chloride activation, using unlabelled substrates, has enabled its site of action to be located. The lower Km for the hydrolysis of simple amide substrates in the presence of Cl- shows that the S sites are involved. Possible involvement of the S' sites is excluded by the finding that the Km for the nucleophile in the transferase reaction is unaffected by chloride. The rates of inhibition by E-64 and iodoacetate are both chloride-dependent and, from the structure of the papain-E-64 complex, it is concluded that chloride binds close to the S2 site. The binding of guanidinium ion, a positively charged inhibitor, to the S site is dependent on chloride. Based on these results, a model is proposed to explain the chloride activation of cathepsin C. The possible physiological role of chloride in the regulation of proteolysis in the lysosome is discussed.

Competitive inhibition of cathepsin C by guanidinium ions and reexamination of substrate inhibition.

Cathepsin C, a lysosomal dipeptidyl aminopeptidase, is competitively and reversibly inhibited by guanidinium ions with a Ki approximately 1.5 mM. Loss of activity is not the result of conformational change, subunit dissociation or altered mobility of the enzyme, but rather reflects a specific binding of guanidinium ions to the active site. The finding that cathepsin C is not inhibited by substrate has allowed the kinetic parameters in the presence of guanidinium ion to be determined. Guanidinium significantly decreases the Km of substrate hydrolysis, without changing Vmax. In a novel application of the transferase reaction, the Km of the nucleophile substrate has been determined (11 mM) and found not to be affected by guanidinium, indicating its inhibition of substrate binding to the S, but not the S', site. Inhibition is suggested to be the result of shielding a negative charge on the enzyme important for interaction with the substrate.

Beta-lactamases as models for protein-folding studies.

This review traces some of the key features of the folding of beta-lactamases and their relevance to the way proteins fold in general. Studies on the enzymes have highlighted the nature and role of equilibrium and transient condensed states. The kinetics of folding are multiphasic, and when monitored by acrylamide quenching of the tryptophan fluorescence, an early phase provides evidence for the transient accumulation of a nonnative intermediate involving burial of tryptophan in a nonpolar environment. Intermediate phases can be understood in terms of progressive folding of different parts of the molecule. The later, slow phases are associated with proline isomerization in the TEM-1 enzyme and, in its P167T mutant form, with isomerization from trans to cis of the E166 T167 peptide bond. Coupled with kinetic and X-ray crystallographic studies of the beta-lactamase from Staphylococcus aureus and its D179Q mutant, it appears that the final stage of folding is that of collapse and packing of the omega-loop on to the main body of the protein.

Stoichiometry and heterogeneity of the pro-region chain in tetrameric human cathepsin C.

The subunit structure and composition of mature human cathepsin C, an oligomeric cysteine proteinase, has been characterised in detail. The heavy chain, light chain and pro-region peptides are shown to be held together solely by non-covalent interactions, and to be present in equimolar ratio, suggesting an important structural role for the residual pro-region chain which is strongly bound to the enzyme. The mass of the light chain, as determined by mass spectrometry, combined with its N-terminal sequence, determines the position of cleavage from the heavy chain. Amino-acid sequencing has led to definition of the 13.5 kDa N-terminal part of the pro-region which remains in the mature enzyme, the C-terminal moiety of 10 kDa being cleaved out and lost from the pro-peptide on activation. The residual pro-region is heterogeneous, a proportion being intact and the remainder being cleaved at alternative positions 58 or 61, yielding two smaller peptides joined by disulphide bond. The proportion of cleaved form was found to vary with tissue and enzyme preparation but did not affect enzyme activity. The molecular masses of the constituent chains after deglycosylation lead to a protein mass of 158 kDa. All four potential glycosylation sites are glycosylated.

A collapsed intermediate with nonnative packing of hydrophobic residues in the folding of TEM-1 beta-lactamase.

The kinetics of refolding of TEM-1 beta-lactamase from solution in guanidine hydrochloride have been investigated on the manual and stopped-flow mixing time scales. The kinetics of change of far-UV circular dichroism and of intrinsic and ANS fluorescence have been compared with changes in the quenching of fluorescence by acrylamide as a probe of the accessibility of solvent to tryptophan. The binding of ANS points to hydrophobic collapse in the very early stages of folding which take place in the burst phase. This is accompanied by regain of 60-65% of native ellipticity, indicating formation of a significant proportion of secondary structure. Also in the burst phase, the tryptophan residues, which are largely exposed to solvent in the native protein, become less accessible to acrylamide, and the intrinsic fluorescence increases markedly. An early intermediate is thus formed in which tryptophan is more buried than in the native protein. Further intermediates are formed over the next 20 s. Quenching by acrylamide increases during this period, as the transient nonnative state is disrupted and the tryptophan residue(s) become(s) reexposed to solvent. The two slowest phases are determined by the isomerization of incorrect prolyl isomers, but double jump tryptophan fluorescence and acrylamide quenching experiments show little, if any, effect of proline isomerization on the earlier phases. Hydrophobic collapse thus occurs to a folding intermediate in which there is a nonnative element of structure which has to rearrange in the later steps of folding, resulting in a nonhierarchical folding pathway. The C-terminal W290 is suggested as being involved in the nonnative intermediate. beta-Lactamase provides further evidence for the occurrence of nonnative intermediates in protein folding.

Kinetic and thermodynamic consequences of the removal of the Cys-77-Cys-123 disulphide bond for the folding of TEM-1 beta-lactamase.

Class A beta-lactamases of the TEM family contain a single disulphide bond which connects cysteine residues 77 and 123. To clarify the possible role of the disulphide bond in the stability and folding kinetics of the TEM-1 beta-lactamase, this bond was removed by introducing a Cys-77-->Ser mutation, and the enzymically active mutant protein was studied by reversible guanidine hydrochloride-induced denaturation. The unfolding and refolding rates were monitored using tryptophan fluorescence. At low guanidine hydrochloride concentrations, the refolding of the wild-type and mutant enzymes followed biphasic time courses. The characteristics of the two phases were not significantly affected by the mutation. Double-jump experiments, in which the protein was unfolded in a high concentration of guanidine hydrochloride for a short time period and then refolded by diluting out the denaturant, indicated that, for both the wild-type and mutant enzymes, the two refolding phases could be ascribed to proline isomerization reactions. Equilibrium unfolding experiments monitored by fluorescence spectroscopy and far-UV CD indicated a three-state mechanism (N<-->H<--U). Both the folded mutant protein (N) and, to a lesser extent the thermodynamically stable intermediate, H. were destabilized relative to the fully unfolded state, U. Removal of the disulphide bond resulted in a decrease of 14.2 kJ/mol (3.4 kcal/mol) in the global free energy of stabilization. Similarly, the mutation also induced a drastic increase in the rate of thermal inactivation.

The rate-limiting step in the folding of the cis-Pro167Thr mutant of TEM-1 beta-lactamase is the trans to cis isomerization of a non-proline peptide bond.

The stability and kinetics of unfolding and refolding of the P167T mutant of the TEM-1 beta-lactamase have been investigated as a function of guanidine hydrochloride concentration. The activity of the mutant enzyme was not significantly modified, which strongly suggests that the Glu166-Thr167 peptide bond, like the Glu166-Pro167, is cis. The mutation, however, led to a significant decrease in the stability of the native state relative to both the thermodynamically stable intermediate and the fully unfolded state of the protein. In contrast to the two slower phases seen in the refolding of the wild-type enzyme, only one phase was detected in the refolding of the mutant, indicating a determining role of proline 167 in the kinetics of folding of the wild-type enzyme. The former phases are replaced by rapid refolding when the enzyme is unfolded for short periods of time, but the latter is independent of the time of unfolding. The monophasic refolding reaction of the mutant is proposed to reflect mainly the trans-->cis isomerization of the Glu166-Thr167 peptide bond.

The role of the C-terminal lysine in the hinge bending mechanism of yeast phosphoglycerate kinase.

Treatment of yeast phosphoglycerate kinase (PGK) with trypsin results in a fourfold increase in the Vmax of this enzyme, without affecting the Km. This activation is shown to be due to the removal of the C-terminal lysine residue. The C-terminal sequence folds back over the N-terminal domain and contacts the extreme N-terminal sequence which folds onto the C-terminal domain, thus making many of the inter-domain contacts in this two domain protein. Previous studies have shown that this C-terminal region is important in mediating the conformational changes required during catalysis by yeast PGK. Observation of the three-dimensional structure of this enzyme suggests that removal of the C-terminal lysine residue will strengthen the interaction between K5 and E413. This indicates that this salt bridge stabilises the enzyme in the higher activity form, while the presence of K415 reduces the strength of that interaction.

The folding and assembly pathway of tumour necrosis factor TNF alpha, a globular trimeric protein.

The mechanisms of folding and assembly of the globular, trimeric protein tumour necrosis factor-alpha (TNF) were studied by chemical cross-linking. This revealed the rapid accumulation of a dimeric intermediate. Under the conditions of renaturation used, formation of the trimer is complete within six minutes. The kinetics of change of intrinsic and 8-anilino-1-naphthalene sulfonic acid fluorescence are first order and, combined with the kinetics of association, reveal the presence of folding steps both before and subsequent to formation of the trimer. Results from gel exclusion chromatography and kinetics, together with the existence of an acid-induced molten globule, support the conclusion that TNF folds and assembles through a trimeric molten globule.

Tumour necrosis factor is in equilibrium with a trimeric molten globule at low pH.

The reversible acid denaturation of tumour necrosis factor, TNF alpha, a trimeric, all-beta protein, leads to significant conformational changes within the molecule. A change in far UV CD spectra reveals a shift in the secondary structure content of the protein, with alpha-helical structure being induced. Loss of ellipticity in the near UV reflects a loss of tertiary interactions. This form of TNF is both compact and trimeric, as revealed by fluorescence anisotropy and sedimentation velocity analysis, respectively. Acid-denatured TNF therefore possesses the defining features of the molten globule intermediate while retaining the ability of the still incompletely folded monomers to exhibit the surface specificity necessary for maintaining the trimeric state.

DNA recognition by the EcoK methyltransferase. The influence of DNA methylation and the cofactor S-adenosyl-L-methionine.

The methyltransferase of the EcoK type I restriction/modification system is trimeric, M2S1, where the S subunit determines the sequence specificity of the enzyme. The methyltransferase has a strong preference for hemimethylated substrate DNA and, therefore, we have investigated the effect of the methylation state of DNA on binding by the enzyme, together with the effects on binding of the cofactor S-adenosyl-L-methionine. Our results indicate that the methyltransferase has two non-interacting S-adenosyl-L-methionine binding sites, each with a dissociation constant of 3.60 (+/- 0.42) microM determined by equilibrium dialysis, or 2.21 (+/- 0.29) microM determined by the displacement of a fluorescent probe. Ultraviolet light-induced crosslinking showed that S-adenosyl-L-methionine binds strongly only to the modification (M) subunits. Changes in the sedimentation velocity of the methyltransferase imply a protein conformational change due to S-adenosyl-L-methionine binding. Gel retardation results show that the binding of S-adenosyl-L-methionine to the methyltransferase enhances binding to both specific and non-specific DNAs, but the enhancement is greater for the specific DNA. Differences in binding affinities contribute to the recognition of the specific nucleotide sequence AAC(N)6GTGC by the methyltransferase in preference to a non-specific sequence. In contrast, although the complexes of unmodified and hemimethylated DNAs with the methyltransferase have different mobilities in non-denaturing gels, there appears to be no contribution of binding affinity to the distinction between these two substrates. Therefore, the preference for a hemimethylated substrate must be due to a difference in catalysis.

Kinetics of folding of the all-beta sheet protein interleukin-1 beta.

The folding of the all-beta sheet protein, interleukin-1 beta, was studied with nuclear magnetic resonance (NMR) spectroscopy, circular dichroism, and fluorescence. Ninety percent of the beta structure present in the native protein, as monitored by far-ultraviolet circular dichroism, was attained within 25 milliseconds, correlating with the first kinetic phase determined by tryptophan and 1-anilinonaphthalene-8-sulfonate fluorescence. In contrast, formation of stable native secondary structure, as measured by quenched-flow deuterium-hydrogen exchange experiments, began after only 1 second. Results from the NMR experiments indicated the formation of at least two intermediates with half-lives of 0.7 to 1.5 and 15 to 25 seconds. The final stabilization of the secondary structure, however, occurs on a time scale much greater than 25 seconds. These results differ from previous results on mixed alpha helix-beta sheet proteins in which both the alpha helices and beta sheets were stabilized very rapidly (less than 10 to 20 milliseconds).

'All-or-none' mechanism of the molten globule unfolding.

The Gdm-HCl-induced unfolding of bovine carbonic anhydrase B and S. aureus beta-lactamase was studied at 4 degrees C by a variety of methods. With the use of FPLC it has been shown that within the transition from the molten globule to the unfolded state the distribution function of molecular dimensions is bimodal. This means that equilibrium intermediates between the molten globule and the unfolded states are absent, i.e. the molten globule unfolding follows the 'all-or-none' mechanism.

A study of the hinge-bending mechanism of yeast 3-phosphoglycerate kinase.

The hinge-bending mechanism proposed as part of the catalytic mechanism for phosphoglycerate kinase (PGK) has been investigated using yeast PGK and the site-directed mutant [H388Q]PGK, where His388 is replaced by Gln. The emission and quenching of fluorescence, supported by the aromatic CD band, show that the mutation in the waist region affects the tryptophan environment in the C-terminal domain. The mutant is also less stable to guanidine denaturation and less cooperative in its unfolding. The effect of substrates on the conformation of PGK was studied using 8-anilino-1-naphthalenesulphonic acid (ANS), a competitive inhibitor of ATP binding to the C-terminal domain, and 8-(2-[(iodoacetyl)ethyl]amino)naphthalene (I-AEDANS), attached to Cys197 on the N-terminal domain. Under the influence of substrates the novel anisotropy decay curves for ANS indicate a 1-5 degrees change in the orientation of the probe, interpreted as a small reorientation of the domains about the waist region. The experimental data are interpreted as a small swivelling of the domains about the waist region under the influence of substrate. The results with AEDANS anisotropy decay are consistent with those for ANS. The enzyme activity of PGK shows a break in the Arrhenius plot at 20 degrees C mirrored by a break in the temperature dependence of tryptophan ellipticity. This is interpreted as a change in protein dynamics associated with destabilisation of the waist region. This destabilisation is shown to have already taken place in the mutant enzyme and in the wild type at pH 5.6, both of which exhibit linear Arrhenius plots. NMR titration curves show that the pH effect must be due to a group other than histidine. The results give further support to the permissive model of hinge bending previously proposed by one of the authors, in which binding of substrate destabilises the waist region. This loosens the hinge which can then swing slightly to bring the domains closer together to make favourable interactions between the domains and the substrates, with the exclusion of water.

Denaturation of stefin B by GuHCl, pH and heat; evidence for molten globule intermediates.

GuHCl, pH and thermal denaturation of the recombinant stefin B was followed by circular dichroism (CD) and size-exclusion chromatography (SEC). CD at 277 nm was taken as an indicator of integral tertiary structure and CD at 222 nm as an indicator of the secondary structure. Compactness was expressed by the volumes of elution on a SEC (Superose 12) column. Data on equilibrium denaturation were recalculated to the fractions of the native state (fN). The results have shown that equilibrium intermediates of the molten globule type exist under conditions of low pH, high temperature or medium GuHCl concentrations, namely A, T and G. Recent findings on structure and energetics of molten globule intermediates are reviewed.