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S Bromberg

Publications and source records attributed to S Bromberg.

13 recordsLinked to original sources
PubMed1999

Immobilized yeast bioreactor systems for continuous beer fermentation

Two different types of immobilized yeast bioreactors were examined for continuous fermentation of high-gravity worts. One of these is a fluidized bed reactor (FBR) that employs porous glass beads for yeast immobilization. The second system is a loop reactor containing a porous silicon carbide cartridge (SCCR) for immobilizing the yeast cells. Although there was some residual fermentable sugar in the SCCR system product, nearly complete attenuation of the wort sugars was achieved in either of the systems when operated as a two-stage process. Fermentation could be completed in these systems in only half the time required for a conventional batch process. Both the systems showed similar kinetics of extract consumption, and therefore similar volumetric productivity. As compared to the batch fermentation, total fusel alcohols were lower; total esters, while variable, were generally higher. The yeast biomass production was similar to that in a conventional fermentation process. As would be expected in an accelerated fermentation system, the levels of vicinal diketones (VDKs) were higher. To remove the VDKs, the young beer was heat-treated to convert the VDK precursors and processed through a packed bed immobilized yeast bioreactor for VDK assimilation. The finished product from the FBR system was found to be quite acceptable from a flavor perspective, albeit different from the product from a conventional batch process. Significantly shortened fermentation times demonstrate the feasibility of this technology for beer production.

Journal Article↗
PubMed1995

Models of cooperativity in protein folding.

What is the basis for the two-state cooperativity of protein folding? Since the 1950s, three main models have been put forward. 1. In 'helix-coil' theory, cooperativity is due to local interactions among near neighbours in the sequence. Helix-coil cooperativity is probably not the principal basis for the folding of globular proteins because it is not two-state, the forces are weak, it does not account for sheet proteins, and there is no evidence that helix formation precedes the formation of a hydrophobic core in the following pathways. 2. In the 'sidechain packing' model, cooperativity is attributed to the jigsaw-puzzle-like complementary fits of sidechains. This too is probably not the basis of folding cooperativity because exact models and experiments on homopolymers with sidechains give no evidence that sidechain freezing is two-state, sidechain complementarities in proteins are only weak trends, and the molten globule model predicted by this model is far more native-like than experiments indicate. 3. In the 'hydrophobic core collapse' model, cooperativity is due to the assembly of non-polar residues into a good core. Exact model studies show that this model gives two-state behaviour for some sequences of hydrophobic and polar monomers. It is based on strong forces. There is considerable experimental evidence for the kinetics this model predicts: the development of hydrophobic clusters and cores is concurrent with secondary structure formation. It predicts compact denatured states with sizes and degrees of disorder that are in reasonable agreement with experiments.

Kinetics↗
PubMed1995

Principles of protein folding--a perspective from simple exact models.

General principles of protein structure, stability, and folding kinetics have recently been explored in computer simulations of simple exact lattice models. These models represent protein chains at a rudimentary level, but they involve few parameters, approximations, or implicit biases, and they allow complete explorations of conformational and sequence spaces. Such simulations have resulted in testable predictions that are sometimes unanticipated: The folding code is mainly binary and delocalized throughout the amino acid sequence. The secondary and tertiary structures of a protein are specified mainly by the sequence of polar and nonpolar monomers. More specific interactions may refine the structure, rather than dominate the folding code. Simple exact models can account for the properties that characterize protein folding: two-state cooperativity, secondary and tertiary structures, and multistage folding kinetics--fast hydrophobic collapse followed by slower annealing. These studies suggest the possibility of creating "foldable" chain molecules other than proteins. The encoding of a unique compact chain conformation may not require amino acids; it may require only the ability to synthesize specific monomer sequences in which at least one monomer type is solvent-averse.

Amino Acid Sequence↗
PubMed1994

Side-chain entropy and packing in proteins.

What role does side-chain packing play in protein stability and structure? To address this question, we compare a lattice model with side chains (SCM) to a linear lattice model without side chains (LCM). Self-avoiding configurations are enumerated in 2 and 3 dimensions exhaustively for short chains and by Monte Carlo sampling for chains up to 50 main-chain monomers long. This comparison shows that (1) side-chain degrees of freedom increase the entropy of open conformations, but side-chain steric exclusion decreases the entropy of compact conformations, thus producing a substantial entropy that opposes folding; (2) there is side-chain "freezing" or ordering, i.e., a sharp decrease in entropy, near maximum compactness; and (3) the different types of contacts among side chains (s) and main-chain elements (m) have different frequencies, and the frequencies have different dependencies on compactness. mm contacts contribute significantly only at high densities, suggesting that main-chain hydrogen bonding in proteins may be promoted by compactness. The distributions of mm, ms, and ss contacts in compact SCM configurations are similar to the distributions in protein structures in the Brookhaven Protein Data Bank. We propose that packing in proteins is more like the packing of nuts and bolts in a jar than like the pairwise matching of jigsaw puzzle pieces.

Drug Stability↗
PubMed1994

Ligation alters the pathway of urea-induced denaturation of the catalytic trimer of Escherichia coli aspartate transcarbamylase.

We have examined the pathway and energetics of urea-induced dissociation and unfolding of the catalytic trimer (c3) of aspartate transcarbamylase from Escherichia coli at low temperature in the absence and presence of carbamyl phosphate (CP; a substrate), N-(phosphonacetyl)-L-Asp (PALA; a bisubstrate analog), and 2 anionic inhibitors, Cl- and ATP, by analytical gel chromatography supplemented by activity assays and ultraviolet difference spectroscopy. In the absence of active-site ligands and in the presence of ATP, c3 dissociates below 2 M urea into swollen c chains that then gradually unfold from 2 to 6 M urea with little apparent cooperativity. Linear extrapolation to 0 M urea of free energies determined in 3 independent types of experiments yields estimates for delta Gdissociation at 7.5 degrees C of about 7-10 kcal m-1 per interface. delta Gunfolding of dissociated chains when modeled as a 2-state process is estimated to be very small, on the order of -2 kcal m-1. The data are also consistent with the possibility that the unfolding of the dissociated monomer is a 1-state swelling process. In the presence of the ligands CP and PALA, and in the presence of Cl-, c3 dissociates at much higher urea concentrations, and trimer dissociation and unfolding occur simultaneously and apparently cooperatively, at urea concentrations that increase with the affinity of the ligand.

Adenosine Triphosphate↗
PubMed1990

Use of analytical gel chromatography to analyze tertiary and quaternary structural changes in E. coli aspartate transcarbamylase.

E. coli aspartate transcarbamylase (ATCase) is a large (310 kDa) protein that undergoes major changes in quaternary structure when substrates and regulatory nucleotides bind. We have used analytical gel chromatography to detect quaternary structure changes in both the holoenzyme and its catalytic subunit (c3), to characterize the quaternary structure of single site mutant proteins and to monitor urea-induced dissociation and unfolding of c3. Binding of the bisubstrate analog PALA (N-(phosphonacetyl)-L-aspartate) to ATCase and c3 has been shown to alter s20.w by -3.3% and + 1.4%, respectively [Howlett, G.J. and Schachman, H.K. (1977), Biochemistry 23, 5077-5083]. The corresponding changes in the chromatographic partition coefficient (sigma) are -2.6 +/- 0.3% and 5.5 +/- 1.9% on Sephacryl S400HR and S200, respectively. Partition coefficients of mutant ATCases with single site mutations in the c chain differ from those of the wild-type protein by +/- 0.5% in small zone experiments; for example, mutations Arg 269----Gly and Glu 239----Gln alter the partition coefficient by 0.4% and -0.5%, respectively. The partition coefficient of mutant Glu 50----Gln is identical to the wild type enzyme. In the presence of saturating PALA, partition coefficients of Glu 50----Gln and Arg 269----Gly, but not Glu 239----Gln are identical to those of the wild type. Results for Glu 239----Gln are consistent with measurements of activity, small angle X-ray scattering and sedimentation coefficient that indicate that mutations at this site shift the quaternary structure towards the R state [Ladjimi and Kantrowitz (1988), Biochemistry 27, 276-83; Vachette and Hervé, cited by Kantrowitz and Lipscomb (1988), Science 241, 669-674; Newell and Schachman (1988), FASEB J. 2, A551]. Results for Glu 50----Gln are also consistent with measurements of activity (Ladjimi et al. (1988), Biochemistry 27, 268-276). The changes in tertiary and quaternary structure that result from urea-induced denaturation of c3 result in larger changes in the partition coefficient. Dissociation into folded monomers in 1-1.75 M urea is accompanied by a 4.6% increase in partition coefficient, while denaturation at greater than 5 M urea gives rise to a 43% decrease on S-300 Sephacryl. The bisubstrate analog PALA suppresses dissociation and increases the cooperativity of the unfolding reaction.

Aspartate Carbamoyltransferase↗
PubMed1983

Suicide in the elderly: the limits of paternalism.

It is the authors' premise that caregivers of the suicidal elderly patient are at times guided by a paternalism that is not always in the best interests of the patient. They advocate instead care based on greater respect for the autonomy of the patient. A case of a suicidal, recently widowed man is presented, and historical and ethical considerations of bereavement and suicide are discussed.

Aged↗
PubMed1982

Sequence specificity of DNA adenine methylase in the protozoan Tetrahymena thermophila.

The sequence specificity of the Tetrahymena DNA-adenine methylase was determined by nearest-neighbor analyses of in vivo and in vitro methylated DNA. In vivo all four common bases were found to the 5' side of N6-methyladenine, but only thymidine was 3'. Homologous DNA already methylated in vivo and heterologous Micrococcus luteus DNA were methylated in vitro by a partially purified DNA-adenine methylase activity isolated from Tetrahymena macronuclei. The in vitro-methylated sequence differed from the in vivo sequence in that both thymidine and cytosine were 3' nearest neighbors of N6-methyladenine.

Adenine↗
PubMed1981

Effect of physical stress on sensitivity of lymphocytes to inhibition by prostaglandin E2.

Physical stress is associated with depressed cellular immune function. We have found that lymphocytes from subjects undergoing either of 2 stressful events, cardiac surgery or childbirth, are more sensitive to inhibition by PGE2. For example, the concentration of PGE2 required for 50% inhibition of 3H-thymidine incorporation (ID50) into phytohemagglutinin-stimulated lymphocytes from patients undergoing cardiac surgery went from 1.5 X 10(-8) M on the day before surgery to 3 X 10(-9) M on the day after surgery. This increase in sensitivity to PGE2 was accompanied by a significantly decreased lymphocyte proliferative response (27 to 68% of control, depending on mitogen dose) and a 50% increase in the percentage of E rosette-positive cells with receptors for the Fc portion of IgG. The increased sensitivity to PGE and the depressed mitogen responses returned to preoperative values by day 10. The depressed mitogen responses of the postoperative patients were completely restored to normal by removal of glass-adherent cells before culture. In addition, the responses of the postoperative patients and the women in labor were partially restored by the addition of indomethacin, a prostaglandin synthetase inhibitor, to the cultures. Thus it would appear that physical stress causes lymphocytes to become more sensitive to prostaglandin E2, and the increased sensitivity to inhibition by this immunomodulator is responsible in part for the depressed cellular immune function after physical stress.

Cell Adhesion↗
PubMed1976

Diethylstilbestrol in the treatment of rape victims.

Despite growing controversy surrounding its use as a "morning after pill," diethylstilbestrol (DES) is prescribed liberally for rape victims. Guidelines for its use in these patients is lacking. Of 150 consecutive rape victims treated at a university medical center, 63 (42 percent) received prescriptions for DES. Of the 55 (87 percent) on whom follow-up was obtained, in 40 (73 percent) there were substantial side effects-nausea or vomiting, or both. At least six (11 percent) did not complete therapy because of these side effects. The authors offer guidelines for use of DES for rape victims and a plan for patient education and follow-up.

Diethylstilbestrol↗