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Dive into the research topics where Bjarni Ásgeirsson is active.

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Featured researches published by Bjarni Ásgeirsson.


Comparative Biochemistry and Physiology B | 1991

Structural and kinetic properties of chymotrypsin from Atlantic cod (Gadus morhua). Comparison with bovine chymotrypsin.

Bjarni Ásgeirsson; Jón B. Bjarnason

1. Two chymotrypsins with isoelectric points pI 6.2 and 5.8 were purified from the pyloric caeca of Atlantic cod using a phenyl-Sepharose column and chromatofocusing chromatography. The apparent molecular weight was 26,000 as judged by SDS-polyacrylamide gel electrophoresis and gel filtration. 2. The cod enzymes differed from bovine chymotrypsin in having a slightly higher molecular weight and more acidic pI points. N-terminal amino acid sequence analysis of cod chymotrypsin B showed considerable similarity with bovine chymotrypsin. 3. Heat stability and stability towards acidic pH were reduced in the cod enzymes. Generally, the cod and bovine chymotrypsins responded similarly to various protease inhibitors. However, the cod chymotrypsins were less sensitive to aprotinin inhibition but more sensitive towards soybean trypsin inhibitor and cysteine. 4. Kinetic properties were examined and the cod enzymes found to be more active towards both ester (N-benzoyl-tyrosine ethyl ester) and amide (N-benzoyl-tyrosine-p-nitroanilide) substrates. The observed differences in kinetic properties are indicative of an adaptive response towards the low temperature environment in which the cod lives.


Biochimica et Biophysica Acta | 1993

Properties of elastase from Atlantic cod a cold-adapted proteinase

Bjarni Ásgeirsson; Jón B. Bjarnason

An intestinal elastase was purified from Atlantic cod (Gadus morhua) of apparent molecular mass 24.8 kDa as determined by SDS-PAGE and with isoelectric point above pI 9.3. Heat stability and stability towards acidic pH was reduced in the cod enzyme as compared with porcine intestinal elastase. N-terminal amino-acid sequence analysis of cod elastase showed considerable similarity with porcine elastase. The cod enzyme was less sensitive to phenylmethylsulfonyl fluoride inhibition than porcine elastase, but sensitivity towards other inhibitors was similar. Kinetic properties were examined using the substrate Suc-Ala-Ala-Ala-p-nitroanilide and the cod enzyme was found to have more than 2-times turnover rate (kcat) as compared with the porcine enzyme, and slightly higher Km values. Thus, the catalytic efficiency (kcat/Km) of Atlantic cod elastase was about 2-times higher than observed with porcine elastase, which indicates an adaptive response towards the low temperature environmental in which the cod lives. Substrate specificity was studied by digestion of oxidized B-chain of insulin and by using synthetic substrates. Digestion was most rapid at the carbonyl side of alanine residues, but also occurred at valine and leucine residues.


Enzyme and Microbial Technology | 2000

Heat-labile bacterial alkaline phosphatase from a marine Vibrio sp.

Jónas B. Hauksson; Ólafur S. Andrésson; Bjarni Ásgeirsson

Psychrophilic organisms have successfully adapted to various low-temperature environments such as cold ocean waters. Catalysts with increased catalytic efficiencies are produced, generally at the expense of thermal stability due to fewer non-covalent stabilizing interactions. A marine bacterial strain producing a particularly heat-labile alkaline phosphatase was selected from a total of 232 strains isolated from North-Atlantic coastal waters. From partial 16S rRNA sequences the strain was characterized as a Vibrio sp. An alkaline phosphatase was purified 151-fold with 54% yield from the culture medium using a single step affinity chromatography procedure on agarose-linked L-histidyldiazobenzylphosphonic acid. The active enzyme was a 55 +/- 6 kDa monomer. The enzyme had optimal activity at pH 10 and was strikingly heat-labile with a half-life of 6 min at 40 degrees C and 30 min at 32 degrees C. This enzyme from Vibrio sp. had a higher turnover number (k(cat)) and higher apparent Michaelis-Menten factor (K(m)) than the enzyme from Escherichia coli, a clear-indication of cold-adaptation. Inorganic phosphate was a competitive inhibitor with a relatively high K(i) value of 1.7 mM. Low affinity for phosphate may contribute to higher turnover rates due to more facile release of product.


Comparative Biochemistry and Physiology B | 1995

Alkaline phosphatase from Atlantic cod (Gadus morhua). Kinetic and structural properties which indicate adaptation to low temperatures

Bjarni Ásgeirsson; Ralf Hartemink; Jan F. Chlebowski

Abstract Alkaline phosphatase was purified from the pyloric caeca of Atlantic cod in a procedure requiring column chromatography on phenyl-Sepharose, Q-Sepharose, Sephacryl S-300, Reactive Red and concanavalin-A-Sepharose. The enzyme is a dimeric glycoprotein of identical 70 kDa subunits and partly associated with membranes. It was inactivated by EDTA and could be reactivated with zinc and magnesium. The catalytic efficiency (kcat/Km) of the cod enzyme was found to be 2.5-fold greater in comparison with the calf intestinal enzyme. The cod enzyme was unstable towards heating above 40°C whereas the calf enzyme remained stable at temperatures up to 55°C. Differences were also found in the response to some commonly used inhibitors, where the cod intestinal alkaline phosphatase resembled most the liver/bone/kidney isozyme from humans. Notably, the sensitivity towards phosphate inhibition was lower with the cod enzyme, and the pH optimum for catalysis was different. We conclude that the observed kinetic differences are indicative of cold-adaptation. Fewer stabilizing interactions in the cod enzyme structure, as evidenced by lower thermal stability, would help to maintain necessary flexibility at the low level of thermal energy in the body of this ectothermic species.


Journal of the Neurological Sciences | 1992

On the role of monocytes/macrophages in the pathogenesis of central nervous system lesions in hereditary cystatin C amyloid angiopathy

Leifur Thorsteinsson; Gudmundur Georgsson; Bjarni Ásgeirsson; María Bjarnadóttir; Isleifur Olafsson; Olafur Jensson; Gunnar Gudmundsson

The pathogenesis of the deposition of a variant cystatin C as amyloid in hereditary cystatin C amyloid angiopathy (HCCAA) is not known. To address this question the synthesis and secretion of cystatin C in cultured monocytes from 9 carriers of the mutated cystatin C gene (5 symptomatic and 4 asymptomatic) was examined. The quantity of cystatin C in cells and supernatants was determined by the ELISA method, Western blots were done and selected samples immunostained for cystatin C. Monocytes from individuals carrying the gene defect synthesized cystatin C that was apparently not truncated, a form found in the cerebral amyloid deposits in HCCAA, but showed a distinctly lower rate of cystatin C synthesis than monocytes from healthy controls. The main difference was that the quantity of cystatin C was significantly lower in the supernatants in monocyte cultures from carriers of the gene defect than from healthy controls, possibly due to a partial block in its secretion. This abnormal processing of the cystatin C could explain the low cerebrospinal fluid levels of cystatin C in HCCAA and might be a part of the pathogenetic pathway of amyloid deposition. Furthermore it could, through a lower extracellular concentration of this inhibitor of cysteine proteinases, contribute to destruction of the amyloidotic blood vessels, leading to the most serious clinical manifestation in HCCAA, intracerebral hemorrhage.


Biochimica et Biophysica Acta | 1996

Structure of chymotrypsin variant B from Atlantic cod, Gadus morhua

Rikke Leth-Larsen; Bjarni Ásgeirsson; Matthías Thórólfsson; Mads Nørregaard-Madsen; Peter Højrup

The amino-acid sequence of chymotrypsin variant B isolated from the pyloric caeca of Atlantic cod has been elucidated. The characterization of the primary structure is based on N-terminal Edman degradation and mass spectrometry of the native protein and enzymatically derived peptides. Chymotrypsin variant B showed 72% sequence identity with the A-variant and 64% and 62%, respectively, with the bovine counterparts A and B, all consisting of 245 amino acids. This new sequence contains a higher proportion of charged residues compared with bovine chymotrypsin but fewer polar hydrogen-bond forming residues which might contribute to its lower thermostability. It also shares the emerging characteristics of other fish serine proteinases which have relatively higher methionine content, including a conserved Met-134 in a loop leading into a domain-connecting strand. The inherent mobility in methionine side-chains may contribute to the maintenance of flexibility at low temperatures. Several amino-acid sequence differences adjacent to the catalytic site are observed in the two cod chymotrypsin variants which also differ in kinetic properties. Unlike the mammalian chymotrypsins, which contain several autolysis sites, cod variant B only contains a single autolysis site. The three-dimensional structures of the A- and B-variants of cod has been modelled on the known crystal structure of bovine alpha-chymotrypsin showing almost superimposable structures.


Biochimica et Biophysica Acta | 2001

Primary structure of cold-adapted alkaline phosphatase from a Vibrio sp. as deduced from the nucleotide gene sequence.

Bjarni Ásgeirsson; Ólafur S. Andrésson

Alkaline phosphatases (AP) are widely distributed in nature, and generally have a dimeric structure. However, there are indications that either monomeric or multimeric bacterial forms may exist. This paper describes the gene sequence of a psychrophilic marine Vibrio AP, previously shown to be particularly heat labile. The kinetic properties were also indicative of cold adaptation. The amino acid sequence of the Vibrio G15-21 AP reveals that the residues involved in the catalytic mechanism, including those ligating the metal ions, have precedence in other characterized APs. Compared with Escherichia coli AP, the two zinc binding sites are identical, whereas the metal binding site, normally occupied by magnesium, is not. Asp-153 and Lys-328 of E. coli AP are His-153 and Trp-328 in Vibrio AP. Two additional stretches of amino acids not present in E. coli AP are found inserted close to the active site of the Vibrio AP. The smaller insert could be accommodated within a dimeric structure, assuming a tertiary structure similar to E. coli AP. In contrast the longer insert would most likely protrude into the interface area, thus preventing dimer formation. This is the first primary structure of a putative monomeric AP, with indications as to the basis for a monomeric existence. Proximity of the large insert loop to the active site may indicate a surrogate role for the second monomer, and may also shape the catalytic as well as stability characteristics of this enzyme.


FEBS Letters | 2012

Inhibition of a cold-active alkaline phosphatase by imipenem revealed by in silico modeling of metallo-β-lactamase active sites

Sandeep Chakraborty; Bjarni Ásgeirsson; Renu Minda; Lipika Salaye; Jean-Marie Frère; Basuthkar J. Rao

We demonstrate the inhibition of the native phosphatase activity of a cold active alkaline phosphatase from Vibrio (VAP) (IC50 of 44 ± 4 (n = 4) μM at pH 7.0 after a 30 min preincubation) by a specific β‐lactam compound (only by imipenem, and not by ertapenem, meropenem, ampicillin or penicillin G). The homologous scaffold was detected by an in silico analysis that established the spatial and electrostatic congruence of the active site of a Class B2 CphA metallo‐β‐lactamase from Aeromonas hydrophila to the active site of VAP. The tested β‐lactam compounds did not inhibit Escherichia coli or shrimp alkaline phosphatase, which could be ascribed to the lower congruence indicated by CLASP. There was no discernible β‐lactamase activity in the tested alkaline phosphatases. This is the first time a scaffold recognizing imipenem in an alkaline phosphatase (VAP) has been demonstrated.


FEBS Journal | 2008

Effects of replacing active site residues in a cold-active alkaline phosphatase with those found in its mesophilic counterpart from Escherichia coli.

Katrín Gudjónsdóttir; Bjarni Ásgeirsson

Alkaline phosphatase (AP) from a North Atlantic marine Vibrio bacterium was previously characterized as being kinetically cold‐adapted. It is still unknown whether its characteristics originate locally in the active site or are linked to more general structural factors. There are three metal‐binding sites in the active site of APs, and all three metal ions participate in catalysis. The amino acid residues that bind the two zinc ions most commonly present are conserved in all known APs. In contrast, two of the residues that bind the third metal ion (numbered 153 and 328 in Escherichia coli AP) are different in various APs. This may explain their different catalytic efficiencies, as the Mg2+ most often present there is important for both structural stability and the reaction mechanism. We have mutated these key residues to the corresponding residues in E. coli AP to obtain the double mutant Asp116/Lys274, and both single mutants. All these mutants displayed reduced substrate affinity and lower overall reaction rates. The Lys274 and Asp116/Lys274 mutants also displayed an increase in global heat stability, which may be due to the formation of a stabilizing salt bridge. Overall, the results show that a single amino acid substitution in the active site is sufficient to alter the structural stability of the cold‐active Vibrio AP both locally and globally, and this influences kinetic properties.


Advances in Experimental Medicine and Biology | 1997

Serine Proteinases from Cold-Adapted Organisms

Magnús M. Kristjánsson; Bjarni Ásgeirsson; Jón B. Bjarnason

Proteins and especially enzymes from organisms that have adapted to extreme conditions in environmental temperatures have been a subject of considerable interest in both basic and applied research for number of years. Sofar most of the research has focused on enzymes from thermophilic microorganisms. As enzymes from thermophiles are generally found to be more thermostable than their counterparts from mesophiles, they have received much attention as experimental model systems for studying the underlying molecular principles in thermostabilization of proteins. Despite much research effort, the question of how thermophilic proteins are stabilized to withstand temperatures close to or above the boiling point of water is, however, still unsolved. Because of their high activity and stability at elevated temperatures, enzymes from thermophiles have also been considered an attractive alternative to mesophilic enzymes, that despite of their often limited thermal stability are used in several industrial applications that require high operational temperatures.

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Basuthkar J. Rao

Tata Institute of Fundamental Research

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Ravindra Venkatramani

Tata Institute of Fundamental Research

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Félix M. Goñi

University of the Basque Country

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Anindya S. Ghosh

Indian Institute of Technology Kharagpur

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Mouparna Dutta

Indian Institute of Technology Kharagpur

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