“Hybridization” between aldolase subunits derived from mammalian and plant origin

A selective reaction of fructose bisphosphate aldolase with fluorescein isothiocyanate in chicken muscle extracts

The present work describes the selective covalent modification of fructose bisphosphate aldolase in crude extracts of chicken breast muscle by fluorescein 5'-isothiocyanate (5'-FITC) at pH 7.0 and 35 degrees C. The modification was observed after 1 min while no other major soluble protein was labeled even after 30 min. We calculated that ca. one 5'-FITC molecule was incorporated into each aldolase tetramer after a 30 min reaction which resulted in a minimal loss of enzyme activity. The "native" structure of aldolase was required for the selective modification by 5'-FITC since high pH, high temperature, and ionic detergents either inhibited or prevented the reaction of 5'-FITC with aldolase. Certain metabolites (ATP, ADP, CTP, GTP, FBP) and erythrosin B also inhibited the 5'-FITC modification of aldolase. In contrast, F-6-P, AMP, NADH, and NAD(+) as well as free lysine and most importantly, the 6'-isomer of FITC exhibited no competition with 5'-FITC for the labeling of aldolase. Alone, the 6'-isomer of FITC did not exhibit preferential reaction when combined with aldolase. 5'-FITC-labeled and -unlabeled aldolases were not distinguished by their ability to bind to muscle myofibrils (MFs) or by their abilities to refold following reversible denaturation in urea. Structural analysis revealed that 5'-FITC-labeled a tryptic peptide corresponding to residues 112-134 in the primary structure of aldolase, a peptide that does not contain lysine, the amino acid believed to be the primary target of this reagent. Unlike chicken and rabbit muscle aldolases, chicken brain and liver aldolase isoforms along with several other aldolases derived from diverse biological sources did not exhibit this highly selective modification by 5'-FITC.

Molecular biology of the C3 photosynthetic carbon reduction cycle

In recent years the enzymes of the C3 photosynthetic carbon reduction (PCR) cycle have been studied using the techniques of molecular biology. In this review we discuss the primary protein sequences and structural predictions that have been made for a number of these enzymes, which, with the input of crystallographic analysis, gives the opportunity to understand the mechanisms of enzyme activity.The genome organisation and gene structure of the PCR enzymes is another area which has recently expanded, and we discuss the regulation of the genes encoding these enzymes and the complex interaction of various factors which influence their expression.

Anaerobically regulated aldolase gene of maize. A chimaeric origin?

The sequence of the anaerobically induced fructose 1,6-bisphosphate aldolase gene of maize is presented. Analysis of the upstream sequences of the aldolase gene reveals a six base-pair sequence (TGGTTT) with perfect homology to one of the sub-regions of the anaerobic regulatory element (ARE) which is responsible for the anaerobic induction of the maize alcohol dehydrogenase 1 gene (Adh1). In the aldolase gene this sequence is located at position -70 relative to the start of transcription, in a small segment proven by functional analysis to be important for expression of the aldolase gene. Since this six base-pair sequence has been shown to be critical for anaerobic induction of the Adh1 mRNA, is in the functional promoter region of aldolase and is also present in a homologous position in Adh2 (another anaerobically-induced gene), we suggest this hexanucleotide is essential for anaerobic regulation of each of these genes. The maize aldolase gene is about 50% homologous at the amino acid level to the animal aldolase gene but has a completely different intron/exon structure. While the rat aldolase gene has nine introns the maize gene has a single large intron near the N terminus of the coding region. Because there is 55% homology downstream from the intron and very little homology upstream, we suggest that the maize gene has acquired a 5' region containing signals for anaerobic regulation and fortuitously adding a new N-terminal region to the protein. We must suppose that the plant gene has lost the remaining introns.

Specific, limited tryptic modification of wheat-germ fructose-bisphosphate aldolase subunits: destruction of catalytic activity but not of ability to establish precise subunit-subunit recognition

We have been using the glycolytic enzyme fructose-bisphosphate aldolase (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphate lyase, EC 4.1.2.13) as a model system to investigate the assembly of oligomeric enzymes. In the present work, we investigate the effect of specific, limited tryptic modification on the properties of aldolase isolated from wheat germ. The wheat-germ enzyme was selected, since several aldolases isolated from animal sources were not readily susceptible to the specific tryptic modification seen with this plant enzyme. We will show that: Low levels of trypsin cause a first-order inactivation of wheat-germ aldolase activity which is associated with a fairly specific cleavage of the enzyme which reduces its subunit molecular weight from 41000 to 39000. The proteolytic modification is greatly inhibited in the presence of the aldolase substrate, fructose bisphosphate. The intact and modified enzymes appear to have similar surface changes, as judged by their behavior during electrophoresis in polyacrylamide gels under non-denaturing conditions. The modified aldolase is not specifically eluted from phosphocellulose columns by fructose bisphosphate under the conditions used in the affinity chromatographic isolation of the intact enzyme, suggesting that the modified enzyme may no longer be able to bind substrate. Although enzymatically inactive, the modified aldolase subunits are able to refold and reassociate into tetrameric combinations following unfolding of the subunits by treatment at low pH; thus, this specific proteolytic modification does not interfere with the ability of wheat-germ aldolase subunits to refold and to establish precise subunit-subunit recognition in vitro.

Hybridization between fructose diphosphate aldolase subunits derived from diverse biological systems: anomolous hybridization behavior of some aldolase subunit types

In the present studies we investigated the abilities of fructose diphosphate aldolase subunits derived from diverse biological sources to form stable heterotetramers with each other in vitro. Aldolase C subunits isolated from chicken brain readily "hybridized" with aldolase subunits derived from lobster muscle and wheat germ following reversible acid dissociation of mixtures of these enzymes; however, appreciable amounts of stable heterotetramers containing chicken C subunits and aldolase subunits isolated from two other invertebrates (Ascaris and squid) were not produced under the same conditions. In contrast to the situation with chicken C subunits, aldolase B subunits isolated from rat liver did not "hybridize" appreciably with lobster muscle or wheat germ aldolase subunits. The present observations are not consistent with the hypothesis that the abilities of different aldolase subunit types to form heterotetramers in vitro is governed solely by the evolutionary relationships which exist between the organisms from which the enzymes are derived.

Purification, subunit structure and immunological comparison of fructose-bisphosphate aldolases from spinach and corn leaves

The cytosol and chloroplast fructose-bisphosphate aldolases from spinach leaves were separated by ion-exchange chromatography on DEAE-cellulose, and were purified by subsequent affinity chromatography on phosphocellulose to apparent homogeneity as judged from polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The two aldolases had specific activities of 7.2 and 7.8 units mg protein-1. Molecular weight determinations by electrophoresis in sodium dodecyl sulfate gels and by sedimentation velocity centrifugation in sucrose gradients showed that the aldolases contained four subunits of Mr 38 000 and 35 000, respectively. Antibodies against the cytosol and chloroplast aldolase from spinach leaves were raised in a guinea pig and in a rabbit, respectively. In the Ouchterlony double-diffusion test, the two aldolases did not cross-react. A small degree of cross-reaction was observed by a test in which immune complexes were adsorbed to a solid-phase support (Staphylococcus aureus Cowan I cells) and nonbound enzyme activity was determined after centrifugation. These results imply major structural differences between the two spinach leaf aldolases. Only one major aldolase could be resolved on DEAE-cellulose from corn leaves. The aldolase was purified and had a specific activity of 6.4 units X mg protein-1. The corn leaf aldolase cross-reacted with the antiserum raised against the chloroplast enzyme from spinach leaves, but not with the other antiserum. Thus, the corn leaf aldolase could be identified as a chloroplast enzyme. Since aldolase activity is mostly restricted to the bundle sheath cells of corn leaf, it was concluded that it is compartmentalized in the chloroplasts of these cells but not in chloroplasts of the mesophyll cells.

Demonstration of physical interactions between consecutive enzymes of the citric acid cycle and of the aspartate-malate shuttle. A study involving fumarase, malate dehydrogenase, citrate synthesis and aspartate aminotransferase

By means of covalently immobilized fumarase and mitochondrial or cytoplasmic malate dehydrogenase we were able to detect physical interactions between different enzymes of the citric acid cycle (fumarase with malate dehydrogenase, malate dehydrogenase with citrate synthase and fumarase with citrate synthase) and between the enzymes of both mitochondrial and cytoplasmic halves of the aspartate-malate shuttle (aspartate amino-transferase and malate dehydrogenase). The interactions between fumarase and malate dehydrogenase were also investigated by immobilizing one enzyme indirectly through antibodies bound to Sepharose-protein A. Our results are consistent with a model in which maximally four molecules of malate dehydrogenase are bound to one fumarase molecule. This complex is able to bind either citrate synthase or aspartate aminotransferase. We propose that these enzymes bind alternatively, in order to allow the cell to perform citric acid cycle or shuttle reactions, according to its needs. The physiological meaning and implications on the regulation of metabolism of the existence of a large citric acid cycle/malate-aspartate shuttle multienzyme complex are discussed.

Study of subunit interactions in immobilized D-glyceraldehyde-3-phosphate dehydrogenase

Under conditions which cause dissociation of soluble tetrameric glyceraldehyde-3-phosphate dehydrogenase (D-glyceraldehyde-3-phosphate:NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12) into inactive dimers, immobilized apoenzymes from yeast and rat skeletal muscle coupled to CnBr-activated Sepharose via one subunit retain 50% of matrix-bound protein with unaltered specific activity. The solubilized dissociated species are inactive. Two molecules of NAD+ (NADH) firmly bound to the immobilized rat muscle tetramer can prevent the dissociation. Immobilized dimer was demonstrated to bind one molecule of coenzyme with high affinity. Using various combinations of immobilized and soluble rat muscle and yeast dimers, we succeeded in reconstituting tetramers, containing one molecule of NAD+ bound either to a matrix-linked or to a non-covalently bound dimer. In the latter case, the dissociation of the tetramer was completely prevented. This suggests that the binding of a single coenzyme molecule is sufficient to stabilize the interdimeric contacts provided the neighbouring dimer is stabilized independently. Such stabilization is produced by the covalent binding of one of the subunits comprising the dimer to the matrix. The structure of the dimer as a whole becomes resistant to the action of the dissociating agent. The effect appears to be cooperative and similar to that of NAD+ or NADH. The dissociation of the immobilized tetramer is, most likely, the result of conformational changes, affecting the structure of the non-covalently bound dimer. Any factor, capable of preventing these changes, would stabilize the interdimeric contacts. The latter conclusion is substantiated by the effect of specific antibodies, which prevent the dissociation of the immobilized tetramer by forming a complex with the dimer, non-covalently bound to the matrix. The evidence obtained in the present investigation supports the conclusion that the isolated dimer of glyceraldehyde-3-phosphate dehydrogenase represents a relatively independent structural and functional 'unit' of the enzyme. It can be stabilized in a catalytically active form by interactions other than those involved in inter-dimeric contacts in the tetramer. The kinetics of the association of immobilized and soluble dimers have been studied. Association rate constants were determined for homologous (yeast-yeast, rat-rat) and heterologous (yeast-rat, yeast-rabbit) dimer combinations. The binding of one molecule of specific antibody to the immobilized dimer was shown to increase the rate constant of association.