Functional analyses of a highly thermostable hexokinase from Pyrobaculum calidifontis

The gene encoding a repressor open reading frame sugar kinase (ROK) family protein from hyperthermophilic crenarchaeon Pyrobaculum calidifontis, Pcal-HK, was cloned and expressed in Escherichia coli. The recombinant protein was produced in soluble and highly active form. Purified Pcal-HK was highly thermostable and existed in a monomeric form in solution. The enzyme was specific to ATP as phosphoryl donor but showed broad specificity to phosphoryl acceptors. It catalyzed the phosphorylation of a number of hexoses, including glucose, glucosamine, N-acetyl glucosamine, fructose and mannose, at nearly the same rate and similar affinity. The enzyme was metal ion dependent exhibiting highest activity at 90-95 °C and pH 8.5. Mg²⁺ was most effective metal ion, which could be partially replaced by Mn²⁺, Ni²⁺ or Zn²⁺. Kinetic parameters were determined at 90 °C and the enzyme showed almost similar catalytic efficiency (k_cat/K_m) towards the above mentioned hexoses. To the best of our knowledge, Pcal-HK is the most active thermostable ROK family hexokinase characterized to date which catalyzes the phosphorylation of various hexoses with nearly similar affinity.

Molecular cloning, expression in Escherichia coli and structural-functional analysis of a pyruvate kinase from Pyrobaculum calidifontis

This manuscript describes recombinant production, characterization and structural analysis of wild-type and mutant Pcal_0029, a pyruvate kinase from Pyrobaculum calidifontis. Recombinant Pcal_0029 was produced in soluble and highly active form in Escherichia coli. Purified protein exhibited divalent metal-dependent activity which increased with the increase in temperature till 85 °C. Recombinant Pcal_0029 was highly thermostable with no significant loss in activity even after an incubation of 120 min at 100 °C. The enzyme exhibited apparent S_0.5 and V_max values of 0.44 ± 0.05 mM and 840 ± 39 units, respectively, towards phosphoenolpyruvate. These values towards adenosine-5'-diphosphate were 0.5 ± 0.07 mM and 870 ± 26 units, respectively. In silico structural analysis and comparison with the characterized enzymes revealed the presence of eight conserved regions. Two substitutions, K130E and S155G, resulted in a 10-fold decrease in activity. Secondary structure analysis indicated similar structures for the wild-type and the mutant enzymes. Bioinformatics analysis revealed disruption of interatomic interactions and hydrogen bond formation, leading to a decreased flexibility and solvent accessibility, which may have led to decrease in activity. To the best of our knowledge, Pcal_0029 is the most thermostable pyruvate kinase reported so far. Moreover, this is the first study on the role of non-catalytic residues in a pyruvate kinase.

Brachial Artery Injury in a Child following Closed Elbow Dislocation: Case Report of a Rare Injury

Elbow dislocation, though a common orthopaedic emergency is rare with brachial artery injury and is even more uncommon in the paediatric age group. We present the case of a child who sustained trauma resulting in closed elbow dislocation with brachial artery injury. Elbow dislocation with brachial artery injury can present with palpable distal pulses and good capillary refill because of rich collaterals at the elbow. But this patient presented with signs of frank ischemia distally, and was managed with ipsilateral reverse cephalic vein graft. He had good volume pulses at one year follow-up. Patients with such presentation should have careful clinical and radiological assessment to exclude complicated elbow dislocation.