The structural features of an ancient ribonuclease from Salmo salar reveal an intriguing case of auto-inhibition.

The superfamily of vertebrate ribonucleases, a large group of evolutionarily related proteins, continues to provide interesting structural and functional information. In particular, the crystal structure of SS-RNase-2 from Salmo salar (SS2), here presented, has revealed a novel auto-inhibition mechanism that enriches the number of inhibition strategies observed in some members of the family. Within an essentially unmodified RNase folding, the SS2 active site cleft is in part obstructed by the collapse of an extra pentapeptide inserted in the C-terminal region. This unexpected intrusion alters the organization of the catalytic triad by pushing one catalytic histidine off the pocket. Possible mechanisms to remove the active site obstruction have also been studied through the production of two mutants that provide useful information on the functionality of this intriguing version of the ribonuclease superfamily.

Towards the identification of the allosteric Phe-binding site in phenylalanine hydroxylase.

The enzyme phenylalanine hydroxylase (PAH) is defective in the inherited disorder phenylketonuria. PAH, a tetrameric enzyme, is highly regulated and displays positive cooperativity for its substrate, Phe. Whether Phe binds to an allosteric site is a matter of debate, despite several studies worldwide. To address this issue, we generated a dimeric model for Phe-PAH interactions, by performing molecular docking combined with molecular dynamics simulations on human and rat wild-type sequences and also on a human G46S mutant. Our results suggest that the allosteric Phe-binding site lies at the dimeric interface between the regulatory and the catalytic domains of two adjacent subunits. The structural and dynamical features of the site were characterized in depth and described. Interestingly, our findings provide evidence for lower allosteric Phe-binding ability of the G46S mutant than the human wild-type enzyme. This also explains the disease-causing nature of this mutant.

Glomerular filtration rate and cardiometabolic risk in an outpatient pediatric population with high prevalence of obesity.

OBJECTIVE: To evaluate the relationship between estimated glomerular filtration rate (eGFR) and cardiometabolic risk factors (CMRF) in an outpatient pediatric population with high prevalence of obesity. DESIGN AND METHODS: eGFR was evaluated in 901 children unselected for chronic kidney disease of whom 694 were overweight/obese (6-16 years) and 207 were age- and sex-matched normal weight (NW). We generated three categories of eGFR: mild-low eGFR (< 20th percentile), high eGFR (>80th percentile) and intermediate eGFR (20-80th percentile), considered as the reference category RESULTS: Children with either mild-low or high eGFR category showed a 2-4 fold higher Odds ratio of high blood pressure, left ventricular hypertrophy, and microalbuminuria compared with children of the intermediate eGFR category. In addition, children with mild-low eGFR levels showed a 1.5-2 fold higher Odds ratio of impaired fasting glucose and high white blood cell count compared with children with intermediate eGFR levels. CONCLUSIONS: In outpatient children with high prevalence of obesity, children with either mildly reduced or high eGFR have an increased burden of CMRF. Children with eGFR < 97 mL/min/1.73 m² show a worse CMR profile. This finding supports the usefulness to assess eGFR to identify children with unfavorable CMR profile.

Structural features of the regulatory ACT domain of phenylalanine hydroxylase.

Phenylalanine hydroxylase (PAH) catalyzes the conversion of L-Phe to L-Tyr. Defects in PAH activity, caused by mutations in the human gene, result in the autosomal recessively inherited disease hyperphenylalaninemia. PAH activity is regulated by multiple factors, including phosphorylation and ligand binding. In particular, PAH displays positive cooperativity for L-Phe, which is proposed to bind the enzyme on an allosteric site in the N-terminal regulatory domain (RD), also classified as an ACT domain. This domain is found in several proteins and is able to bind amino acids. We used molecular dynamics simulations to obtain dynamical and structural insights into the isolated RD of PAH. Here we show that the principal motions involve conformational changes leading from an initial open to a final closed domain structure. The global intrinsic motions of the RD are correlated with exposure to solvent of a hydrophobic surface, which corresponds to the ligand binding-site of the ACT domain. Our results strongly suggest a relationship between the Phe-binding function and the overall dynamic behaviour of the enzyme. This relationship may be affected by structure-disturbing mutations. To elucidate the functional implications of the mutations, we investigated the structural effects on the dynamics of the human RD PAH induced by six missense hyperphenylalaninemia-causing mutations, namely p.G46S, p.F39C, p.F39L, p.I65S, p.I65T and p.I65V. These studies showed that the alterations in RD hydrophobic interactions induced by missense mutations could affect the functionality of the whole enzyme.

Glucokinase (GCK) mutations and their characterization in MODY2 children of southern Italy.

Type 2 Maturity Onset Diabetes of the Young (MODY2) is a monogenic autosomal disease characterized by a primary defect in insulin secretion and hyperglycemia. It results from GCK gene mutations that impair enzyme activity. Between 2006 and 2010, we investigated GCK mutations in 66 diabetic children from southern Italy with suspected MODY2. Denaturing High Performance Liquid Chromatography (DHPLC) and sequence analysis revealed 19 GCK mutations in 28 children, six of which were novel: p.Glu40Asp, p.Val154Leu, p.Arg447Glyfs, p.Lys458_Cys461del, p.Glu395_Arg397del and c.580-2A>T. We evaluated the effect of these 19 mutations using bioinformatic tools such as Polymorphism Phenotyping (Polyphen), Sorting Intolerant From Tolerant (SIFT) and in silico modelling. We also conducted a functional study to evaluate the pathogenic significance of seven mutations that are among the most severe mutations found in our population, and have never been characterized: p.Glu70Asp, p.His137Asp, p.Phe150Tyr, p.Val154Leu, p.Gly162Asp, p.Arg303Trp and p.Arg392Ser. These seven mutations, by altering one or more kinetic parameters, reduced enzyme catalytic activity by >40%. All mutations except p.Glu70Asp displayed thermal-instability, indeed >50% of enzyme activity was lost at 50°C/30 min. Thus, these seven mutations play a pathogenic role in MODY2 insurgence. In conclusion, this report revealed six novel GCK mutations and sheds some light on the structure-function relationship of human GCK mutations and MODY2.

Usefulness of the high triglyceride-to-HDL cholesterol ratio to identify cardiometabolic risk factors and preclinical signs of organ damage in outpatient children.

OBJECTIVE: To evaluate whether the high triglyceride-to-HDL cholesterol (TG-to-HDL-C) ratio is associated with cardiometabolic risk (CMR) factors and preclinical signs of organ damage in an outpatient population of white children and adolescents. RESEARCH DESIGN AND METHODS: The study population included 884 subjects (aged 6-16 years), of whom 206 (23%) were normal weight, 135 (15%) were overweight, and 543 (61%) were obese. Biochemical variables were analyzed in the whole sample, whereas homocysteine and left ventricular (LV) geometry and function were evaluated in 536 and 258 children, respectively. RESULTS: The percentage of pubertal children (P < 0.001), as well as measurements of BMI, waist circumference, homeostasis model assessment of insulin resistance, white blood cell count, alanine aminotransferase (ALT), systolic blood pressure (P < 0.0001, for all), creatinine (P < 0.001), and diastolic blood pressure (P < 0.02), increased from the lowest to the highest tertile of the TG-to-HDL-C ratio. Age, sex, homocysteine, and glomerular filtration rate did not change. Moreover, interventricular septum thickness, relative wall thickness, and LV mass index (P = 0.01 to P < 0.0001) increased across tertiles of the TG-to-HDL-C ratio. Children with a TG-to-HDL-C ratio ≥2.0 showed a two- to threefold higher risk of elevated ALT levels and concentric LV hypertrophy than those with a TG-to-HDL-C ratio <2.0, independent of confounding factors. CONCLUSIONS: The high TG-to-HDL-C ratio is associated with several CMR factors and preclinical signs of liver and cardiac abnormalities in the outpatient, white pediatric population. Thus, a TG-to-HDL-C ratio ≥2.0 may be useful in clinical practice to detect children with a worsened CMR profile who need monitoring to prevent cardiovascular disease in adulthood.

Natural phenylalanine hydroxylase variants that confer a mild phenotype affect the enzyme's conformational stability and oligomerization equilibrium.

Hyperphenylalaninemias are genetic diseases prevalently caused by mutations in the phenylalanine hydroxylase (PAH) gene. The wild-type PAH enzyme is a homotetramer regulated by its substrate, cofactor and phosphorylation. We reproduced a full-length wild-type protein and seven natural full-length PAH variants, p.I65M, p.N223Y, p.R297L, p.F382L, p.K398N, p.A403V, and p.Q419R, and analyzed their biochemical and biophysical behavior. All mutants exhibited reduced enzymatic activity, namely from 38% to 69% of wild-type activity. Biophysical characterization was performed by size-exclusion chromatography, light scattering and circular dichroism. In the purified wild-type PAH, we identified the monomer in equilibrium with the dimer and tetramer. In most mutants, the equilibrium shifted toward the dimer and most tended to form aggregates. All PAH variants displayed different biophysical behaviors due to loss of secondary structure and thermal destabilization. Specifically, p.F382L was highly unstable at physiological temperature. Moreover, using confocal microscopy with the number and brightness technique, we studied the effect of BH4 addition directly in living human cells expressing wild-type PAH or p.A403V, a mild mutant associated with BH4 responsiveness in vivo. Our results demonstrate that BH4 addition promotes re-establishment of the oligomerization equilibrium, thus indicating that the dimer-to-tetramer shift in pA403V plays a key role in BH4 responsiveness. In conclusion, we show that the oligomerization process and conformational stability are altered by mutations that could affect the physiological behavior of the enzyme. This endorses the hypothesis that oligomerization and folding defects of PAH variants are the most common causes of HPAs, particularly as regards mild human phenotypes.