Mohamad Hekarl Uzir

Chemoenzymatic and microbial dynamic kinetic resolutions

This review tracks a decade of dynamic kinetic resolution developments with a biocatalytic inclination using enzymatic/microbial means for the resolution part followed by the racemization reactions either by means of enzymatic or chemocatalyst. These fast developments are due to the ability of the biocatalysts to significantly reduce the number of synthetic steps which are common for conventional synthesis. Future developments in novel reactions and products of dynamic kinetic resolutions should consider factors that are needed to be extracted at the early synthetic stage to avoid inhibition at scale-up stage have been highlighted.

Thermodynamics and inhibition studies of lipozyme TL IM in biodiesel production via enzymatic transesterification.

In order to characterize enzyme activity and stability corresponding to temperature effects, thermodynamic studies on commercial immobilized lipase have been carried out via enzymatic transesterification. An optimum temperature of 40 degrees C was obtained in the reaction. The decreasing reaction rates beyond the optimum temperature indicated the occurrence of reversible enzyme deactivation. Thermodynamic studies on lipase denaturation exhibited a first-order kinetics pattern, with considerable stability through time shown by the lipase as well. The activation and deactivation energies were 22.15 kJ mol(-1) and 45.18 kJ mol(-1), respectively, implying more energy was required for the irreversible denaturation of the enzyme to occur. At water content of 0.42%, the initial reaction rate and FAME yield displayed optimum values of 3.317 g/L min and 98%, respectively.

Kinetic Studies on Fermentative Production of Biofuel from Synthesis Gas Using Clostridium ljungdahlii

The intrinsic growth, substrate uptake, and product formation biokinetic parameters were obtained for the anaerobic bacterium, Clostridium ljungdahlii, grown on synthesis gas in various pressurized batch bioreactors. A dual-substrate growth kinetic model using Luong for CO and Monod for H2 was used to describe the growth kinetics of the bacterium on these substrates. The maximum specific growth rate (μ max = 0.195 h−1) and Monod constants for CO (K s,CO = 0.855 atm) and H2 (K s,H2 = 0.412 atm) were obtained. This model also accommodated the CO inhibitory effects on cell growth at high CO partial pressures, where no growth was apparent at high dissolved CO tensions (P CO ∗ > 0.743 atm). The Volterra model, Andrews, and modified Gompertz were, respectively, adopted to describe the cell growth, substrate uptake rate, and product formation. The maximum specific CO uptake rate (q max = 34.364 mmol/gcell/h), CO inhibition constant (K I = 0.601 atm), and maximum rate of ethanol (R max = 0.172 mmol/L/h at P CO = 0.598 atm) and acetate (R max = 0.096 mmol/L/h at P CO = 0.539 atm) production were determined from the applied models.

Saccharomyces cerevisiae: a potential stereospecific reduction tool for biotransformation of mono- and sesquiterpenoids

Terpenes and terpenoids are among the key impact substances in the food and fragrance industries. Equipped with pharmacological properties and applications as ideal precursors for the biotechnological production of natural aroma chemicals, interests in these compounds have been escalating. Hence, the syntheses of new derivatives that can show improved properties are often called for. Stereoselective biotransformation offers several benefits to increase the rate of production, in terms of both the percentage yield and its enantiomeric excesses. Bakers yeast (Saccharomyces cerevisiae) is broadly used as a whole cell stereospecific reduction biocatalyst, due to its capability in reducing carbonyls and carbon–carbon double bonds, which also extends its functionality as a versatile biocatalyst in terpenoid biotransformation. This review provides some insights on the development and prospects in the reductive biotransformation of monoterpenoids and sesquiterpenoids using S. cerevisiae, with an overview of strategies to overcome the common challenges in large‐scale implementation. Copyright

Modelling on the effect of diffusive and convective substrate transport for biofilm

A one-dimensional biofilm model was developed based on the basic principle of conservation of mass. Three simple, generic processes were combined in the model which includes microbial growth, diffusive and convective mass transport. The final model could generate a quantitative description of the relationship between the microbial growth and the consumption of substrate (oxygen) within the fixed biofilm thickness. Mass transfer resistance contributes large influence on the substrates and microbial concentration across the biofilm thickness due to the effect of biofilm structure.

Clostridium ljungdahlii for production of biofuel from synthesis gas

ABSTRACT Lowering the redox potential of the growth medium is a critical step in the cultivation of obligate anaerobes for production of biofuels from synthesis gas. In this study, the simultaneous effects of reducing solutions (sodium sulfide and/or cysteine-HCl) and initial medium pH on the fermentation of synthesis gas using Clostridium ljungdahlii was investigated. The results suggested the plausible provision of more electrons into the culture in the presence of 5.07 mM cysteine-HCl at the medium pH 5.9. This culture demonstrated enhanced ethanol production (48%) and ethanol to acetate production ratio (24%) compared to the cells cultivated in standard growth medium recommended by ATCC.

The feasibility of growing cells of Saccharomyces cerevisiae for citronellol production in a continuous-closed-gas-loop bioreactor (CCGLB)

The present work aims to address the gas-phase biotransformation of geraniol into citronellol using growing cells of Saccharomyces cerevisiae (bakers yeast) in a continuous-closed-gas-loop bioreactor (CCGLB). This study revealed that the gaseous geraniol had a severe effect on the production of biomass during the growing cell biotransformation resulting in the decrease in the specific growth rate from 0.07 to 0.05 h⁻¹. The rate of reaction of the growing cell biotransformation was strongly affected by agitation and substrate flow rates. The highest citronellol concentration of 1.18 g/L and initial rate of reaction of 7.06 × 10⁻⁴ g/min g(cell) were obtained at 500 rpm and 8 L/min, respectively.

Modeling of polycaprolactone production from ε-caprolactone using neural network

In this paper, extensive study of ring-opening polymerization e-caprolactone (e-CL) using lipase Novozym 435 as catalyst in flask level and reactor level were conducted. The polymerization rates increase with an increase in time up to 4 h after which there has been a steep decrease for all temperature from 50 to 100 °C in the flask level. The conclusion out of flask level and reactor level study is that a uniform trend is obtained at 70 °C. A multilayer feed-forward neural network (FANN) model was trained with an error back-propagation algorithm. Reaction time, temperature were used as the input parameters and molecular weight is the output for the flask level study where as reactor impeller speed was also included for reactor level study. Two FANN models with modeling performances of 2-10-1 in the flask level and 3-9-1 FANN1 and 2-13-1 FANN2 (excluding reactor impeller speed) for the reactor level study were obtained.

Kinetic modeling of hydrogen production rate by photoautotrophic cyanobacterium A. variabilis ATCC 29413 as a function of both CO2 concentration and oxygen production rate

ABSTRACT Hydrogen production by cyanobacteria could be one of the promising energy resources in the future. However, there is very limited information regarding the kinetic modeling of hydrogen production by cyanobacteria available in the literature. To provide an in-depth understanding of the biological system involved during the process, the Haldane’s noncompetitive inhibition equation has been modified to determine the specific hydrogen production rate (HPR) as a function of both dissolved CO2 concentration (CTOT) and oxygen production rate (OPR). The highest HPR of 15 was found at xCO2 of 5% vol/vol and the rate consequently decreased when the CTOT and OPR were 0.015 k mol m−3 and 0.55 mL h−1, respectively. The model provided a fairly good estimation of the HPR with respect to the experimental data collected.

Light irradiance and spectral distribution effects on cyanobacterial hydrogen production

Light is an essential energy source for photosynthetic cyanobacteria. Changes in both light irradiance and spectral distribution will affect their photosynthetic productivity. Compared to the light irradiance, little investigations have been carried out on the effect of light spectra towards cyanobacterial hydrogen production. Hence, this work aims to investigate the effects of both light quantity and quality on biohydrogen productivity of heterocystous cyanobacterium, A.variabilis. Under white light condition, the highest hydrogen production rate of 31 µmol H2 mg chl a -1 h-1 was achieved at 70 µE m-2 s-1. When the experiment was repeated at the same light irradiance but different light spectra of blue, red and green, the accumulations of hydrogen were significantly lower than the white light except for blue light. As the light irradiance was increased to 350 µE m-2 s-1, the accumulated hydrogen under the blue light doubled that of the white light. Besides that, an unusual prolongation of the hydrogen production up to 120 h was observed. The results obtained suggest that blue light could be the most desirable light spectrum for cyanobacterial hydrogen production.