Qingshan Huang

Aeration and mass transfer optimization in a rectangular airlift loop photobioreactor for the production of microalgae

Effects of superficial gas velocity and top clearance on gas holdup, liquid circulation velocity, mixing time, and mass transfer coefficient are investigated in a new airlift loop photobioreactor (PBR), and empirical models for its rational control and scale-up are proposed. In addition, the impact of top clearance on hydrodynamics, especially on the gas holdup in the internal airlift loop reactor, is clarified; a novel volume expansion technique is developed to determine the low gas holdup in the PBR. Moreover, a model strain of Chlorella vulgaris is cultivated in the PBR and the volumetric power is analyzed with a classic model, and then the aeration is optimized. It shows that the designed PBR, a cost-effective reactor, is promising for the mass cultivation of microalgae.

Improving the activity of Trichoderma reesei cel7B through stabilizing the transition state.

Trichoderma reesei (Tr.) cellulases, which convert cellulose to reducing sugars, are a promising catalyst used in the lignocellulosic biofuel production. Improving Tr. cellulases activity, though very difficult, is highly desired due to the recalcitrance of lignocellulose. Meanwhile, it is preferable to enhance the cellulases promiscuity so that substrates other than cellulose can also be hydrolyzed. In this work, an attempt is made to improve the catalytic activity of a major endogluanase Tr. Cel7B against xylan which crosslinks with cellulose in lignocellulose. By using quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations, the transition state of the xylo‐oligosaccharide hydrolysis is identified. Then, mutations are introduced and their effect on the transition state stabilization is ranked based on the free energy calculations. Seven top ranked mutants are evaluated experimentally. Three mutants A208Q, A222D, and G230R show a higher activity than the wild‐type Tr. Cel7B in the hydrolysis of xylan (by up to 47%) as well as filter paper (by up to 50%). The combination of the single mutants can further improve the enzyme activity. Our work demonstrates that the free energy method is effective in engineering the Tr. Cel7B activity against xylan and cellulose, and thus may also be useful for improving the activity of other Tr. cellulases. Biotechnol. Bioeng. 2016;113: 1171–1177.

Arginine side chain stacking with peptide plane stabilizes the protein helix conformation in a cooperative way

A combined experimental and computational study is performed for arginine side chain stacking with the protein α‐helix. Theremostability measurements of Aristaless homeodomain, a helical protein, suggest that mutating the arginine residue R106, R137 or R141, which has the guanidino side chain stacking with the peptide plane, to alanine, destabilizes the protein. The R‐PP stacking has an energy of ∼0.2‐0.4 kcal/mol. This stacking interaction mainly comes from dispersion and electrostatics, based on MP2 calculations with the energy decomposition analysis. The calculations also suggest that the stacking stabilizes 2 backbone‐backbone h‐bonds (i→i‐4 and i‐3→i‐7) in a cooperative way. Desolvation and electrostatic polarization are responsible for cooperativity with the i→i‐4 and i‐3→i‐7 h‐bonds, respectively. This cooperativity is supported by a protein α‐helices h‐bond survey in the pdb databank where stacking shortens the corresponding h‐bond distances.