Date of Award

5-9-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Peng George Wang

Second Advisor

Hao Xu

Third Advisor

Jun Yin

Abstract

Dihydroxyacetone phosphate (DHAP)-dependent aldolases have been intensively studied and widely used in the synthesis of carbohydrates and complex polyhydroxylated molecules. However, the strict specificity toward donor substrate DHAP greatly hampers their synthetic utility. We transformed DHAP dependent aldolases mediated in vitro reactions into bioengineered Escherichia coli (E. coli). Such flask-to-cell transformation addressed several key issues plaguing in vitro enzymatic synthesis: 1) it solves the problem of DHAP availability by in vivo hijacking DHAP from glycolysis pathway of bacterial system, 2) it circumvents purification of recombinant aldolases and phosphatase, and 3) it dephosphorylates resultant aldol adducts in vivo, thus eliminating the additional step for phosphate removal and achieving in vivo phosphate recycling. The engineered E. coli strains tolerate a wide variety of aldehydes as acceptor, and provide a set of biologically relevant polyhydroxylated molecules in gram scale. Sialic acids exist in abundance in glycan chains of glycoproteins and glycolipids on the surface of all eukaryotic cells and some prokaryotic cells. Their presence affects the molecular properties and structure of glycoconjugates, modifies their functions and interactions with other molecules. The sialylation status, referring to the expression levels and linkages of sialic acids on the cell surface, is determined by the dynamic balance between sialylation and desialylation (removal of sialic acids). Sialylation is mainly regulated through expression and activity of sialyltransferases. And the mainstream idea attributes desialylation to the sialidases. However, more and more emerging evidences support the existence of ROS/RNS mediated chemical desialylation process under some pathological conditions. We used electrochemical oxidation of sialic acid conjugates to mimic ROS mediated chemical desialylation. Such electrochemical desialylation mimicry reveals that 1) β-linked sialic acid is much more difficult to de desialylated than α-linked sialic acid, 2) electron withdrawing residue and bulky underlying residue can facilitate the desialylation, 3) α- 2,3-linked sialic acid is easier to be desialylated than α-2,6- and α-2,8-linked sialic acid. This information is highly valuable for identifying the ROS species participated in ROS mediated desialylation and unveiling corresponding mechanisms. The mechanism of ROS mediated desialylation was proposed to go through radical decarboxylation.

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