NAD+ -Dependent Dehydrogenase PctP and Pyridoxal 5′-Phosphate Dependent Aminotransferase PctC Catalyze the First Postglycosylation Modification of the Sugar Intermediate in Pactamycin Biosynthesis

Modifications of Protein-Bound Substrates by Trans-Acting Enzymes in Natural Products Biosynthesis

Enzymatic modifications of small molecules are a common phenomenon in natural product biosynthesis, leading to the production of diverse bioactive compounds. In polyketide biosynthesis, modifications commonly take place after the completion of the polyketide backbone assembly by the polyketide synthases and the mature products are released from the acyl-carrier protein (ACP). However, exceptions to this rule appear to be widespread, as on-line hydroxylation, methyl transfer, and cyclization during polyketide assembly process are common, particularly in trans-AT PKS systems. Many of these modifications are catalyzed by specific domains within the modular PKS systems. However, several of the on-line modifications are catalyzed by stand-alone proteins. Those include the on-line Baeyer-Villiger oxidation, α-hydroxylation, halogenation, epoxidation, and methyl esterification during polyketide assembly, dehydrogenation of ACP-bound short fatty acids by acyl-CoA dehydrogenase-like enzymes, and glycosylation of ACP-bound intermediates by discrete glycosyltransferase enzymes. This review article highlights some of these trans-acting proteins that catalyze enzymatic modifications of ACP-bound small molecules in natural product biosynthesis.

Modifications of Acyl Carrier Protein-Bound Glycosylated Polyketides in Pactamycin Biosynthesis

The potent antitumor antibiotic pactamycin is an aminocyclopentitol-containing natural product produced by the soil bacterium Streptomyces pactum. Recent studies showed that the aminocyclopentitol unit is derived from N-acetyl-D-glucosamine, which is attached to an acyl carrier protein (ACP)-bound polyketide by a glycosyltransferase enzyme, PtmJ. Here, we report a series of post-glycosylation modifications of the sugar moiety of the glycosylated polyketide while it is still attached to the carrier protein. In vitro reconstitution of PtmS (an AMP-ligase), PtmI (an ACP), PtmJ, PtmN (an oxidoreductase), PtmA (an aminotransferase), and PtmB (a putative carbamoyltransferase) showed that the N-acetyl-D-glucosamine moiety of the glycosylated polyketide is first oxidized by PtmN and then transaminated by PtmA to give ACP-bound 3-amino-3-deoxy-N-acetyl-D-glucosaminyl polyketide. The amino group is then coupled with carbamoyl phosphate by PtmB to give a urea functionality. We also show that PtmG is a deacetylase that hydrolyses the C-2 N-acetyl group to give a free amine.

Biosynthesis of cyclitols

Review covering up to 2021Cyclitols derived from carbohydrates are naturally stable hydrophilic substances under ordinary physiological conditions, increasing the water solubility of whole molecules in cells. The stability of cyclitols is derived from their carbocyclic structures bearing no acetal groups, in contrast to sugar molecules. Therefore, carbocycle-forming reactions are critical for the biosynthesis of cyclitols. Herein, we review naturally occurring cyclitols that have been identified to date and categorize them according to the type of carbocycle-forming enzymatic reaction. Furthermore, the cyclitol-forming enzymatic reaction mechanisms and modification pathways of the initially generated cyclitols are reviewed.

The chemistry and biology of natural ribomimetics and related compounds

Natural ribomimetics represent an important group of specialized metabolites with significant biological activities. Many of the activities, e.g., inhibition of seryl-tRNA synthetases, glycosidases, or ribosomes, are manifestations of their structural resemblance to ribose or related sugars, which play roles in the structural, physiological, and/or reproductive functions of living organisms. Recent studies on the biosynthesis and biological activities of some natural ribomimetics have expanded our understanding on how they are made in nature and why they have great potential as pharmaceutically relevant products. This review article highlights the discovery, biological activities, biosynthesis, and development of this intriguing class of natural products.

Functional Characterization of 3-Aminobenzoic Acid Adenylation Enzyme PctU and UDP-N-Acetyl-d-Glucosamine: 3-Aminobenzoyl-ACP Glycosyltransferase PctL in Pactamycin Biosynthesis

Pactamycin is an antibiotic produced by Streptomyces pactum with antitumor and antimalarial properties. Pactamycin has a unique aminocyclitol core that is decorated with 3-aminoacetophenone, 6-methylsaliciate, and an N,N-dimethylcarbamoyl group. Herein, we show that the adenylation enzyme PctU activates 3-aminobenzoic acid (3ABA) with adenosine triphosphate and ligates it to the holo form of the discrete acyl carrier protein PctK to yield 3ABA-PctK. Then, 3ABA-PctK is N-glycosylated with uridine diphosphate-N-acetyl-d-glucosamine (UDP-GlcNAc) by the glycosyltransferase PctL to yield GlcNAc-3ABA-PctK. Because 3ABA is known to be a precursor of the 3-aminoacetophenone moiety, PctU appears to be a gatekeeper that selects the appropriate 3-aminobenzoate starter unit. Overall, we propose that acyl carrier protein-bound glycosylated 3ABA derivatives are biosynthetic intermediates of pactamycin biosynthesis.

Glycosylation of acyl carrier protein-bound polyketides during pactamycin biosynthesis

Glycosylation is a common modification reaction in natural products biosynthesis and has been known to be a post assembly line tailoring process in glycosylated polyketide biosynthesis. Here, we show that in pactamycin biosynthesis glycosylation can take place on an acyl carrier protein (ACP)-bound polyketide intermediate. Using in vivo gene inactivation, chemical complementation, and in vitro pathway reconstitution we demonstrate that the 3-aminoacetophenone moiety of pactamycin is derived from 3-aminobenzoic acid by a set of discrete polyketide synthase proteins via a 3-[3-aminophenyl]3-oxopropionyl-ACP intermediate. This ACP-bound intermediate is then glycosylated by an N-glycosyltransferase, PtmJ, providing a sugar precursor for the formation of the aminocyclopentitol core structure of pactamycin. This is the first example of glycosylation of a small molecule while tethered to a carrier protein. Additionally, we demonstrate that PtmO is a hydrolase that is responsible for the release of the ACP-bound product to a free β-ketoacid that subsequently undergoes decarboxylation.

The secondary metabolite pactamycin with potential for pharmaceutical applications: biosynthesis and regulation

The antitumor antibiotic pactamycin is a highly substituted aminocyclopentitol-derived secondary metabolite produced by the soil bacterium Streptomyces pactum. It has exhibited potent antibacterial, antitumor, antiviral, and antiprotozoal activities. Despite its outstanding biological activities, the complex chemical structure and broad-spectrum toxicity have hampered its development as a therapeutic, limiting its contribution to biomedical science to a role as a molecular probe for ribosomal function. However, a detailed understanding of its biosynthesis and how the biosynthesis is regulated has made it possible to tactically design and produce new pactamycin analogues, some of which have shown improved pharmacological properties. This mini-review describes the biosynthesis, regulation, engineered production, and biological activities of pactamycin and its congeners. It also highlights the suitability of biosynthetic methods as a feasible approach to generate new analogues of complex natural products and underscores the importance of utilizing biosynthetic enzymes as tools for chemoenzymatic production of structurally diverse bioactive compounds.