Polysaccharide utilization loci from Bacteroidota encode CE15 enzymes with possible roles in cleaving pectin-lignin bonds.

Lignocellulose is a renewable but complex material exhibiting high recalcitrance to enzymatic hydrolysis, which is attributed, in part, to the presence of covalent linkages between lignin and polysaccharides in the plant cell wall. Glucuronoyl esterases from carbohydrate esterase family 15 (CE15) have been proposed as an aid in reducing this recalcitrance by cleaving ester bonds found between lignin and glucuronoxylan. In the Bacteroidota phylum, some species organize genes related to carbohydrate metabolism in polysaccharide utilization loci (PULs) which encode all necessary proteins to bind, deconstruct, and respond to a target glycan. Bioinformatic analyses identified CE15 members in some PULs that appear to not target the expected glucuronoxylan. Here, five CE15 members from such PULs were investigated with the aim of gaining insights on their biological roles. The selected targets were characterized using glucuronoyl esterase model substrates and with a new synthetic molecule mimicking a putative ester linkage between pectin and lignin. The CE15 enzyme from Phocaeicola vulgatus was structurally determined by X-ray crystallography both with and without carbohydrate ligands with galacturonate binding in a distinct conformation than that of glucuronate. We further explored whether these CE15 enzymes could act akin to pectin methylesterases on pectin-rich biomass but did not find evidence to support the proposed activity. Based on the evidence gathered, the CE15 enzymes in the PULs expected to degrade pectin could be involved in cleavage of uronic acid esters in rhamnogalacturonans.IMPORTANCEThe plant cell wall is a highly complex matrix, and while most of its polymers interact non-covalently, there are also covalent bonds between lignin and carbohydrates. Bonds between xylan and lignin are known, such as the glucuronoyl ester bonds that are cleavable by CE15 enzymes. Our work here indicates that enzymes from CE15 may also have other activities, as we have discovered enzymes in PULs proposed to target other polysaccharides, including pectin. Our study represents the first investigation of such enzymes. Our first hypothesis that the enzymes would act as pectin methylesterases was shown to be false, and we instead propose that they may cleave other esters on complex pectins such as rhamnogalacturonan II. The work presents both the characterization of five novel enzymes and can also provide indirect information about the components of the cell wall itself, which is a highly challenging material to chemically analyze in fine detail.

Left Frontal Lobe Cardioembolic Stroke as a Co-presentation of Bicuspid Aortic Valve With Severe Stenosis and a Basal Interventricular Septal Aneurysm in a Young Female Patient.

Bicuspid aortic valve (BAV) and interventricular septum (IVS) aneurysms are common congenital heart defects affecting 1.3% and 0.3% of the population, respectively. The coexistence of membranous types of IVS aneurysm and BAV is even rarer. We report a case of a 48-year-old woman with a history of BAV and severe aortic stenosis who had a seizure in a grocery store and was brought to the emergency department (ED). An MRI of the brain without contrast revealed a left frontal lobe acute lacunar infarct, suggestive of embolic origin. A transesophageal echocardiogram confirmed a basal IVS aneurysm measuring 12.2 mm × 16 mm without intracardiac shunting or thrombi. We diagnosed her with cardioembolic stroke as a complication of BAV and IVS aneurysm and initiated anticoagulation as she did not qualify for surgical intervention. This report emphasizes that IVS aneurysms associated with BAV, although often asymptomatic, may cause adverse outcomes such as cardioembolic stroke. Therefore, timely detection by non-invasive imaging, including echocardiography, CT scans, and MRI, and appropriate intervention are essential to improving health outcomes and survival.