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Engineering the stereoisomeric structure of seed oil to mimic human milk fat
Thursday, 2019/10/17 | 07:44:18

Harrie van Erp, Fiona M. Bryant, Jose Martin-Moreno, Louise V. Michaelson, Govindprasad Bhutada, and Peter J. Eastmond

PNAS, first published September 30, 2019 https://doi.org/10.1073/pnas.1907915116

Significance

In human milk fat, saturated fatty acids are esterified to the middle position on the glycerol backbone giving the triacylglycerol molecules an unusual stereochemistry that assists nutrient absorption in the infant gut. However, the fat used in most infant formulas is derived from plants, which esterify saturated fatty acids to the outer positions. Here, we have engineered the metabolism of an oilseed plant so that it accumulates triacylglycerol with more than 70% of the saturated fatty acid palmitate in the middle position, thereby mimicking human milk fat stereoisomeric structure. Applying this technology to oilseed crops (or oleaginous microorganisms) might provide a source of human milk fat substitute for infant nutrition.

Abstract

Human milk fat substitute (HMFS) is a class of structured lipid that is widely used as an ingredient in infant formulas. Like human milk fat, HMFS is characterized by enrichment of palmitoyl (C16:0) groups specifically at the middle (sn-2 or β) position on the glycerol backbone, and there is evidence that triacylglycerol (TAG) with this unusual stereoisomeric structure provides nutritional benefits. HMFS is currently made by in vitro enzyme-based catalysis because there is no appropriate biological alternative to human milk fat. Most of the fat currently used in infant formulas is obtained from plants, which exclude C16:0 from the middle position. In this study, we have modified the metabolic pathway for TAG biosynthesis in the model oilseed Arabidopsis thaliana to increase the percentage of C16:0 at the middle (vs. outer) positions by more than 20-fold (i.e., from ∼3% in wild type to >70% in our final iteration). This level of C16:0 enrichment is comparable to human milk fat. We achieved this by relocating the C16:0-specific chloroplast isoform of the enzyme lysophosphatidic acid acyltransferase (LPAT) to the endoplasmic reticulum so that it functions within the cytosolic glycerolipid biosynthetic pathway to esterify C16:0 to the middle position. We then suppressed endogenous LPAT activity to relieve competition and knocked out phosphatidylcholine:diacylglycerol cholinephosphotransferase activity to promote the flux of newly made diacylglycerol directly into TAG. Applying this technology to oilseed crops might provide a source of HMFS for infant formula.

 

See https://www.pnas.org/content/early/2019/10/07/1907915116

Fig. 3.

Disruption of ER-resident LPAT2 increases C16:0 incorporation into the sn-2 position of TAG. (A) Diagram of LPAT2 locus showing positions of T-DNA insertions in mutant alleles. Effect of lpat2 mutant backgrounds on (B) the percentage of C16:0 esterified to the sn-2 position of TAG, verses sn-1+3, and (CLPAT2 transcript abundance in seeds expressing ∆CTS-LPAT1. WT, wild type; L11, homozygous ProGLY:∆CTS-LPAT1 line. Values are the mean ± SE of measurements made on separate batches of dry seeds in B and developing siliques in C from 3 plants of each genotype (n = 3). LPAT2 expression was normalized to the geometric mean of 3 reference genes and expressed relative to WT. a, b, and c denote values significantly (P < 0.05) different from L11 (ANOVA + Tukey HSD test).

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