Lipid transfer protein

From Brewing Forward

Lipid transfer proteins (LTPs) are ubiquitous plant lipid-binding proteins that were originally identified by their ability to catalyze the transfer of lipids between membranes. LTPs are abundant soluble proteins of the aleurone layers from barley endosperm and are involved in stress response.[1][2][3] There are two members of the lipid transfer protein family expressed in barley, LTP1 and LTP2.[4] With regard to brewing, lipid transfer protein 1 (LTP1) survives the malting and brewing process and it is the second most abundant protein found in beer, in modified forms.[5][6][7][8] LTP2 is a minor protein in beer.[7][9] LTPs are resistant to degradation by malt and yeast proteases.[10][4][9]

LTP1 in native (unmalted) barley has little effect on beer foam.[2] However, during malting it is glycated via Maillard reactions — grain sugars such as glucose or maltose may react (bind) with the lysine residues of LTP1.[11][12] During mashing, further modification via acylation occurs.[9] Then, boiling the wort causes LTP1 to denature (unfold).[4] The glycation, acylation, and unfolding all increase the foam-promoting effect.[12][5][13] Specifically, the modified LTP1 has excellent foam generation, although it has poor foam stabilizing properties on its own; foam is stabilized by other proteins.[4] Glycation is suggested to stabilize the unfolded protein, keeping it soluble in the wort/beer. Some of the LTP1 remains undenatured from the boil, and this portion plays an important lipid-binding role, mopping up foam-negative lipid materials.[9] Besides its foam benefits, LTP1 from beer has also been found to exhibit high antioxidative activity, being a highly-efficient scavenger towards all reactive oxygen species (ROS) after it has been unfolded.[13] As such, LTP1 is an important antioxidant in beer. See Protein for more information.

Technical information[edit]

The 9 kDa LTP1 contains eight cysteines, representing 9% of the total polypeptide sequence. The 7 kDa LTP2 also has eight cysteines (12%). The cysteines in the native forms (in raw grain) of these proteins form inter- and intra-molecule disulfide bonds. These proteins are denatured and linearized during the brewing process. This may be explained by the secondary structure and polymorphism of the proteins and glycation variants resulted from brewing conditions. The multiple forms of beer LTP, owing to the glycation process, have been well revealed by proteomics. Glycation with sugars such as glucose and xylose often occurs with glycine and lysine residues via the Maillard reaction. D-Glucose reacts with a free amino group in amino acids (e.g. lysine), giving a Schiff base that rapidly rearranges to form a more stable (1-deoxy-D-fructose-1-yl)-amino acid derivative called an Amadori compound. Barley LTP1 contains nine glycines and four lysines, while LTP2 contains 11 glycines and three lysines, which is about 13% of the whole protein.[14]

The high content of thiol cysteines in LPT1 is the basis for its radical scavenging and antioxidant activities. However, native barley LTP1 would not have antioxidant activity because all its thiol groups are occupied in the formation of disulfide bonds. The labeling of LTP1 thiols in beer demonstrated that the disulfide bonds in the native LTP1 were disrupted and linearized, most likely due to denaturing steps of malting, wort boiling, and brewing. These free thiols were maintained during brewing and in packaged beer by a variety of factors. One of them could be the glycation of glycine and lysine residues with sugars such as glucose and xylose via the Maillard reaction. A possible working mechanism for its ROS-scavenging ability is proposed: LTP thiol(s) is oxidized to the sulfenic acid by oxidants such as H2O2, which results in the destruction of a peroxide molecule in 1:1 stoichiometry. The free thiol can be recovered by two sequential reactions (reactions 2 and 3). The reaction 2 generates a disulfide (LTP-SSR) through reaction with a small molecule (HS-R) such as yeast thioredoxin. The reaction 3 uses sulfite or phenolic compounds to generate free thiol from the disulfide for the next round elimination of ROS.[13] Beers with higher levels of free thiols taste better.

See also[edit]

References[edit]

  1. ↑ Steiner E, Gastl M, Becker T. Protein changes during malting and brewing with focus on haze and foam formation: a review. Eur Food Res Technol. 2011;232:191–204.
  2. ↑ a b Perrocheau L, Bakan B, Boivin P, Marion D. Stability of barley and malt lipid transfer protein 1 (LTP1) toward heating and reducing agents: relationships with the brewing process. J Agric Food Chem. 2006;54(8):3108−3113.
  3. ↑ Ali M, Hasan H, Bux H, et al. Chapter 11: Role of transcription factors in drought mediating pathways in wheat. In: Ozturk M, Gul A, eds. Climate Change and Food Security with Emphasis on Wheat. Academic Press; 2020:177–192.
  4. ↑ a b c d Evans DE, Bamforth CW. Beer foam: achieving a suitable head. In: Beer: A Quality Perspective. Academic Press; 2009:1−60.
  5. ↑ a b Jin B, Li L, Liu GQ, Li B, Zhu YK, Liao LN. Structural changes of malt protein during boiling. Molecules. 2009;14(3):1081–1097.
  6. ↑ Lund MN, Lametsch R, Sørensen MB. Increased protein–thiol solubilization in sweet wort by addition of proteases during mashing. J Inst Brew. 2014;120(4):467–473.
  7. ↑ a b Iimure T, Nankaku N, Kihara M, Yamada S, Sato K. Proteome analysis of the wort boiling process. Food Res Int. 2012;45(1):262–271.
  8. ↑ Krottenthaler M, Back W, Zarnkow M. Wort production. In: Esslinger HM, ed. Handbook of Brewing: Processes, Technology, Markets. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2009.
  9. ↑ a b c d Stanislava G. Barley grain non-specific lipid-transfer proteins (ns-LTPs) in beer production and quality. J Inst Brew. 2007;113(3):310–324.
  10. ↑ Gorjanović S, Sužnjević D, Beljanski M, et al. Effects of lipid-transfer protein from malting barley grain on brewers yeast fermentation. J Inst Brew. 2004;110(4):297–302.
  11. ↑ Jégou S, Douliez PJ, Mollé D, Boivin P, Marion D. Evidence of the glycation and denaturation of LTP1 malting and brewing process. J Agric Food Chem. 2001;49(10):4942–4949.
  12. ↑ a b Kerr ED, Fox GP, Schulz BL. Grass to glass: Better beer through proteomics. In: Cifuentes A, ed. Comprehensive Foodomics. Elsevier; 2020:407–416.
  13. ↑ a b c Wu MJ, Clarke FM, Rogers PJ, et al. Identification of a protein with antioxidant activity that is important for the protection against beer ageing. Int J Mol Sci. 2011;12(9):6089–6103.
  14. ↑ Wu MJ, Rogers PJ, Clarke FM. 125th anniversary review: The role of proteins in beer redox stability. J Inst Brew. 2012;118(1):1–11.
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