Protein: Difference between revisions

To delve further into the chemistry, many proteins contain "thiol" groups, each made up of a sulfur and hydrogen (–SH) branch from an amino acid in the protein or polypeptide. These thiol groups can bond to each other, forming a "disulfide bridge" by linking the sulfur atoms together, removing the hydrogen atoms. This bonding occurs very rapidly under oxidative conditions; oxygen in the mash oxidizes the free thiols, creating links between molecules, thereby forming aggregates.<ref name=lund/> Certain enzymes in grain catalyze the oxidation of thiols, and these may include sulphydryl oxidase, glutathione oxidase, glutathione peroxidase and phospholipid-hydroperoxide glutathione peroxidase (but neither peroxidases nor lipoxygenases).<ref name=stephenson>Stephenson WH, Biawa JP, Miracle RE, Bamforth CW. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2003.tb00168.x Laboratory-scale studies of the impact of oxygen on mashing.] ''J Inst Brew.'' 2003;109(3):273–283.</ref> It's interesting that thiol oxidation is actually used in many scientific studies as a measure of wort oxidation, which is of particular interest to many breweries due to its correlation with gel formation.<ref name=celus/> Disulfide bonds may be broken by reducing (oxygen-scavenging) compounds such as [[sulfite]], resulting in the regeneration of the original protein thiol groups. Regenerated thiol groups can participate in new reaction cycles with reactive oxygen species (ROS) and, taken as a whole, act as catalysts for the removal of ROS by sulfite.<ref name=lundmn>Lund MN, Andersen ML. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2011-0620-01 Detection of Thiol Groups in Beer and Their Correlation with Oxidative Stability.] ''J Am Soc Brew Chem.'' 2011;69(3):163–169.</ref> This antioxidant mechanism includes [[lipid transfer protein]] 1 (LTP1) as an important thiol-containing protein. See [[Oxidation]] for more information.

Hops contain a vast array of different proteins — over 1000 unique proteins have been identified.<ref name=kerr/>  Despite this, hop proteins are not present in beer.<ref name=picariello/>

Protein thiols, which are derived from cysteine residues in proteins, are believed to possess antioxidative capacity in beer during storage.<ref name=lund>Lund MN, Lametsch R, Sørensen MB. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.155 Increased protein–thiol solubilization in sweet wort by addition of proteases during mashing.] ''J Inst Brew.'' 2014;120(4):467–473.</ref> The free thiol concentration in twelve different beers has previously been quantified to be 13–46 μM (7). This concentration is dependent not only on the protein concentration but also on the redox status of the beer. The heat-stable malt proteins that are preserved during brewing and end up in the beer are typically disulfide-rich proteins. In order for the disulfides to be reduced and active as thiol antioxidants, they need to be reduced during fermentation and then preserved by an oxygen-limited packaging.

the major contribution to the protein thiol concentration in beer is therefore believed to be derived primarily from LTP1.<ref name=lund/>

 

 

Lipid transfer protein 1 (LTP1), which is the second-most abundant protein in beer, contains eight cysteine residues and is believed to play an important role in the antioxidative mechanism in beer. Beer LTP1 has been shown to exhibit antioxidative activity in two different assays and to scavenge one of the dominating radical compounds in beer, the 1-hydroxyethyl radical, at a rate similar to other reactive compounds in beer such as hop bitter acids. The native form of LTP1 in barley is stabilized by four disulfide bonds and, if these disulfide bonds are not reduced to free thiols, they are not reactive toward the 1-hydroxyethyl radical.<ref name=lundm/> Disulfide bonds are reducible by different chemical reagents, as well as by compounds produced by the yeast such as SO2 and thioredoxin. LTP1 has been found in its reduced form in fresh beer, which suggests that LTP1 is reduced during the brewing process, possibly during wort boiling or during fermentation by compounds produced by the yeast, and that it has the potential to act as an antioxidant in beer.