Taste is simple, right? Table salt is salty. Sugar is sweet. Coffee is bitter. And lemons are sour.
And of course we mustn't forget umami, the fifth and youngest (most recently agreed on) taste. Described in 1908 by the Japanese scientist Kikunae Ikeda, it was only accepted quite recently, so that most languages do not even have their own name for it but have adopted the Japanese term.
Yet there is more to taste than simple salts (like NaCl), carbohydrates (like sucrose), alkaloids (like caffeine), and organic acids (like citric acid) or acid salts (monosodium glutamate). Peptides, despite being subjected to intensive scrutiny of the many biological functions they perform, are rarely considered in light of their taste. However, peptides essentially cover the whole range of the established tastes, and contribute significantly to the complex flavour of much of the food we eat every day.
In general, and not surprisingly, peptides with acidic residues such as aspartic or glutamic acid tend to have a sour taste. Sourness is the taste that detects acidity, through the detection of protons (hydrogen ions) that are released when the carboxylic groups of the peptide dissociate.
Salty peptides are few and far between and are often accompanied by a bitter aftertaste. The 1980s saw a flurry of research around the newly discovered L-ornithine-taurine dipeptide, which was reported to have a salty taste without the presence of any sodium. Some controversy erupted over whether the taste was actually due to the peptide, or residual contaminant NaCl, and no salty peptide so far is being used as a salt substitute.
The hydrophobic amino acids phenylalanine, tryptophan, leucine, and tyrosine have bitter tastes, and similarly peptides rich in hydrophobic residues (especially if they are at the C-terminus) are bitter also. One of the most bitter peptides described is the octapeptide Arg-Arg-Pro-Pro-Pro-Phe-Phe-Phe, with a bitterness comparable to that of strychnine (one of the most bitter molecules known). While humans have an innate aversion to bitter tasting molecules (as protection from ingestion of poisonous substances, such as strychnine), the rejection of bitter foods is not absolute. Foods such as beer, tea, and coffee can be highly bitter, yet are beloved world-wide. In addition, bitter peptides are found in a variety of aged or fermented foodstuffs, including cheese and meaty products such as ham, and other foods containing fermented proteins.
The archetypal umami tastant is of course glutamate, but many peptides have been claimed to be "umami peptides". The so-called "delicious peptide" - the octapeptide Lys-Gly-Asp-Glu-Glu-Ser-Leu-Ala - was suggested to have an umami potency higher than glutamate itself. Unfortunately, upon re-examination of this peptide and its fragments, no real umami taste could be detected, casting the existence of umami peptides into doubt. Yet they have not disappeared: recent research describes umami peptides from peanut hydrolysate and soybean paste. If these results are confirmed, these umami peptides could be very desirable flavour-enhancing alternatives to the sometimes reviled monosodium glutamate.
But it is probably a sweet peptide that we are most familiar with - the dipeptide aspartame (L-aspartyl-L-phenylalanine methyl ester) is the most used non-caloric sweetener in the world. Discovered accidentally when a researcher licked his (contaminated) finger to lift a piece of paper, aspartame was found to be 200 times sweeter than sucrose (table sugar). Following on from this serendipitous discovery, scientists explored many alternatives more rigorously. They found that Asp cannot be substituted by any other residue, whereas Phe can be replaced by some (but not all) hydrophobic amino acids. Interestingly, they also found that all the other possible chiral isomers (D-L, L-D, and D-D) are not sweet at all, but quite bitter. Other modifications, on the other hand, have led to the discovery of super-aspartame molecules, such as neotame. It has a 3,3-dimethylbutyl group attached to the amino group of the aspartic acid, and is around 10,000 times as sweet as sucrose. Most jurisdictions have now approved its use in food.
Proteins and peptides make up a large part of the foods we eat every day, and it is clear that they play a significant role in the complex chemical interplay that is taste. Mostly, the peptides seem to contribute sweet, bitter, and sour tastes, but some evidence suggests salty and umami peptides exist also. While the taste of the sour and salty peptides is probably simply due to the presence of the charged terminals and side chains, bitter and sweet receptors are clearly activated by specific electronic and conformational features of a specific peptide (as demonstrated by the various isomers of aspartame). Peptides are therefore extremely useful tools for researching taste receptor function and leading to a better understanding of taste and taste perception.