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Methionine
**Methionine** is a sulfur-containing essential amino acid that the human body cannot synthesize, making dietary intake mandatory. It functions both as the initiating residue of protein synthesis and as the body's primary methyl donor, and it is frequently the limiting amino acid in legumes and pulses.
Biological role
Methionine carries a sulfur atom in its side chain, a structural feature it shares only with cysteine among the standard amino acids. Beyond its role as a building block of proteins, methionine is converted to S-adenosylmethionine (SAM), the cofactor that donates methyl groups in reactions ranging from DNA methylation to neurotransmitter and creatine metabolism.
Methionine also occupies a unique position in translation: the start codon AUG codes for methionine, so nearly every polypeptide chain begins with a methionine residue. This makes it both a metabolic intermediate and the literal first step of building a new protein.
Methionine as a limiting amino acid
An amino acid is “limiting” when its scarcity caps how much complete protein the body can assemble from a food, regardless of how abundant the other amino acids are. In most legumes and pulses, that constraint is methionine — usually grouped with cysteine as the total sulfur amino acids. In an analysis of new pea (Pisum sativum L.) genotypes, the combined methionine-plus-cysteine content averaged only 2.6 g per 100 g of protein, giving a chemical score of 46% — the lowest of any amino acid measured (Molecules, 2024, PMID 39519674).
Collagen is similarly low in sulfur amino acids, which is one reason collagen and most single-pulse proteins are classed as incomplete protein sources.
Relevance to potato protein
Unlike pulses, potato protein is not constrained by its sulfur amino acids in the same way, which is part of why its measured quality runs high. The Digestible Indispensable Amino Acid Score (DIAAS) for potato protein isolates has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID 33133540). A favorable methionine contribution is one factor distinguishing potato protein from the legume isolates it is often compared against. For a broader overview of the ingredient and how it is processed, see what is potato protein.
Where a diet leans heavily on pulses, pairing them with a methionine-richer source — grains, animal proteins, or potato protein — is the conventional way to cover the shortfall, the principle behind complementary protein combinations.
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