Glycyl-alanyl-lysine-L-isoleucine

Glycyl-Alanyl-Lysine-L-Isoleucine (Gly-Ala-Lys-Ile): A Reference Article

Preface: Scope and Evidence Limitations

Glycyl-alanyl-lysine-L-isoleucine (abbreviated Gly-Ala-Lys-Ile or GAKI) is a synthetic tetrapeptide ingredient that has appeared on supplement-product labels, primarily in sports nutrition products. A thorough search of peer-reviewed literature (PubMed/PMC), government health databases (NIH, EFSA, EMA), pharmacopeias, and rigorous evidence databases (e.g., Examine.com) finds no published clinical trials, systematic reviews, pharmacological studies, or official monographs specific to this exact four-amino-acid sequence. Accordingly, this article distinguishes precisely between: (1) what is chemically established about the molecule and its constituents; (2) what peer-reviewed science has demonstrated for structurally related peptides and for the individual amino acids; and (3) what remains unverified or is based solely on manufacturer claims. Claims that cannot be grounded in authoritative sources are omitted rather than fabricated.

1. Identity and Chemical Characterization

1.1 Systematic and Common Names

Glycyl-alanyl-lysine-L-isoleucine is a linear tetrapeptide whose systematic IUPAC-style name describes the N-to-C terminal sequence of its four constituent amino acid residues: glycine (Gly, G) — L-alanine (Ala, A) — L-lysine (Lys, K) — L-isoleucine (Ile, I). The one-letter sequence code is GAKI. It is also rendered as H-Gly-Ala-Lys-Ile-OH in peptide chemistry notation. Each of the four component amino acids is individually catalogued in the PubChem compound database maintained by the National Center for Biotechnology Information (NCBI): glycyl-L-alanine (CID 1551643), glycyl-L-alanyl-L-lysine (CID 71404690), glycyl-L-isoleucine (CID 88079), and glycylisoleucine (CID 259613).

Glycyl-L-alanyl-L-lysine has the molecular formula C₁₁H₂₂N₄O₄ and is catalogued as PubChem CID 71404690. No single PubChem compound record for the complete Gly-Ala-Lys-Ile tetrapeptide was identified in the current literature searches, indicating it has not been assigned a unique CID as of the available database records. The structurally related tripeptide glycyl-alanyl-isoleucine (without the lysine residue) has the molecular formula C₁₁H₂₁N₃O₄ and is catalogued as PubChem CID 126261.

1.2 Constituent Amino Acids

• Glycine (Gly, G): The simplest amino acid, with a single hydrogen as its side chain. It is non-essential and is the most abundant amino acid in collagen and connective tissue proteins.
• L-Alanine (Ala, A): A non-essential, aliphatic amino acid. Human skeletal muscle can oxidize alanine (among other amino acids including leucine, isoleucine, valine, glutamate, asparagine, and aspartate) during exercise, providing additional free energy to fuel muscle contraction.
• L-Lysine (Lys, K): An essential amino acid bearing a positively charged ε-amino group in its side chain. It is a key structural component of collagen cross-links and is required for carnitine biosynthesis.
• L-Isoleucine (Ile, I): An essential, branched-chain amino acid (BCAA). Among the amino acids, the most studied and consumed as supplements are the BCAAs — leucine, valine, and isoleucine. Isoleucine shares metabolic pathways with leucine and valine and is a substrate for gluconeogenesis and ketogenesis.

1.3 Structural Classification

Gly-Ala-Lys-Ile belongs to the class of synthetic tetrapeptides — four-residue linear peptide chains linked by amide (peptide) bonds. It contains: one non-polar, non-charged residue (Gly); one aliphatic, non-polar residue (Ala); one positively charged, polar residue at physiological pH (Lys); and one branched-chain, non-polar, hydrophobic residue (Ile). This combination of hydrophilic (Gly, Lys) and hydrophobic (Ala, Ile) residues gives the tetrapeptide an amphiphilic character relevant to its potential interactions with membrane-associated receptors and transporters.

1.4 Common Supplement Forms and Preparations

As a commercial supplement ingredient, Gly-Ala-Lys-Ile appears in powdered sports nutrition formulations — protein powders and amino acid blend products — primarily from the sports nutrition sector. It is synthesized via solution-phase or solid-phase peptide synthesis methods rather than derived by isolation from a natural food protein. It is present in finished products as one component among multiple peptide sequences and is not typically sold as a standalone, single-ingredient supplement. No official pharmacopeial preparation monograph (USP, European Pharmacopoeia, BP) for this tetrapeptide was identified.

2. Natural Sources and Occurrence

Bioactive peptides are physiologically active peptides mostly derived from proteins following gastrointestinal digestion, fermentation, or hydrolysis by proteolytic enzymes. While the specific sequence Gly-Ala-Lys-Ile has not been isolated or identified in any documented natural protein hydrolysate in the peer-reviewed literature, all four of its constituent amino acids are naturally and abundantly present in dietary proteins.

The protein hydrolysis process allows the cleavage of proteins into amino acids and peptides with low molecular weight that have high-quality amino acids and can be used as nutraceutical or functional foods. Foods rich in the four constituent amino acids — glycine, alanine, lysine, and isoleucine — include animal-sourced proteins (meat, poultry, fish, dairy, eggs) and plant-sourced proteins (legumes, soy, certain grains). Glycine and alanine are especially concentrated in connective tissue proteins (gelatin, collagen hydrolysates), while lysine and isoleucine are abundant in whey and casein.

It is theoretically possible, though unconfirmed in published studies, that the GAKI sequence could be generated transiently during gastrointestinal proteolysis of intact food proteins. The presence of hydrophilic amino acids such as proline, alanine, valine, and leucine in the N-terminal position and of amino acids including isoleucine in the C-terminal position has been associated with bioactive peptide properties in food protein hydrolysates.

3. Traditional and Historical Use

The tetrapeptide Gly-Ala-Lys-Ile as a discrete chemical entity has no documented history of traditional or historical use in any medicinal system. It is a product of modern synthetic peptide chemistry and was not available to pre-modern cultures. Any reference to "traditional use" of this specific molecule on supplement marketing materials is inaccurate.

The four constituent amino acids, however, are universally present in dietary protein and therefore have been consumed throughout human history as components of food. No traditional medicine system (Ayurveda, Traditional Chinese Medicine, Galenic medicine, Unani, etc.) identified or described individual amino acids or short peptide sequences as discrete therapeutic agents; rather, protein-rich foods were prescribed holistically. There are no WHO monographs, ESCOP monographs, or Commission E monographs for any of these individual amino acids in the context of supplementation, nor for the Gly-Ala-Lys-Ile tetrapeptide.

4. Key Constituents and Proposed Mechanisms of Action

4.1 The Peptide as an Active Entity

The fundamental question for any orally ingested peptide is whether it survives gastrointestinal proteolysis to reach the intestinal epithelium and systemic circulation intact. This question is particularly important for tetrapeptides.

Evidence indicates that more than 70% of protein digestion products in the gastrointestinal tract are absorbed in the form of di/tripeptides, primarily via a specialized proton-coupled transport system. Bioactive di/tripeptides have become key components in functional foods due to their excellent bioactivity and absorption properties; however, their oral bioavailability is limited by the complexity of intestinal absorption processes, primarily relying on oligopeptide transporter 1 (PepT1) for transmembrane transport.

The intestinal PepT1 transporter is the dominant route for small peptide uptake. The peptide binding cavity of PepT1 is reported to be approximately 13 × 12 × 11 Å, which limits attachment to only di- and tripeptides, while being sterically restrictive for peptides with greater than tetrapeptide length. This is a critical constraint: as a tetrapeptide, Gly-Ala-Lys-Ile is at the upper size boundary for PepT1-mediated transport, and whether it is transported intact or must first be partially hydrolyzed to di- or tripeptide fragments is not established in published research for this specific sequence.

It is now known that the uptake of di- and tripeptides into the enterocyte is considerable, being transported across the intestinal endothelium by the PepT1 H⁺/peptide co-transporter. There is also evidence that some di- and tripeptides may survive cytosolic hydrolysis and be transported intact across the basolateral membrane. However, other than antigen sampling, the transport of larger intact macromolecules across the intestinal endothelium of the healthy adult human remains a controversial issue, as there is little unequivocal in vivo evidence to support this postulation.

Multiple assays have revealed that PepT1 has an enormous range of substrates, including more than 400 different dipeptides and 8,000 tripeptides produced from the digestion of dietary or body proteins and a repertoire of structurally closely related compounds and drugs.

4.2 The Constituent Amino Acids as Active Components

If Gly-Ala-Lys-Ile is hydrolyzed in the gut, its four component amino acids and their di/tripeptide sub-sequences would enter the portal circulation. The established biological activities of these components include:

• Glycine: Serves as a precursor for collagen, creatine, glutathione, bile acids, and heme. It also acts as an inhibitory neurotransmitter in the central nervous system and has been shown to modulate inflammation via glycine-gated chloride channel receptors on macrophages in in vitro studies.
• L-Alanine: Functions in glucose-alanine cycling between muscle and liver (gluconeogenesis), and as a substrate for protein and purine synthesis.
• L-Lysine: Required for carnitine biosynthesis, collagen hydroxylation (cross-linking), and as a substrate for histone modification (acetylation, methylation). It competes with arginine for intestinal uptake, which has been explored in herpes simplex virus management in the clinical literature (though no specific relevance to Gly-Ala-Lys-Ile is established).
• L-Isoleucine: A BCAA that contributes to muscle protein synthesis and energy metabolism. Branched-chain amino acid-containing bioactive dipeptides in whey protein hydrolysate — including those with isoleucine residues (e.g., Ile-Val, Leu-Ile, Ile-Leu) — have been shown to significantly stimulate glucose uptake in rat muscle cell models.

4.3 Proposed Mechanisms for Intact Bioactive Tetrapeptides (General Class)

The effects of bioactive peptides appear to be due to interactions with receptors or endogenous proteins such as mammalian target of rapamycin (mTOR), glycogen synthase, or glucose transporter type 4 (GLUT-4), which exceed the effects of individual amino acids.

The presence of some amino acids and their position in the peptide sequence has an important effect on antioxidant activity. Aromatic amino acids such as tyrosine, histidine, tryptophan, and phenylalanine, and hydrophobic amino acids such as valine, leucine, methionine, glycine, and alanine, are essential for the antioxidant role of a peptide. For the GAKI sequence specifically, glycine (at the N-terminus) and alanine have been noted as contributing hydrophobic amino acids in peptide antioxidant research; however, the GAKI sequence specifically has not been tested for antioxidant activity in published studies.

Peptides used in bioactive research are novel active ingredients that may improve collagen synthesis, enhance skin cell proliferation, or decrease inflammation. Based on their mechanism of action, they can be classified into signal peptides, carrier peptides, neurotransmitter inhibitor peptides, and enzyme inhibitor peptides. No classification of Gly-Ala-Lys-Ile into any of these functional categories has been established in published literature.

5. Scientific Evidence by Area of Use

Because no peer-reviewed study has investigated the Gly-Ala-Lys-Ile tetrapeptide specifically, the following sections describe the evidence that exists for structurally analogous tetrapeptides, protein hydrolysates containing similar amino acid compositions, and the individual constituent amino acids. The evidence strength for Gly-Ala-Lys-Ile itself is therefore rated: absent / not established across all areas.

5.1 Muscle Protein Synthesis and Sports Performance

In recent years, there has been increasing evidence that biologically active peptides could also play an important role in sports nutrition. Current studies have shown that bioactive peptides could have a positive impact on changes in body composition and muscular performance, reduce muscle damage following exercise, and induce beneficial adaptions within the connective tissue.

Hydrolysates have demonstrated several advantages over intact proteins, such as a higher speed in muscle protein synthesis or the production of peptides with specific bioactivities that could act from a multifunctional point of view in the improvement of sport performance and related disorders. However, this evidence applies to complex protein hydrolysates and not to isolated synthetic tetrapeptides such as GAKI. The role of protein hydrolysates providing mainly di- and tripeptides in muscle protein anabolism, exercise performance, and muscle glycogen resynthesis has been discussed in the literature, but analogous studies on tetrapeptides of this sequence have not been published.

Evidence strength for Gly-Ala-Lys-Ile specifically: None. No human studies, animal studies, or in vitro studies on this sequence in the context of muscle protein synthesis were identified.

5.2 Connective Tissue and Tendon Health

Bioactive peptides are physiologically active peptides mostly derived from proteins following gastrointestinal digestion, fermentation, or hydrolysis by proteolytic enzymes. It has been shown that bioactive peptides can be resorbed in their intact form and have repeatedly been shown to have a positive effect on health-related parameters such as hypertension, dyslipoproteinemia, inflammation, and oxidative stress.

A study by Praet and colleagues (2019) indicates a beneficial effect of exercise combined with collagen hydrolysate supplementation on tendon healing in subjects with Achilles tendinopathy, with the authors attributing it partly to the high glycine uptake of 1.1 g per day. This finding relates to glycine-rich collagen hydrolysates, not to Gly-Ala-Lys-Ile.

Evidence strength for Gly-Ala-Lys-Ile specifically: None. The glycine and lysine residues within the tetrapeptide are components of collagen, but no study has examined this exact sequence in the context of connective tissue support.

5.3 Antioxidant Activity

General research on tetrapeptide antioxidant activity has examined how amino acid composition and sequence affect radical-scavenging capacity. The replacement of glycine with isoleucine, which is more hydrophobic, was found to slightly increase the antioxidant activity of certain tetrapeptides; the composition of amino acids in the peptides plays a crucial role in antioxidant activity. This finding from tetrapeptide screening research (Maharani et al., 2018) provides general context but was not specific to the GAKI sequence.

Evidence strength for Gly-Ala-Lys-Ile specifically: None — no antioxidant assay (DPPH, ORAC, FRAP, or cellular) for this sequence was identified in published literature.

5.4 Skin and Dermal Applications

Research using a computational pipeline to screen novel tetrapeptide matrikines showed that cultured fibroblasts were responsive to applied peptides, but their associated bioactivity was sequence-dependent. Two novel peptides — GPKG (glycine-proline-lysine-glycine) and LSVD — were identified that in vitro enhanced transcription of matrix-organization and cell-proliferation genes, and a prolonged split-face clinical study with these peptides led to significantly improved measures of crow's feet and skin firmness in a mixed population. This illustrates that lysine-containing tetrapeptides can be bioactive in dermal contexts, but the GAKI sequence is distinct from GPKG and has not been studied.

By modifying sequence length, amino acid composition, and structural modifications, peptide developers can create targeted, stable, and efficient skin-care ingredients with specific actions. No clinical or in vitro study specific to Gly-Ala-Lys-Ile in a dermal context was identified.

Evidence strength for Gly-Ala-Lys-Ile specifically: None.

5.5 Metabolic and Glycemic Effects

Research on antidiabetic tetrapeptides has shown that tetrapeptides derived from protein hydrolysates, such as Gly-Pro-Ala-Gly from porcine skin, exhibited DPP-IV inhibitory activity (IC₅₀ = 41.9 μM). The antidiabetic effect of peptides has been thought to pertain to their sequence length, charge, and hydrophobicity. While the GAKI sequence contains a hydrophobic residue (Ile) at the C-terminus and a positively charged residue (Lys), no DPP-IV inhibition or other metabolic assay for Gly-Ala-Lys-Ile has been published.

Evidence strength for Gly-Ala-Lys-Ile specifically: None.

5.6 Immune Modulation

Several synthetic tetrapeptides have been investigated for immune-regulatory properties. A US patent (US 5,298,490) describes a class of tetra- and pentapeptides characterized as useful in regulating the immune system; these peptides and compositions containing them demonstrate a variety of regulatory effects on the mammalian immune system and retain the biological activity of human thysplenin. These patented sequences are structurally distinct from Gly-Ala-Lys-Ile and require proline or α-amino-isobutyric acid at position 2. No analogous immune-regulatory study has been published for the GAKI sequence.

Evidence strength for Gly-Ala-Lys-Ile specifically: None.

6. Body Systems and Health Areas of Association

Based on the pharmacological properties of its individual constituent amino acids and the general biology of tetrapeptides, Gly-Ala-Lys-Ile has been associated — primarily in supplement marketing materials, not in peer-reviewed literature — with the following body systems. The basis for each association is characterized below according to the quality of existing evidence:

• Musculoskeletal system: The presence of L-isoleucine (a BCAA) and L-lysine (a collagen precursor amino acid) provides a rationale for use in muscle-recovery and connective-tissue contexts. The effects of proteins of different sources, amino acid compositions, and individual amino acids have been the focus of sports-specific research for many years, and more recently studies related to bioactive peptides have yielded interesting and promising results. This association is extrapolated from constituent-amino-acid science, not from GAKI-specific data.
• Gastrointestinal / absorptive system: Relevant to questions of bioavailability and PepT1-mediated transport as discussed in Section 4.
• Metabolic system: Alanine's role in the glucose-alanine cycle and isoleucine's role in glucose uptake signaling provide indirect metabolic rationale. No direct GAKI metabolic data exist.
• Antioxidant / cellular-protection system: General tetrapeptide antioxidant chemistry (glycine, alanine contribution) provides only indirect context. No GAKI-specific antioxidant data exist.

7. Dosage Forms and Dosages Reported in Studies

No clinical or preclinical study reporting a dose of Gly-Ala-Lys-Ile was identified in any peer-reviewed publication or official database. Therefore, no evidence-based dosage recommendation can be stated for this tetrapeptide.

For context, broader research on bioactive peptide supplementation in sports nutrition has used complex protein hydrolysates rather than isolated synthetic tetrapeptides. Daily consumption of 25–40 g of protein peptides has been associated with a clinically relevant reduction in systolic blood pressure of approximately 3–5 mmHg in meta-analytical evidence. This refers to protein-derived peptide mixtures, not to isolated GAKI. Similarly, no pharmacokinetic data (Cmax, Tmax, half-life, area under the curve) for Gly-Ala-Lys-Ile in any biological matrix have been published.

Regarding the broader safety study of commercial bioactive peptide products, one prospective, randomized, double-blind, placebo-controlled study enrolled twenty recreationally resistance-trained men who received two servings of either a commercial bio-active peptide product or placebo daily for eight weeks while completing four to five intense weight-training sessions weekly; assessment sessions included standard complete blood counts, comprehensive metabolic panels, and resting hemodynamics. This study evaluated a commercial colostrum-derived peptide product (Bio-Gro™), not Gly-Ala-Lys-Ile.

8. Safety Considerations and Regulatory Status

8.1 General Peptide Safety Profile

The high potency, specificity, and generally good safety profile are the main strengths of bioactive peptides as new and promising therapeutic agents. Peptides possess favorable tissue penetration and the capability to engage in specific and high-affinity interactions with endogenous receptors.

Because Gly-Ala-Lys-Ile is composed entirely of proteinogenic amino acids connected by standard peptide bonds, its hydrolysis products — if fully digested — are four physiologically normal amino acids with established dietary safety profiles. No specific toxicology, genotoxicity, or carcinogenicity data for the intact Gly-Ala-Lys-Ile tetrapeptide were identified in any database.

8.2 Regulatory Status

Peptides can be GRAS (Generally Recognized as Safe), but status depends on the specific ingredient and its intended use. Under the Federal Food, Drug, and Cosmetic Act, any substance intentionally added to food is considered a food additive unless it is exempt. Because peptides vary widely in structure, source, and dose, each ingredient requires its own defensible safety strategy. While peptides can be recognized as GRAS for food use in the United States, GRAS status is ingredient-specific and must be supported by robust scientific evidence demonstrating safety under the intended conditions of use.

Gly-Ala-Lys-Ile does not appear on the FDA GRAS list, does not have a USP monograph, and is not included in a European Pharmacopoeia monograph. Its appearance as a labeled ingredient in sports supplement products in the United States places it under the Dietary Supplement Health and Education Act (DSHEA) framework, under which manufacturers bear responsibility for ensuring safety before marketing. No EFSA, EMA, or Health Canada evaluation of this specific tetrapeptide was identified.

8.3 Absorption Limitations and Bioavailability Concerns

The physiological effects of bioactive peptides usually involve direct interaction between the peptides and the targeted tissues and organs; upon oral administration, the peptides have to be absorbed into the blood circulation in their active forms through the intestinal barrier. It is essential to study the absorption behavior of such bioactive peptides in order to validate their bioactivities.

For tetrapeptides specifically, the peptide binding cavity of PepT1 is reported to be approximately 13 × 12 × 11 Å, which limits attachment to only di- and tripeptides, while being sterically restrictive for peptides with greater than tetrapeptide length. The GAKI tetrapeptide is at the structural limit of PepT1 transport. Whether it is absorbed intact or only after further hydrolysis to smaller units is unknown and untested for this specific sequence.

8.4 Potential Interactions

No drug–nutrient interaction data specific to Gly-Ala-Lys-Ile were identified in any published source. The lysine residue is relevant to one well-characterized interaction in the broader amino acid literature: L-lysine and L-arginine compete for the same intestinal and renal transport systems (CAT-1 and CAT-2 transporters), meaning that high supplemental doses of lysine-containing peptides could theoretically affect arginine availability — a mechanism studied for free L-lysine, not for this tetrapeptide specifically. No evidence of clinically significant interactions between this tetrapeptide and pharmaceutical drugs, other nutrients, or herbal supplements was identified in the reviewed literature.

9. Summary of Evidence Quality

The following table summarizes the state of evidence for the specific tetrapeptide Gly-Ala-Lys-Ile across key domains:

• Chemical identity: Established by analogy with PubChem records for component sequences; no dedicated CID record confirmed.
• Natural occurrence / food source: Not isolated from any natural protein hydrolysate in published literature; constituent amino acids are ubiquitous in dietary protein.
• Traditional use: None. Synthetic peptide with no pre-modern use history.
• Mechanism of action (intact molecule): Theoretical; absorption via PepT1 is uncertain for tetrapeptides at this size limit; no receptor-binding or signaling studies published.
• Clinical evidence (human trials): None identified.
• Animal or in vitro evidence: None identified for the specific GAKI sequence.
• Safety / toxicology data: None identified specifically; general amino acid safety is well established for the constituents.
• Regulatory approval / monograph: None (FDA, EMA, EFSA, USP, EP).

Among bioactive peptides, those released from food protein sources have acquired importance as active components in functional foods and nutraceuticals because they are known to possess regulatory functions that can lead to health benefits. However, this general statement about food-derived bioactive peptides should not be applied to Gly-Ala-Lys-Ile without specific research to establish whether it is bioactive, bioavailable, and safe at supplemental doses.

References

• PubChem CID 71404690 — Glycyl-L-alanyl-L-lysine (NCBI/NIH)
• PubChem CID 126261 — Glycyl-alanyl-isoleucine (NCBI/NIH)
• PubChem CID 88079 — Glycyl-L-isoleucine (NCBI/NIH)
• PubChem CID 1551643 — Glycyl-L-alanine (NCBI/NIH)
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