Ghrelin Octanoylation: The Lipid Tag Behind GHS-R1a Binding

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Introduction

A small fatty acid can switch a peptide on. Ghrelin is a 28-residue peptide, and whether it can engage its receptor, GHS-R1a, depends on one eight-carbon octanoyl group attached to a single serine. Without that group, the same chain of amino acids does essentially nothing at the receptor.

If you're researching ghrelin or the growth hormone secretagogue receptor, ghrelin octanoylation is the first concept to get right. This article walks through what the tag is, which enzyme adds it, how that enzyme picks its substrate, and why the receptor cares.

What ghrelin is and how it is made

In short: ghrelin is cut out of a larger precursor, and only afterwards does it receive its lipid tag.

Ghrelin was reported in 1999 by Kojima and colleagues as a 28-residue peptide purified from rat stomach, as described in the 2021 Open Biology review by Davis and colleagues. The gene first yields a 117-residue preproghrelin. That is processed into a 94-residue proghrelin, which is cleaved after Arg-28 by the prohormone convertase PC1/3 to release the mature peptide.

The same gene produces several related products, including unacylated ghrelin and obestatin, according to the Wikipedia entry on ghrelin. The name itself comes from "growth hormone-releasing peptide," which reflects the assay that led to its discovery.

Order of events matters here. The octanoyl group is added to the proghrelin precursor in the endoplasmic reticulum, before the convertase cut. So the finished peptide carries the tag from the start.

Ser-3 and the 126 Da shift

In short: the tag is a medium-chain fatty acid on the side chain of the third residue, and mass spectrometry can see it.

The third residue of ghrelin is serine, and its side-chain hydroxyl carries an n-octanoyl group, an eight-carbon fatty acid. When the original peptide was purified, its mass was 3315 Da. Synthetic ghrelin built without the modification measured 3189 Da. That 126 Da gap is the octanoyl group, as summarized in the Davis review.

Because a fatty acid is joined to a peptide, ghrelin is sometimes described as a proteolipid. The modification was unprecedented when it was found, and ghrelin remains the only known octanoylated protein in the human proteome.

Serine isn't the only residue that tolerates the chemistry in principle. In enzyme studies, replacing Ser-3 with threonine still allows some acylation, but less of it. The hydroxyl group is the reactive handle. The surrounding geometry matters too.

A shorter splice variant called mini-ghrelin, truncated after the first 13 residues, has also been reported to be octanoylated and active in cell and mouse studies. That fits the substrate picture below: the recognition elements sit at the very start of the sequence, so the rest of the chain can change without losing the tag. Think of the first four residues as a handle that the enzyme grips, with the remaining residues largely along for the ride during acylation itself.

GOAT: the enzyme that adds the tag

In short: ghrelin O-acyltransferase, or GOAT, is the single enzyme known to octanoylate ghrelin.

GOAT, also called MBOAT4, was identified in 2008 by two independent groups. It belongs to the membrane-bound O-acyltransferase family, a set of enzymes that sit in membranes and move fatty acyl groups onto lipids and proteins. Ghrelin is its only known substrate in humans.

Several features of the enzyme have been mapped:

• Location. GOAT works in the endoplasmic reticulum, where proghrelin is processed.
• Catalytic residues. An invariant histidine, His-338, is proposed to act as the general base. Mutating it abolishes activity. A nearby asparagine, Asn-307, interacts with the acyl donor.
• Architecture. Mouse GOAT has 11 transmembrane domains. A computational model of the human enzyme shows an internal channel running from the ER lumen to the cytoplasm, which would let the peptide and the fatty acyl donor approach from opposite sides.

The preferred acyl donor is octanoyl-CoA, the eight-carbon (C8) form. In cells supplied with different fatty acids, GOAT can use chains from C2 to C10. The octanoyl-CoA pool can come from medium-chain fatty acids or from the breakdown of longer ones, per the same review. These channel and mechanism details are partly model-based, so treat them as working hypotheses rather than a settled structure.

How GOAT recognizes its substrate

In short: GOAT reads the first few residues of ghrelin, and the fourth, phenylalanine, matters a great deal.

Recognition depends on the N-terminal motif Gly-Ser-Ser-Phe, often extended to GSSFL. Work summarized in a review of GOAT inhibitor discovery found that mutating the glycine at position 1 or the phenylalanine at position 4 almost completely blocks octanoylation. A peptide of just the first five residues can serve as a substrate.

The enzyme is also stereoselective. It distinguishes the configuration of both Ser-3 and Phe-4, and swapping Phe-4 for its D-isomer abolishes binding.

GOAT is also inhibited by its own product. Octanoylated ghrelin inhibits the enzyme with a modest IC50 of about 7 µM. Substituting Ser-3 with 2,3-diaminopropanoic acid (Dap) produced substrate mimics that act as submicromolar inhibitors in vitro, with IC50 values reported as low as 62 nM. These compounds are laboratory tools for studying the enzyme.

Why the lipid tag matters for GHS-R1a binding

In short: the receptor recognizes the acylated form; the unacylated form does not activate it.

GHS-R1a is a G protein-coupled receptor with 377 amino acids and seven transmembrane helices. A splice variant, GHS-R1b, is truncated and lacks helices 6 and 7. It cannot bind ghrelin.

Acylation at Ser-3 is required for ghrelin to bind and activate GHS-R1a. Unacylated ghrelin, called des-acyl ghrelin, does not activate the receptor. A study of the rodent hippocampus by Murtuza and Isokawa makes the same point and adds a twist. A fluorescent des-acyl ghrelin bound GHS-R1a in living brain slices. It did not bind in fixed slices, in receptor knockout tissue, or when a GOAT inhibitor was present. The authors concluded that local GOAT activity can acylate des-acyl ghrelin on site. They also noted that how GOAT reaches extracellular ghrelin remains an open question.

That's why the lipid tag gets so much attention. It's the difference between a peptide that engages the receptor and one that does not. For readers comparing other receptor-active peptides, the ghrelin-mimetic class overview covers synthetic compounds that act at the same receptor without needing an acyl group, and Ipamorelin vs Hexarelin compares two of them structurally.

Acyl and des-acyl ghrelin in the lab

In short: the two forms behave differently, and keeping them apart is a real analytical problem.

Des-acyl ghrelin isn't just a throwaway precursor. Reported effects in the literature include actions on insulin release, cell survival and adipogenesis, though some findings depend on context and its mechanism is undefined. Its pathway does not involve GHS-R1a activation.

The octanoyl ester is also chemically fragile. Serum esterases can remove it, and the Davis review reports that α2-macroglobulin deacylates ghrelin with a Km of 24 µM. Ester bonds can also hydrolyze on their own in solution, which is one of the common peptide degradation pathways. For anyone measuring acyl ghrelin in vitro, sample handling and buffer conditions therefore shape the result as much as the assay itself.

Conclusion

Ghrelin octanoylation is compact biochemistry. A single enzyme, GOAT, attaches an eight-carbon group to serine 3 in the endoplasmic reticulum. That tag adds 126 Da, depends on a GSSF recognition motif, and is required for the peptide to bind and activate GHS-R1a. Without it, the receptor does not respond.

Open questions remain, from the exact structure of GOAT to how acylation happens at distant signalling sites. For now, the Ser-3 lipid tag is the clearest example of how a small chemical modification can define a peptide's receptor pharmacology in the lab.

Frequently Asked Questions

What is ghrelin octanoylation?

It is the attachment of an eight-carbon octanoyl group to the side chain of serine 3 of ghrelin. The enzyme GOAT catalyzes it in the endoplasmic reticulum, and it adds 126 Da to the peptide's mass.

Does des-acyl ghrelin bind GHS-R1a?

Not in a way that activates the receptor. Acylation at Ser-3 is required for ghrelin to bind and activate GHS-R1a. Des-acyl ghrelin has reported effects of its own, but its mechanism is not well defined.

Is ghrelin the only octanoylated protein?

Ghrelin is the only known octanoylated protein in the human proteome, and it is the only known substrate of GOAT there.

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