GHRP-2: Molecular Structure

Molecular Structure and Properties#

GHRP-2 (Growth Hormone Releasing Peptide-2, INN: Pralmorelin, developmental code: KP-102) is a synthetic hexapeptide growth hormone secretagogue with a molecular weight of 817.9 Da and the molecular formula C45H55N9O6. Its CAS registry number is 158861-67-7. The peptide is composed of six amino acid residues arranged in a linear sequence with a C-terminal amide modification, incorporating three non-proteinogenic D-amino acids that confer enhanced metabolic stability and receptor selectivity compared to earlier generation growth hormone releasing peptides.

GHRP-2 was developed through iterative structure-activity relationship optimization of the original growth hormone secretagogue scaffold identified by Cyril Bowers and colleagues in the 1980s, representing a second-generation GHRP with improved pharmacological properties relative to the first-generation peptides GHRP-6 and GHRP-1.

Amino Acid Sequence#

Primary Sequence#

The complete amino acid sequence of GHRP-2 is:

D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2

The sequence consists of six residues with the following positional assignments:

C-Terminal Amidation#

The C-terminal lysine residue is amidated (Lys-NH2), converting the terminal carboxyl group to a carboxamide. This modification is standard in many bioactive peptides and serves two functions: it eliminates the negative charge of the free carboxylate at physiological pH, and it provides resistance to carboxypeptidase degradation. The amide modification is essential for full biological activity; the free acid form of GHRP-2 shows substantially reduced GHS-R1a binding affinity.

D-Amino Acid Modifications#

The incorporation of three D-amino acids at positions 1, 2, and 5 is a defining structural feature of GHRP-2 and a key innovation relative to earlier growth hormone secretagogues. D-amino acids are mirror images of the naturally occurring L-amino acids and are not recognized by most mammalian proteases, which have evolved active sites that are stereospecific for L-configured substrates.

Protease Resistance#

The strategic placement of D-amino acids at the N-terminal (positions 1 and 2) and at an internal position (position 5) provides comprehensive protection against enzymatic degradation. D-Alanine at position 1 blocks aminopeptidase attack on the N-terminus, which is the primary degradation pathway for unprotected peptides in plasma. D-Phenylalanine at position 5 disrupts endopeptidase recognition of the internal peptide backbone. Together, these modifications extend the circulating half-life of GHRP-2 to approximately 25-30 minutes, which is substantially longer than unmodified hexapeptides that are typically degraded within minutes.

Conformational Effects#

D-amino acid substitutions also alter the preferred backbone conformations of the peptide. The alternation of D and L residues in the sequence creates a tendency toward extended or beta-strand conformations rather than the helical or turn structures favored by all-L sequences. This conformational bias is believed to pre-organize the peptide into a geometry that is complementary to the GHS-R1a binding pocket, enhancing binding affinity without requiring the energetic cost of conformational adjustment upon receptor engagement.

2-Naphthylalanine Modification#

The D-2-naphthylalanine (D-2-Nal) residue at position 2 is one of the most pharmacologically significant modifications in GHRP-2. 2-Naphthylalanine is a synthetic amino acid in which the phenyl side chain of phenylalanine is replaced by a larger bicyclic naphthalene ring system. The "2-" designation indicates that the alanine backbone is attached at the 2-position of the naphthalene ring (beta-naphthylalanine), as opposed to the 1-position (alpha-naphthylalanine).

Structure-Activity Relationships#

The 2-naphthyl side chain provides a substantially larger hydrophobic surface area than phenylalanine or tryptophan. Structure-activity studies have demonstrated that replacement of D-2-Nal with other aromatic residues (D-Phe, D-Trp, or D-1-Nal) at position 2 significantly reduces GHS-R1a binding affinity and GH-releasing potency. The extended aromatic surface of the naphthyl group makes favorable van der Waals contacts with hydrophobic residues in the receptor binding pocket that cannot be achieved by smaller aromatic groups.

The D-configuration of 2-Nal at this position is also critical. The L-2-Nal epimer shows markedly reduced activity, indicating that the spatial orientation of the bulky naphthyl group relative to the peptide backbone must match the three-dimensional requirements of the receptor binding site.

Comparison with GHRP-6#

GHRP-6, the first-generation growth hormone releasing peptide (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2), differs from GHRP-2 at positions 1 and 2. GHRP-6 has L-Histidine at position 1 and D-Tryptophan at position 2, whereas GHRP-2 has D-Alanine and D-2-Naphthylalanine at these positions. The substitution of D-2-Nal for D-Trp at position 2, combined with the D-Ala for His substitution at position 1, accounts for much of GHRP-2's improved GH-releasing potency and selectivity profile compared to GHRP-6.

GHS-R1a Receptor Binding Pharmacology#

Receptor Description#

GHRP-2 exerts its primary biological effects through activation of the growth hormone secretagogue receptor type 1a (GHS-R1a), also designated the ghrelin receptor. GHS-R1a is a 366-amino acid seven-transmembrane domain G protein-coupled receptor (GPCR) that belongs to the rhodopsin-like (Class A) GPCR superfamily. The receptor is primarily expressed on somatotroph cells of the anterior pituitary gland and on neurons within the hypothalamic arcuate nucleus and ventromedial hypothalamus.

GHS-R1a exhibits constitutive activity, meaning it maintains a baseline level of signaling even in the absence of ligand binding. This constitutive activity is physiologically significant and contributes to basal GH secretory tone. GHRP-2 acts as a full agonist at GHS-R1a, further increasing receptor signaling above the constitutive baseline.

Binding Site and Mechanism#

GHRP-2 binds to a site on GHS-R1a that overlaps with but is not identical to the binding pocket engaged by the endogenous ligand ghrelin (acylated 28-amino acid peptide). Mutagenesis studies have identified several transmembrane domain residues critical for GHRP-2 binding, particularly within transmembrane helices III, V, VI, and VII. Key interactions include hydrophobic contacts between the aromatic residues of GHRP-2 (D-2-Nal, Trp, D-Phe) and hydrophobic residues lining the receptor binding pocket, as well as electrostatic interactions involving the positively charged lysine side chain and the amidated C-terminus.

Unlike ghrelin, GHRP-2 does not require an acyl modification for receptor activation. Ghrelin requires an octanoyl (C8) fatty acid group on its Ser3 residue for GHS-R1a binding, a post-translational modification catalyzed by the enzyme ghrelin O-acyltransferase (GOAT). The ability of GHRP-2 to activate GHS-R1a without lipid modification simplifies its synthesis and enhances its chemical stability.

Signal Transduction#

Upon GHRP-2 binding, GHS-R1a couples primarily to Gq/11 heterotrimeric G proteins, activating phospholipase C-beta (PLC-beta). PLC-beta catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 triggers release of calcium from endoplasmic reticulum stores through IP3 receptors, while DAG activates protein kinase C (PKC). The resultant elevation in intracellular calcium concentration promotes fusion of GH-containing secretory granules with the plasma membrane and exocytosis of GH into the perivascular space.

Secondary signaling pathways activated by GHS-R1a include the MAPK/ERK1/2 cascade and, under some conditions, coupling to Gi/o proteins and modulation of adenylyl cyclase activity. The relative contribution of these pathways may depend on the specific agonist, with evidence suggesting that different GHS-R1a ligands can stabilize distinct receptor conformations that preferentially engage different G protein subtypes (biased agonism).

Chemical Properties#

Metabolism and Half-Life#

GHRP-2 has an estimated circulating half-life of approximately 25 to 30 minutes following intravenous administration in humans. Following subcutaneous injection, peak plasma concentrations are reached at approximately 15 to 30 minutes, with a slower absorption phase extending the apparent half-life. The peptide is cleared primarily through renal filtration and enzymatic degradation by circulating and tissue-associated peptidases.

Despite the protective D-amino acid substitutions, GHRP-2 remains susceptible to some endopeptidase activity, particularly at the L-amino acid bonds between positions 3-4 (Ala-Trp) and 4-5 (Trp-D-Phe). The degradation products lose GHS-R1a binding activity. The relatively short half-life necessitates multiple daily administrations in research protocols aimed at sustained GH elevation, and has motivated the development of non-peptide GHS-R1a agonists with longer durations of action.

Stability Characteristics#

GHRP-2 is supplied as a lyophilized (freeze-dried) white to off-white powder for research and diagnostic applications. Key stability considerations include:

• Lyophilized storage: The powder is stable for extended periods (months to years) when stored at -20 degrees C protected from moisture and light. Short-term storage at 2-8 degrees C is acceptable for weeks to months.
• Reconstituted solution: Once dissolved in aqueous solution, GHRP-2 should be stored at 2-8 degrees C and used within a defined timeframe, typically within 2-4 weeks when reconstituted with bacteriostatic water. Sterile water preparations should be used more promptly.
• pH stability: GHRP-2 is most stable in slightly acidic to neutral pH ranges (pH 4-7). The amide bond at the C-terminus is susceptible to base-catalyzed hydrolysis under strongly alkaline conditions.
• Protease sensitivity: Although more resistant than unmodified peptides, GHRP-2 is degraded by serum proteases over time. Biological samples containing GHRP-2 should be processed with protease inhibitors for accurate measurement.
• Freeze-thaw cycles: Repeated freeze-thaw cycles should be avoided as they can promote aggregation and degradation. Aliquoting reconstituted solution into single-use portions is recommended.

Related Reading#

• GHRP-2 overview and research guide
• Peptides similar to GHRP-2
• GHRP-2 research evidence