VIP: Molecular Structure

Molecular Structure#

Vasoactive Intestinal Peptide (VIP) is a 28-amino acid linear neuropeptide with a molecular weight of 3326.8 Da and the molecular formula C147H237N43O43S1. It was first isolated from porcine small intestine in 1970 by Sami Said and Viktor Mutt and subsequently characterized as a member of the secretin/glucagon superfamily of regulatory peptides. The CAS registry number for VIP is 37221-79-7.

The primary structure of VIP consists of the following residues in sequential order: His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Thr-Arg-Leu-Arg-Lys-Gln-Met-Ala-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Ile-Leu-Asn. In single-letter amino acid notation, this corresponds to HSDAVFTDNYTRLRKQMAVKKYLNSILN. The peptide contains a single methionine residue at position 17, which is susceptible to oxidation and can affect biological activity. No disulfide bonds or post-translational modifications are present in the mature peptide under physiological conditions.

Secretin/Glucagon Superfamily Classification#

VIP belongs to the secretin/glucagon superfamily, a family of structurally related peptide hormones and neuropeptides that share significant sequence homology and signal through class B (secretin family) G-protein coupled receptors. Other members of this superfamily include pituitary adenylate cyclase-activating polypeptide (PACAP), secretin, glucagon, glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2), growth hormone-releasing hormone (GHRH), gastric inhibitory polypeptide (GIP), and peptide histidine methionine (PHM-27).

The evolutionary conservation of VIP across vertebrate species is notable. Human VIP is identical to porcine, bovine, and canine VIP, indicating strong selective pressure to preserve its amino acid sequence. Rat and mouse VIP differ from human VIP at only a single position (position 5, where valine is replaced by alanine in rodents), further underscoring the functional importance of the peptide's primary structure.

The N-terminal region (residues 1-7) of VIP shares the greatest homology with other superfamily members, particularly PACAP-27 and secretin. The first six residues of VIP (HSDAVF) are identical to those of PACAP, which explains their shared affinity for VPAC1 and VPAC2 receptors. This N-terminal conservation reflects the critical role of this region in receptor binding and activation across the superfamily.

Three-Dimensional Structure and Alpha-Helical Domain#

Structural studies using circular dichroism (CD) spectroscopy and nuclear magnetic resonance (NMR) have revealed that VIP adopts a predominantly random coil conformation in aqueous solution but undergoes a conformational transition to an alpha-helical structure in membrane-mimetic environments such as trifluoroethanol/water mixtures, detergent micelles, and lipid bilayers. This environment-dependent structural behavior is characteristic of many bioactive peptides and is biologically relevant because receptor binding occurs at the cell membrane interface.

The alpha-helical domain of VIP is concentrated in the C-terminal half of the molecule, spanning approximately residues 13-28. This region forms an amphipathic helix in which hydrophobic residues (Leu13, Met17, Val19, Tyr22, Leu23, Ile26, Leu27) align along one face while polar and charged residues (Arg14, Lys15, Gln16, Lys20, Lys21, Asn24, Ser25, Asn28) occupy the opposite face. This amphipathic character facilitates interaction with the lipid membrane and the transmembrane domains of VPAC receptors.

The N-terminal region (residues 1-12) remains more flexible and extended, serving as the primary pharmacophore for receptor activation. Structure-activity relationship studies have demonstrated that the N-terminal histidine at position 1 is essential for receptor activation, while truncation of even the first two residues (His1-Ser2) converts VIP from a full agonist to an antagonist at VPAC receptors. The C-terminal helix is critical for high-affinity receptor binding, likely through interactions with the extracellular N-terminal domain of the receptor, consistent with the two-domain binding model established for class B GPCRs.

Enzymatic Degradation and Short Half-Life#

VIP has an extremely short circulating half-life of approximately 1-2 minutes following intravenous administration. This rapid clearance is the result of extensive enzymatic degradation by multiple peptidases present in plasma, vascular endothelium, and peripheral tissues.

The primary enzymes responsible for VIP degradation include:

• Neutral endopeptidase (NEP, EC 3.4.24.11): Also known as neprilysin or enkephalinase, NEP is a zinc metalloprotease expressed on the surface of endothelial cells, epithelial cells, and other cell types. NEP cleaves VIP at multiple internal sites, with preferential cleavage at the Ser-Asp (positions 2-3), Tyr-Thr (positions 10-11), and Arg-Lys (positions 14-15) bonds.
• Dipeptidyl peptidase IV (DPP-IV, EC 3.4.14.5): DPP-IV is a serine protease that cleaves dipeptides from the N-terminus of peptides containing a proline or alanine residue at position 2. Although VIP has serine rather than proline at position 2, DPP-IV can still cleave the His1-Ser2 dipeptide, albeit at a slower rate than for its preferred substrates.
• Aminopeptidases: Membrane-bound aminopeptidases contribute to N-terminal degradation of VIP, sequentially removing residues from the exposed amino terminus.
• Mast cell tryptase and chymase: In tissues containing mast cells, these serine proteases can degrade VIP at multiple sites, potentially modulating local VIP concentrations in inflammatory microenvironments.

The rapid degradation of VIP represents the single most significant barrier to its therapeutic development. To address this limitation, numerous approaches have been investigated, including the development of protease-resistant VIP analogs (through D-amino acid substitution, N-methylation, or cyclization), PEGylation to reduce enzymatic access and renal clearance, encapsulation in liposomes or nanoparticles, and conjugation with albumin or long-lived carrier proteins.

Biosynthesis and Processing#

VIP is encoded by the VIP gene located on human chromosome 6q25. The gene spans approximately 9 kilobases and contains 7 exons. Transcription produces a prepro-VIP mRNA that is translated into a 170-amino acid preproprotein. Signal peptide cleavage yields the 148-residue proVIP, which is subsequently processed by prohormone convertases (PC1/3 and PC2) at dibasic cleavage sites to release the mature 28-amino acid VIP peptide along with its co-encoded peptide, peptide histidine methionine (PHM-27, also known as PHI in rodents where the C-terminal residue is isoleucine rather than methionine).

Both VIP and PHM/PHI are stored in dense-core secretory granules in neurons and neuroendocrine cells and are co-released upon stimulation. The co-expression and co-release of these two peptides from the same precursor provides a mechanism for coordinated neuropeptide signaling, as both peptides activate VPAC receptors, though with different affinities and potencies.

Receptor Binding Properties#

VIP binds to two receptor subtypes, VPAC1 (VIPR1) and VPAC2 (VIPR2), with low nanomolar affinity (Kd values in the 0.5-5 nM range for both receptors). Both are class B G-protein coupled receptors that couple predominantly to Gs-alpha, activating adenylate cyclase and increasing intracellular cAMP. VIP binds PAC1 receptors with 100 to 1000-fold lower affinity than PACAP, making PAC1 activation negligible at physiological VIP concentrations.

The binding interaction follows the two-domain model characteristic of class B GPCRs. The C-terminal alpha-helical domain of VIP first engages the extracellular N-terminal domain of the receptor (the "affinity trap"), positioning the N-terminal pharmacophore of VIP to insert into the receptor transmembrane domain junction and trigger G-protein activation. This model explains why C-terminal truncation reduces binding affinity while N-terminal truncation eliminates efficacy without necessarily abolishing binding.

Related Reading#

• VIP overview and research guide
• Peptides similar to VIP
• VIP research evidence