Rubiscolin

The rubiscolins are a group of opioid peptides that are formed during digestion of the ribulose bisphosphate carboxylase/oxygenase (Rubisco) protein from spinach leaves. Two of them are known,^[1] acting as weak agonists of the delta opioid receptor selective for the G protein signaling pathway.^[2]

Background

Rubisco is a key protein in carbon fixation and is found in photosynthetic organs such as leaves in very high concentration. It usually accounts for 40% of the protein mass of a plant.^[3] With all forms of RuBisCO (not only plants, but also bacteria) taken into account, RuBisCO is possibly the most abundant type of protein on Earth.^[4]

The production of similar peptides from digestion of other species' Rubisco has not yet been reported.

Overview

Studies have been conducted on rubiscolin structure and biological responses following its digestion.^[5][6] The tertiary structure and biological function of spinach-derived rubiscolin has been analyzed in the laboratory.^[5]

When rubiscolin is digested, studies have shown that rubiscolin has the potential to bind to δ opioid receptors in the body.^[5] The analysis of the amino acids responsible for this agonistic relationship of rubiscolin with δ opioid receptors can lead to replication of these proteins in the lab.^[5] Rubiscolin has the capability to bind to δ opioid receptors following its digestion.^[5] Rubiscolin-5 and -6 are unusual delta opioid receptor agnoists in that they mainly activate the G protein signaling pathway at the receptor, mostly leaving the β-arresting pathway alone.^[2]

Types of rubiscolin

Several peptides are known to be produced after digestion of rubiscolin in various ways.

• For a key to the single-letter sequences, see Amino acid § Table of standard amino acid abbreviations and properties.
• All peptides sequences are assumed to start with NH₂- and end with -COOH, unless otherwise stated.

Rubiscolin-5

• Sequence: YPLDL
• From: ?

Rubiscolin-6

• Sequence: YPLDLF
• From: ?
• Delta opioid peptide. Can have an anxiolytic effect via activation of sigma1 and dopamine D1 receptors.^[7]

Other bioactive Rubisco-derived peptides

MRW

• Sequence: MRW
• From: peptin digestion.
• Binds angiotensin-converting enzyme (ACE). Reduces the blood pressure of hypertensive rats, 2 hours after ingestion.^[6]

MRWRD

• Sequence: MRWRD
• From: peptin digestion.
• Binds angiotensin-converting enzyme (ACE). Reduces the blood pressure of hypertensive rats, 4 hours after ingestion.^[6]

IAYKPAG

• Sequence: IAYKPAG
• From: peptin digestion.
• Binds angiotensin-converting enzyme (ACE). Reduces the blood pressure of hypertensive rats, 4 hours after ingestion.^[6]

LRIPVA

• Sequence: LRIPVA
• From: peptin digestion.
• Binds angiotensin-converting enzyme (ACE). No effects on blood pressure of hypertensive rats.^[6]