E-64 is an epoxide

Epoxide

An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...

 which can irreversibly inhibit a wide range of cysteine peptidases

Cysteine protease

Proteases are enzymes that degrade polypeptides. Cysteine proteases have a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic dyad. The first step is deprotonation of a thiol in the enzyme's active site by an adjacent amino acid with a basic side chain, usually a...

.

The compound was first isolated and identified from Aspergillus japonicus in 1978.
, and has since been shown to inhibit many cysteine peptidases

Cysteine protease

Proteases are enzymes that degrade polypeptides. Cysteine proteases have a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic dyad. The first step is deprotonation of a thiol in the enzyme's active site by an adjacent amino acid with a basic side chain, usually a...

 (e.g. papain

Papain

Papain, also known as papaya proteinase I, is a cysteine protease enzyme present in papaya and mountain papaya .-Papain family:...

, cathepsin B

Cathepsin B

Cathepsin B is an enzymatic protein belonging to the peptidase families. In humans, it is coded by the CTSB gene.- Function :...

, cathepsin L

Cathepsin L

Cathepsin L can refer to:* Cathepsin L1, previously called cathepsin L* Cathepsin L2 or cathepsin V...

, calpain

Calpain

A calpain is a protein belonging to the family of calcium-dependent, non-lysosomal cysteine proteases expressed ubiquitously in mammals and many other organisms. Calpains constitute the C2 family of protease clan CA in the MEROPS database...

 and staphopain).

The low toxic effects of the inhibitor, in addition to its effective mechanism of action, makes E-64 a good template for drugs to treat diseases where high levels of a cysteine proteases

Cysteine protease

Proteases are enzymes that degrade polypeptides. Cysteine proteases have a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic dyad. The first step is deprotonation of a thiol in the enzyme's active site by an adjacent amino acid with a basic side chain, usually a...

 are the primary cause.

Structure and mechanism of inhibition

E-64 possesses a trans-epoxysuccinic acid group coupled to a modified dipeptide

Dipeptide

A dipeptide is a molecule consisting of two amino acids joined by a single peptide bond.Dipeptides are produced from polypeptides by the action of the hydrolase enzyme dipeptidyl peptidase. Dietary proteins are digested to dipeptides and amino acids, and the dipeptides are absorbed more rapidly...

. The covalent attachment of E-64 to the active site cysteine

Cysteine

Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...

 occurs via nucleophillic attack from the thiol

Thiol

In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...

 group of the cysteine on C2 of the epoxide. Early studies suggested that the amino-4-guanidinobutane bound in the S3' subsite and the leucyl group in the S2' subsite, however published crystal structures of E-64 complexed with papain

Papain

Papain, also known as papaya proteinase I, is a cysteine protease enzyme present in papaya and mountain papaya .-Papain family:...

and actinidum indicated that E-64 binds via the S subsites.