A tripeptide epoxy ketone compound having the following structural formula, or a salt thereof, wherein: R 1, R 2 and R 3 are each independently selected from the group consisting of H, C 1-6 alkyl-D, halogenated C 1-6 alkyl-D, C 1-6 hydroxy alkyl, C 1-6 mercapto alkyl, C 1-6 alkoxy alkyl, aryl, aralkyl, heteroaryl, heteroaralkyl, where D is N(R a)(R b) or absent, R a and R b are each independently selected from the group consisting of H, OH, C 1-6 alkyl, halogenated C 1-6 alkyl and a protective group for the N-terminal; R 4 and R 5 are each independently selected from the group consisting of H, OH, C 1-6 alkyl, halogenated C 1-6 alkyl and aralkyl; R 6 is selected from the group consisting of H, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 hydroxy alkyl, C 1-6 alkoxy group, halogenated C 1-6 alkoxy group, C O-C 1-6 alkyl, C NH-C 1-6 alkyl, and aralkyl; X is O; L is or is absent, where R is selected from the group consisting of H, C 1-6 alkyl and halogenated C 1-6 alkyl; the ring A is a six membered saturated aliphatic heterocycle, where the heterocycle contains N and optionally 0-3 heteroatoms selected from O and S, and is optionally substituted by those selected from the group consisting of R 8, R 9 and B 1; R 8 and R 9 are each independently selected from the group consisting of H, OH, C 1-6 alkyl, C 1-6 alkoxy group, C 1-6 hydroxy alkyl, C 1-6 mercapto alkyl, C 1-6 alkyl-D, aryl, heterocyclic aryl, cycloalkyl and heterocyclic group, which groups may be optionally substituted by those selected from the group consisting of halogen, nitro, amino, CN-, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 alkoxy group and halogenated C 1-6 alkoxy group, and each of which groups may be optionally fused to one or more aryl or heterocyclic aryl, or be fused to one or more saturated or partially unsaturated cycloalkyl or heterocycle; B is and/or L is or where R is H, C 1-6 alkyl or halogenated C 1-6 alkyl, B 1 and B 2 are each independently 0, S, or N(R c), or are absent, and R c is selected from the group consisting of H, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 alkoxy group, halogenated C 1-6 alkoxy group; R 7 is selected from the group consisting of H, C 1-6 alkyl, C 1-6 alkenyl, C 1-6 alkynyl, carbocyclic group, heterocyclic group, aryl, C 1-6 aralkyl, heteroaryl, C 1-6 heteroaralkyl, heterocyclic group-M-, carbocyclic group-M-, which groups may be optionally substituted by those selected from the group consisting of halogen, nitro, amino, CN-, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 alkoxy group and halogenated C 1-6 alkoxy group, where M is absent or is a C 1-12 alkyl, wherein, each of the amino acids is an L-amino acid, unless particularly specified. R 1, R 2 and R 3 are each independently selected from the group consisting of H, C 1-6 alkyl-D, halogenated C 1-6 alkyl-D, C 1-6 hydroxy alkyl, C 1-6 mercapto alkyl, C 1-6 alkoxy alkyl, aryl, aralkyl, heteroaryl, heteroaralkyl, where D is N(R a)(R b) or absent, R a and R b are each independently selected from the group consisting of H, OH, C 1-6 alkyl, halogenated C 1-6 alkyl and a protective group for the N-terminal; R 4 and R 5 are each independently selected from the group consisting of H, OH, C 1-6 alkyl, halogenated C 1-6 alkyl and aralkyl; R 6 is selected from the group consisting of H, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 hydroxy alkyl, C 1-6 alkoxy group, halogenated C 1-6 alkoxy group, C O-C 1-6 alkyl, C NH-C 1-6 alkyl, and aralkyl; X is O; L is or is absent, where R is selected from the group consisting of H, C 1-6 alkyl and halogenated C 1-6 alkyl; the ring A is a six membered saturated aliphatic heterocycle, where the heterocycle contains N and optionally 0-3 heteroatoms selected from O and S, and is optionally substituted by those selected from the group consisting of R 8, R 9 and B 1; R 8 and R 9 are each independently selected from the group consisting of H, OH, C 1-6 alkyl, C 1-6 alkoxy group, C 1-6 hydroxy alkyl, C 1-6 mercapto alkyl, C 1-6 alkyl-D, aryl, heterocyclic aryl, cycloalkyl and heterocyclic group, which groups may be optionally substituted by those selected from the group consisting of halogen, nitro, amino, CN-, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 alkoxy group and halogenated C 1-6 alkoxy group, and each of which groups may be optionally fused to one or more aryl or heterocyclic aryl, or be fused to one or more saturated or partially unsaturated cycloalkyl or heterocycle; B is and/or L is or where R is H, C 1-6 alkyl or halogenated C 1-6 alkyl, B 1 and B 2 are each independently 0, S, or N(R c), or are absent, and R c is selected from the group consisting of H, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 alkoxy group, halogenated C 1-6 alkoxy group; R 7 is selected from the group consisting of H, C 1-6 alkyl, C 1-6 alkenyl, C 1-6 alkynyl, carbocyclic group, heterocyclic group, aryl, C 1-6 aralkyl, heteroaryl, C 1-6 heteroaralkyl, heterocyclic group-M-, carbocyclic group-M-, which groups may be optionally substituted by those selected from the group consisting of halogen, nitro, amino, CN-, C 1-6 alkyl, halogenated C 1-6 alkyl, C 1-6 alkoxy group and halogenated C 1-6 alkoxy group, where M is absent or is a C 1-12 alkyl, wherein, each of the amino acids is an L-amino acid, unless particularly specified.