These class of beta lactamases basically contains Metallo beta lactamases
or Metallo enzymes.
Metalloenzymes
Metals play roles in approximately one-third of the known enzymes. Metals
may be a co-factor or they may be incorporated into the molecule, and these are
known as metalloenzymes. Amino Acids in peptide linkage posses groups that can form
coordinate-covalent bonds with the metal atom. The free amino and carboxy group
bind to the metal affecting the enzymes structure resulting in its active conformation
. Metals main function is to serve in electron transfer. Many enzymes can serve
as electrophiles and some can serve as nucleophilic groups. This versatility explains
metals frequent occurrence in enzymes. Some metalloenzymes include hemoglobins,
cytochromes, phosphotransferases, alcohol dehydrogenase, arginase, ferredoxin, and
cytochrome oxidase.
Carboxypeptidase A is a zinc metalloenzyme that breaks peptide linkages
in the digestion of proteins. The zinc ion that the enzyme contains in its active
site plays a key role in that function.
Metalloenzymes can be regulated in several ways since they are such
a diverse group. One way metalloenzymes are regulated is the pH level. The pH level
can disrupt the electron flow that the metal would normally help facilitate. In
this way the pH level could inhibit the overall effectiveness of the metalloenzyme.
Transition state analogs play a key role in the competitive inhibition
of metalloenzymes because they mimic the structure of the substrates transition
state in the reaction of enzyme and substrate.
Metalloenzymes such as the ones containing zinc can also be regulated
by diet. The source of zinc in humans is almost entirely through diet. Without proper
intake of metals such as zinc in a persons diet, the activity of the enzyme would
be inhibited.
Structure:
Metalloenzymes are proteins which function as an enzyme and contain
metals that are tightly bound and always isolated with the protein. In proteins
such as hemoglobins and cytochromes, the metal is Fe2+ or Fe3+, and it is part of
the heme prosthetic group. In other metalloenzymes the metal is built into the structure
of the enzyme molecule. The metal ion can not be removed with out destroying the
structure of the enzyme. Metals built into the molecule include: most phosphotransferases,
containing Mg2+; alcohol dehydrogenase, Zn2+; arginase, Mn2+; ferredoxin, Fe2+;
and cytochrome oxidase, Cu2+ .
Metals are usually found in the active site of the enzyme. The metals
resemble protons (H+) in that they are electrophiles that are able to accept an
electron pair to form a chemical bond. In this aspect, metals may act as general
acids to react with anionic and neutral ligands .
Metal's larger size relative to protons is compensated for by their
ability to react with more than one ligand. Metals typically react with two, four,
or six ligands. A ligand is whatever molecule the metal interacts with. If a metal
is bound with two ligands it will form a linear complex. If the metal reacts with
four ligands the metal will be set in the center of a square that is planer or it
will form a tetrahedral structure, and when six ligands react, the metal sits in
the center of an octahedron.
Amino acids in their peptide linkage in proteins possess groups with
the ability to bind to the metal resulting in coordinate-covalent bonds. The free
amino and carboxyl groups in a protein can bind to the metal and this may bind the
protein to a specific, active conformation . The fact that metals bind to several
ligands is important in that metals play a role in bringing remote parts of the
amino acid sequence together and help establish an active conformation of the enzyme.
Zinc is the metal incorporated in carboxypeptidase A. The zinc atom
serves as a metal ion catalyst and promotes hydrolysis. The substrate fits into
the hydrophobic pocket in carboxypeptidase A and zinc binds to the carboxyl group
of the substrate to help stabilize the enzyme-substrate complex. In this example
the zinc ion acts a generalized acid and stabilizes the developing O- as water attacks
the carbonyl.
Zinc can also perform a different role in enzymes like the role it performs
in carbonic anhydrase. Here the metal binds H2O and makes it acidic enough to lose
a proton and form a Zn-OH group. The zinc metal serves as a nucleophile to the substrate.
Since zinc has the ability to act as an electrophile or as the source of a nucleophilic
group it is incorporated and used by many enzymes
Function and Role:
Hemoglobins
A four-subunit molecule, containing a iron atom in each subunit, in
which each subunit binds a single molecule of oxygen. Hemoglobin transports oxygen
from the lungs to the capillaries of the tissue.
Cytochromes
Cytochromes are integral membrane proteins. Cytochromes contain iron
which serves to carry electrons between two segments of the electron-transport chain.
The iron is reversibly oxidizable and serves as the actual electron acceptor for
the cytochrome.
Phosphotransferase
The Mg2+ atom serves again in electron transfer.
Alcohol Dehydrogenase
A zinc metalloenzyme with broad specificity. They oxidize a range of
aliphatic and aromatic alcohols to their corresponding aldehydes and ketones using
NAD+ as a coenzyme.
Arginase
The metal atom of Mn2+ is used in electron transfer.
Ferredoxin
An electron transferring proteins involved in one-electron transfer
processes.
Cytochrome Oxidase
The copper ions easily accommodate electron removed from a substrate
and can just as easily transfer them to a molecule of oxygen
Eg : CARBOXYPEPTIDASE A
Carboxypeptidase A (CPA) is a zinc metalloenzyme that undergoes a large
conformational change upon binding of the substrate that serves the purpose of bringing
together the components of the active site. It is important to see that the zinc
metal ion plays a key role in the catalytic process . Carboxypeptidase A is an exopeptidase
which hydrolyzes the oligopeptides one at a time from the C-terminal end of the
polypeptide chain. CPA is specific for large hydrophobic side chains while its closely
related complimentary digestive enzyme, Carboxypeptidase B (CPB), is specific to
basic residues. This complementary relationship between CPA and CPB is very similar
to that of the closely related group of non-metalloenzymes of the digestive system,
chymotrypsin and trypsin. However, chymotrypsin and trypsin are endopeptidases that
catalyze the hydrolysis of non-terminal peptide bonds . As was stated CPA preferentially
hydrolyzes peptides when the terminal residue is hydrophobic, either aromatic or
branched aliphatic groups make favorable substituents. The binding is also stereospecific,
as the side group must be in the L-configuration.
