The generation of enzymes with new catalytic activities remains a major challenge. So far, several different strategies have been developed to tackle this problem, including site-directed mutagenesis, random mutagenesis (directed evolution), antibody catalysis, computational redesign, and de novo methods. Using these techniques, a broad array of novel enzymes has been created. In this article, we will focus on two categories of novel enzyme: ribozymes and abzymes.
Ribozyme
Ribozymes are RNA molecules that have a catalytic function, which belong to biocatalysts. Its discovery breaks the traditional notion that enzymes are proteins. The substrates of ribozymes can be different molecules, and some substrates are parts of the same RNA molecule. Ribozymes have many functions, some of which can cleave RNA, others can cleave DNA, and some also have activities such as RNA ligase and phosphatase. Compared with protein enzymes, ribozymes have a lower catalytic efficiency and are relatively primitive catalytic enzymes. Most ribozymes participate in RNA self-cleavage and processing by catalyzing the hydrolysis of phosphate and phosphodiester bonds. Compared with general antisense RNA, ribozymes have a more stable spatial structure and are less vulnerable to RNase attack. More importantly, the ribozyme, after cutting off the mRNA, can be released from the hybrid strand and recombine and cleave other mRNA molecules.
With the in-depth study of ribozymes, the potential of ribozymes in genetic diseases, tumors and viral diseases has been recognized. For example, for HIV, the transcriptional information is from RNA rather than DNA. Ribozymes can cleave RNA at specific sites, rendering it inactive. If a ribozyme that specifically recognizes HIV RNA is present in the virus-infected cell, it can establish a first line of defense against invasion. Even if HIV does indeed enter the cell and replicate, RNA can also cut off HIV RNA without affecting its own RNA at different stages of the viral life cycle. In another example, leukemia is a malignant tumor of the hematopoietic system and there is currently an absence of effective treatments. The discovery of ribozymes, especially hammerhead ribozymes, has brought new hope for the gene therapy of leukemia.
Abzyme
Abzymes, also known as catalytic antibodies, are a class of catalytic immunoglobulins, i.e., catalytically active antibodies prepared by a series of chemical and biotechnological methods, which have both a corresponding immunological activity and an enzyme-like activity that can catalyze a chemical reaction. Abzymes have typical enzyme reaction characteristics, specificity of binding to ligands, including stereospecificity, and specificity of catalytic reactions of antibody enzymes, which can achieve or even exceed the specificity of natural enzymes, and also have high catalytic efficiency.
The conversion of antibodies into enzymes mainly through the induction method, introduction method, and copy method. The induction method utilizes the reaction of transition state analogues to produce monoclonal antibodies for haptens and screens monoclonal antibodies with high catalytic activity, namely, abzyme. The introduction method uses catalytic engineering and protein engineering to introduce catalytic genes into the antigen binding sites of specific antibodies, so that it obtains the catalytic function. The copy method is mainly designed based on the antigen-antibody complementarity during the antibody production process. Pollack et al. used nitrophenol phosphorylcholine ester as a hapten to induce monoclonal antibodies. After screening, they found abzymes that accelerated the hydrolysis reaction by 12,000 times.
Abzymes can catalyze a variety of chemical reactions, including ester hydrolysis, amide hydrolysis, acyl transfer, photoinduced reactions, redox reactions, metal chelation reactions, and the like. Some of these reactions did not exist in the past in the presence of a biocatalyst that catalyzed them, and even allowed thermodynamically unviable reactions to proceed. The study of antibody enzymes provides people with a reasonable way to design proteins that are suitable for the market, that is, artificially designed enzymes. It is a new field of enzyme engineering. Using the animal immune system to produce highly specific antibodies, a series of highly specific antibody enzymes can be obtained to enrich the abzyme. As a result, there are a large number of highly targeted and highly effective drugs. The study of specific antibody enzymes made it possible to produce drugs with high purity and specificity.
Reference:
Woycechowsky K J, Vamvaca K, Hilvert D. Novel enzymes through design and evolution. Adv Enzymol Relat Areas Mol Biol, 2007, 75(75):241-294.
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