BIOCHEMISTRY AND HUMAN NUTRITION

Module 2. Enzymes

Lesson 14
ZYMOGENS AND RIBOZYMES

14.1 Introduction (Zymogens)

• Several enzymes are synthesised as larger inactive precursor forms called proenzymes or zymogens.
• Activation of zymogens involves irreversible hydrolysis of one or more peptide bonds.
• The biochemical change usually occurs in a lysosome where a specific part of the precursor enzyme is cleaved in order to activate it. The amino acid chain that is released upon activation is called the activation peptide.
• The pancreas secretes zymogens partly to prevent the enzymes from digesting proteins in the cells in which they are synthesised.
• Fungi also secrete digestive enzymes into the environment as zymogens. The external environment has a different pH than inside the fungal cell and this changes the zymogen's structure into an active enzyme.

14.2 Digestive Enzymes as Zymogens

The digestive enzymes trypsin, chymotrypsin and elastase produced as zymogens in the pancreas.

• They are transported to the small intestine as their zymogens forms and activated there by cleavage of specific peptide bonds.
• Trypsin is synthesized initially as zymogen trypsinogen. It is cleaved (and hence activated) in the intestine by the enzyme enteropeptidase which is only produced in the intestine. Once activated trypsin can cleave and activate further trysinogen molecules as well as other zymogens, such as chymotrysinogen and proelastases.
  
• The peptidase in the stomach is pepsin. Pepsin works optimally in the acidic environment of the stomach, being active at pH 2-3, but becoming inactivated when the pH is above 5. The chief cells at the base of the gastric glands secrete the zymogen, which is called pepsinogen. Pepsinogen is partially activated by hydrocholoric acid (HCl), which is secreted by the parietal cells. This partially active enzyme then cleaves the peptide from other pepsinogen molecules to form active pepsin.

Fig. 14.1 Digestive enzymes as zymogens

14.3 Biological Significance of Zymogens

• Zymogens: Inactive Precursor Proteins. A clinically important mechanism of controlling enzyme activity is the case of protease enzymes involved (predominantly) in food digestion and blood clotting.
• Activation of zymogens by proteolytic cleavage result in irreversible activation.
• Zymogen forms allow proteins to be transported or stored in inactive forms that can be readily converted to active forms in response to some type of cellular signal.
• Thus they represent a mechanism whereby the levels of an enzyme/protein can be rapidly increased (post-translationally). Other examples of zymogens include proinsulin, procollagen and many blood clotting enzymes

14.4 Ribozymes

• Before the discovery of ribozymes, enzymes, which are defined as catalytic proteins were the only known biological catalysts.
• The first ribozymes were discovered in the 1980s by Thomas R. Cech, who was studying RNA splicing in the ciliated protozoan Tetrahymena thermophila and Sidney Altman, who was working on the bacterial RNase P complex.
• These ribozymes were found in the intron of an RNA transcript, which removed itself from the transcript, as well as in the RNA component of the RNase P complex, which is involved in the maturation of pre-tRNAs. Ribozymes often have divalent metal ions such as Mg²⁺ as cofactors.
• A ribozyme (from ribonucleic acid enzyme, also called RNA enzyme or catalytic RNA) is an RNA molecule that catalyzes a chemical reaction. Many natural ribozymes catalyze either the hydrolysis of one of their own phosphodiester bonds, or the hydrolysis of bonds in other RNAs. They have also been found to catalyze the aminotransferase activity of the ribosome.
• Investigators studying the origin of life have produced ribozymes in the laboratory that are capable of catalyzing their own synthesis under very specific conditions, such as an RNA polymerase ribozyme.
• Some ribozymes may play an important role as therapeutic agents, as enzymes which tailor defined RNA sequences, as biosensors, and for applications in functional genomics and gene discovery.

14.5 Activity

• Although most ribozymes are quite rare in the cell, their roles are sometimes essential to life. For example, the functional part of the ribosome, the molecular machine that translates RNA into proteins, is fundamentally a ribozyme, composed of RNA tertiary structural motifs that are often coordinated to metal ions such as Mg²⁺ as cofactors. There is no requirement for divalent cations in a five-nucleotide RNA that can catalyze trans-phenylalanation of a four-nucleotide substrate which has three base complementary sequence with the catalyst