Intracellular and Extracellular Enzymes

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Intracellular and Extracellular Enzymes

Introduction

Enzymes are the protein substances that speed up the chemical reactions taking place in the bodies of living organisms. The presence of enzymes is essential for life to continue. Without enzymes, metabolism will stop, and life will stop to exist.

Enzymes can catalyse chemical reactions both in the cytoplasm or organelles within the cell as well as in the extracellular environment. It must be kept in mind that enzymes working in both these environments are essentially synthesized within the cell cytoplasm.

In this article, we will discuss intracellular and extracellular enzymes with the help of various examples.

Intracellular Enzymes

As the name indicates, such enzymes are present inside the cell membrane. Intracellular enzymes may be present freely in the cytoplasmic fluid of the cell or they may be bound to some organelles such as ribosomes. Enzymes may also present within the membrane-bound organelles within the cell such as mitochondria, lysosomes, nucleus, etc.

Let us first discuss the synthesis of intracellular enzymes. Later, various examples of enzymes present at different locations within the cell are discussed.

Synthesis

Almost all enzymes are protein in nature. Just like the rest of the proteins, they are synthesized by ribosomes in a process called gene expression. It comprises two steps: transcription and translation.

The information for the synthesis of enzymes is located on various chromosomes within the nucleus. This information is copied in the form of mRNA in the process of gene transcription. The molecules of mRNA carry this information to the ribosomes present in the cell cytoplasm.

The mRNA molecules are bind by the ribosomes and the information in nucleotide sequence is translated in the form of an amino acid sequence of polypeptides.

The polypeptide chain thus formed is released within the rough endoplasmic reticulum where it undergoes folding and attains its final shape.

Cytoplasmic enzymes are released without any further change. On the other hand, enzymes that are to be transported to other organelles are sent to the Golgi apparatus for packaging.

Golgi apparatus packages enzymes into vesicles that are then sent to the membrane-bound organelles to be a part of their structure.

Some of the examples of intracellular enzymes are mentioned below.

Cytoplasmic Enzymes

The cytoplasm is fluid present within the cell membrane. All the organelles of the cell are dispersed in the cytoplasm. The cytoplasm is the main hub of cellular metabolism. The majority of metabolic processes take place within the cytoplasm of cells. The important intracellular enzymes that catalyse the metabolic process within the cytoplasm are as follows.

Glycolysis

Glycolysis is an anaerobic process by which one glucose molecule is broken down into two molecules of pyruvic acid. It is the basic reaction of carbohydrate metabolism and an important source of energy for organisms, especially in anaerobic conditions.

Glycolysis consists of ten steps. Each step is catalysed by a separate enzyme. All these enzymes are intracellular, present in the cytoplasm. Below is a list of these cytoplasmic enzymes.

  • Hexokinase
  • Phosphoglucose Isomerase
  • Phosphofructokinase (PFK)
  • Aldolase
  • Isomerase
  • Triosephosphate dehydrogenase
  • Phosphoglycerokinase
  • Mutase
  • Enolase
  • Pyruvate kinase

Gluconeogenesis

Gluconeogenesis is a process by which glucose is synthesized from non-carbohydrate sources. The non-carbohydrate precursors include pyruvate, lactate, glycerol, and some amino acids called glucogenic amino acids. 

Gluconeogenesis is not a mere reversal of what happens in glycolysis. Some reactions of glycolysis are irreversible. Thus, some different reactions are needed to bypass these irreversible steps.

The first three reactions of gluconeogenesis occur within the mitochondria. The rest of the reactions take place within the cytoplasm. Following cytoplasmic enzymes are needed for gluconeogenesis in addition to what is already mentioned in glycolysis.

  • Malate dehydrogenase (cytoplasmic)
  • PEP carboxykinase
  • Fructose 1,6- bisphosphatase
  • Glucose 6-phosphatase

Glycogen Metabolism

Glycogen is a glucose storage molecule found in animal cells. It is found in all the cells of the body. However, major stores are found in liver and muscle cells.

When the body has excess glucose, it is converted into glycogen to be stored. This process is called glycogenesis.

However, when the body needs glucose, glycogen stores are broken down to release glucose in a process called glycogenolysis.

Most of the enzymes for the synthesis and degradation of glycogen are located within the cellular cytoplasm. These intracellular enzymes include:

  • UDP-glucose phosphorylase
  • Glycogen synthase
  • Glycogenin
  • Branching enzymes
  • Glycogen phosphorylase
  • Debranching enzyme

Some lysosomal and mitochondrial enzymes are also used which will be discussed later.

Urea Cycle

The urea cycle is a metabolic process by which two molecules of ammonia and one carbon dioxide molecule are converted into one urea molecule. Ammonia is a highly toxic product. In humans, it is converted into urea, a less toxic nitrogenous waste, to be excreted in the urine. The urea cycle takes place in the cellular cytoplasm. The intracellular cytoplasmic enzymes catalyse various steps involved in this process.

A list of enzymes involved in the urea cycle is as follows.

  • Arginosuccinate Synthetase
  • Arginosuccinase
  • Arginase

One mitochondrial enzyme is also used in this process that will be discussed in the subsequent heading.

Amino Acid Metabolism

Various enzymes of amino acid metabolism are also present in the cytoplasm. These enzymes catalyse reactions involved in the synthesis and degradation of various amino acids and their conversion to different nitrogen-based products. The list of these enzymes is very large and cannot be mentioned here.

Miscellaneous Enzymes

The list of intracellular enzymes present in the cytoplasm is never-ending. It is because the cytoplasm is the main metabolic centre of each cell. The majority of the cellular metabolic reactions occur within the cytoplasm.

Other than the ones participating in the metabolic process mentioned above, following cytoplasmic enzymes are also important.

  • ATPase: This enzyme breaks down ATP to ADP releasing energy that is needed for various processes.
  • Carbonic anhydrase: It catalyses the reversible reaction between carbon dioxide and water to make carbonic acid. This reaction plays an important role in the acid-base balance within the body.
  • Catalase: This enzyme saves the body from harmful reactive oxygen species. It converts the toxic hydrogen peroxide into non-toxic water and oxygen molecules. This enzyme is predominantly found in the liver cells of animals.

Mitochondrial Enzymes

Mitochondria is also a site of a large number of metabolic reactions within the cells. Therefore, it also contains a large population of enzymes. It is said that mitochondria contain a battery of enzymes that extract energy from various molecules and provides it to the cell in the form of ATP.

A brief detail of important metabolic processes along with the intracellular enzymes needed for them is given below.

Tricarboxylic Acid Cycle

It is the final pathway for the oxidative metabolites of carbohydrates, fatty acids, and amino acids. During this cycle, pyruvate, and other carboxylic acids, obtained from various sources are oxidized to simple carbon dioxide molecules and the energy is released. The enzymes involved in this cycle are as follows.

  • Citrate synthase
  • Aconitase
  • Isocitrate dehydrogenase
  • Alpha-ketoglutarate dehydrogenase complex
  • Succinate thiokinase
  • Succinate dehydrogenase
  • Fumarase
  • Malate dehydrogenase

Before starting the tricarboxylic acid cycle, pyruvate is converted to acetic acid by an intracellular enzyme complex called pyruvate dehydrogenase. It is also located within the mitochondria.

Gluconeogenesis

The first three reactions of gluconeogenesis occur within the mitochondria. The enzymes needed for these reactions are as follows.

  • Pyruvate carboxylase
  • Malate dehydrogenase (mitochondrial)

Urea Cycle

Some reactions of the urea cycle also occur in mitochondria. These include the synthesis of carbamoyl phosphate and its transfer to ornithine to form citrulline. The enzymes involved in these reactions are:

  • Carbamoyl phosphate synthetase
  • Ornithine transcarbamoylase

In addition to the above-mentioned reaction, several other anabolic and catabolic reactions also occur in mitochondria, catalysed by intracellular enzymes. Because of keeping the writing simple, all those reactions cannot be discussed here.

Nuclear Enzymes

The nucleus not only contains the genetic material of a cell but also contains certain enzymes. These enzymes are useful in the metabolic process taking place within the nucleus such as transcription, DNA duplication, mitosis, etc.

Intracellular enzymes present inside the nucleus include the following:

  • DNA Polymerase
  • RNA Polymerase
  • Nucleoside dehydrogenase
  • Nucleoside phosphorylase
  • Helicase
  • Ligase
  • Topoisomerase
  • Telomerase
  • DNA primase
  • Endonucleases
  • Exonucleases
  • DNA repair enzymes

These enzymes are also sometimes called nuclear proteins. They are made in the cytoplasm and enter the nucleus via nuclear pores.

Lysosomal Enzymes

Lysosomes are the membrane-bound organelles found in animal cells. They contain a collection of enzymes for intracellular digestion and detoxification of phagocytosed molecules. They are involved in the breakdown of various large-sized molecules.

Some examples of lysosomal enzymes are mentioned below.

  • Collagenases: They cause the digestion of collagen fibres within the lysosomal vesicle.
  • Elastases: These proteolytic enzymes cause the digestion of elastin fibres.
  • Phospholipases: They are involved in the breakdown of phospholipids such as lecithin.
  • Fatty acyl esterase: It is responsible for the digestion of fatty acyl esters.
  • Glycosidase: It causes lysosomal glycogenolysis.
  • Galactosidase: It is involved in the digestion of galactose sugar.
  • Nucleases: They cause the breakdown of DNA and RNA.
  • Catalase: It is also present in the lysosome and is responsible for the detoxification of hydrogen peroxide.

Ribosomal Enzymes

Ribosomes are the protein-making machinery of the cell. They contain enzymes needed for the translation of mRNA to make proteins. Following intracellular enzymes are present within the ribosomes.

  • Peptidyl transferase
  • Termination enzyme

Extracellular Enzymes

As the name indicates, extracellular enzymes are present in the extracellular fluid. They catalyse chemical reactions taking place outside the cells. They may be present in tissue spaces, in body fluids like saliva and blood, and cavities of organs like the stomach, intestine, etc.

In this portion, we will discuss the synthesis and action of some major extracellular enzymes.

Synthesis

Like intracellular enzymes, the extracellular ones are also synthesized by ribosomes in the process of translation.

Once synthesized, they are transferred to the Golgi apparatus from the rough endoplasmic reticulum. Here, the enzymes are packed into membrane-bound vesicles and are directed towards the cell membrane.

The Golgi vesicles fuse with the cell membrane and dump the enzymes into the extracellular space.

The extracellular enzymes either work in these spaces between the cells or are transported via specialized ducts into different body organs for their function as seen in the case of pancreatic enzymes.

Salivary Enzymes

Saliva is a watery fluid produced by salivary glands and released into the oral cavity via salivary ducts. It contains important enzymes that are necessary for the digestion of food. Digestion begins in the oral cavity and is initiated by extracellular enzymes present in the saliva.

The important salivary enzymes and their functions are as follows.

  • Esterase: It causes hydrolysis of esters present in food releasing alcoholic acids. These esters are present in food as flavouring agents.
  • Alpha-Amylase: It is a carbohydrate digestion enzyme that causes hydrolysis of starch. It breaks down starch into simpler sugars like sucrose.
  • Lipase: It causes hydrolysis of triglycerides present in the food particles resulting in the synthesis of free fatty acids.
  • Carbonic anhydrase: It is an important enzyme present in saliva that causes the synthesis of bicarbonate ions from carbon dioxide and water molecules.
  • Lysozymes: This enzyme has antimicrobial activity. It can cause lysis of peptidoglycans present in bacterial cell walls, thus protecting the body from their harmful effects.

Salivary enzymes are mixed with food in the oral cavity. They continue to perform their action as the food bolus moves from the oral cavity to the stomach via the oesophagus.

Pepsin

Pepsin is an extracellular enzyme released by chief cells of the stomach into gastric juice. This enzyme is released in an inactive form called pepsinogen. The inactive variant is converted to an active enzyme by abundant hydrochloric acid present in the stomach.

The activated enzyme causes partial digestion of proteins. It is a protease enzyme that digests protein particles present in food to form simpler polypeptides. These polypeptides are later digested by another enzyme to generate simpler products.

Pepsin is released in an inactive form to protect cells from its proteolytic action. It can only be activated in the presence of an acidic environment that is provided by the HCL in the stomach antrum.

Trypsin

It is another enzyme involved in protein digestion. Like pepsin, it is released in an inactive form called trypsinogen. It is produced by the pancreas and released in the duodenum along with other enzymes in pancreatic juice.

Once in the duodenum, it is activated by another enzyme called enterokinase. Enterokinase is released by the cells of the duodenum. It cleaves a peptide from the amino terminal of trypsinogen to form trypsin.

Trypsin acts on polypeptides and converts them into simpler peptides. They are further acted upon by other enzymes to be converted into simpler products.

Trypsin also causes activation of other enzymes in the pancreatic juice such as chymotrypsinogen and proelastase etc.

Chymotrypsin

It is a pancreatic enzyme released in an inactive chymotrypsinogen form. Trypsin from the pancreas causes activation of this enzyme. It is also involved in protein digestion within the small intestine.

Elastases and Collagenases

These are proteolytic extracellular enzymes released by the pancreas into the small intestine. They act upon elastin and collagen fibres present in the food particles. These extracellular enzymes cause complete digestion of protein fibres into the constituent amino acids.

Pancreatic Amylase

It is another extracellular enzyme released by the pancreas. It is an enzyme involved in carbohydrate digestion. Just like salivary amylase, it causes hydrolysis of starch present in food. It converts starch into simpler sugars like disaccharides and trisaccharides.

Lipolytic Pancreatic Enzymes

Pancreas also releases extracellular enzymes for the digestion of lipid particles present in food. The enzymes and their actions are listed below.

  • Pancreatic lipase: It causes digestion and breakdown of triglycerides present in food.
  • Phospholipases: These enzymes cause the breakdown of phospholipids. The majority of phospholipids are found in the cell membranes. Thus, phospholipids provide an access to the cells present in food by degrading their cell membranes.
  • Cholesterol ester hydrolase: It causes hydrolysis and breakdown of cholesterol esters present in food.
  • Bile salt-activated lipase: It is another enzyme having an action similar to pancreatic lipase. The only difference is that it is converted into its active form by the action of bile salts. Bile salts are made in the liver, stored in the gall bladder, and released into the small intestine via the bile duct.

Pancreatic Nucleases and Nucleosidases

These extracellular enzymes are involved in the digestion of nucleic acids like DNA and RNA.

Nuclease acts upon long strands of nucleic acids and releases individual nucleotides by cleaving phosphodiester bonds.

Nucleosidase then acts upon individual nucleotides and cause their breakdown into nitrogenous base and sugar.

It should be kept in mind that all the pancreatic enzymes are made by pancreatic parenchymal cells. They are then released out of the cell into the pancreatic duct. The pancreatic enzyme along with water and other components together make the pancreatic juice that is then released into the duodenum. All these enzymes act extracellularly in the lumen of the small intestine and cause the digestion of various food particles.

Intestinal Enzymes

In addition to the digestive enzymes mentioned above, intestinal cells also make and release some enzymes that complete the process of digestion. These are extracellular enzymes released by the cells lining the intestines into the intestinal lumen. Some important enzymes are as follows.

  • Peptidase: Peptidase is released by cells lining the small intestine. It is also known as protease or proteinase because It breaks the peptide linkages between small peptides and releases individual amino acids. Thus, the process of protein digestion is completed, and amino acids are absorbed by cells lining the small intestine.
  • Sucrase: Sucrase is a digestive enzyme that is released by villi of the epithelium of the small intestine. In the intestine, it is released in the form of sucrase-isomaltase. It causes the breakdown of sucrose into its subunits glucose and fructose that are later absorbed by intestinal cells.
  • Maltase: Starch is converted into maltose by pancreatic amylases. In the intestine, another enzyme called maltase completes the digestion of starch. Maltase is an alpha-glucosidase enzyme that causes the digestion of maltose into two glucose molecules. It breaks the alpha-glucosidase linkages of complex carbohydrates.
  • Lactase: Lactose is milk sugar mostly present in milk and milk products and it is responsible for the sweetness of the milk. The enzyme lactase causes the breakdown of lactose into glucose and galactose.

Summary

  • Enzymes are tiny little protein substances that catalyse chemical reactions taking place in our body. Enzymes that are present inside the cell membrane are called intracellular enzymes and the extracellular enzymes are those which are present outside the cell. Intracellular enzymes may reside in the cytoplasmic fluid or they may be bound to cellular organelles.
  • The synthesis of enzymes takes place within the cytoplasm. Ribosomes make enzymes through a process called gene expression that involves transcription and translation. Some examples of intracellular enzymes are cytoplasmic enzymes, mitochondrial enzymes, and nuclear enzymes.
  • In the cytoplasm, Hexokinase, Phosphoglucose isomerase, phosphofructokinase, and other enzymes catalyse the process of glycolysis. Various other cytoplasmic enzymes speed up the processes of gluconeogenesis, glycogen metabolism, urea cycle, and amino acid metabolism. The intracellular enzymes present in the mitochondria are involved in the tricarboxylic acid cycle in which pyruvate and other carboxylic acids are oxidized to simple carbon dioxide molecules.
  • The enzymes of mitochondria are also involved in gluconeogenesis and the urea cycle. Another example of intracellular enzymes is nuclear enzymes. These enzymes are present within the nucleus and are involved in transcription, mitosis, and DNA duplication, etc. Some lysosomal and ribosomal enzymes are also present inside the cell.
  • The extracellular enzymes are present outside the cells, in the extracellular fluid. They are present in tissue spaces, body fluids, and organ cavities. Extracellular enzymes are also synthesized by ribosomes of the cells and after further processing in ER and Golgi apparatus, they go into the extracellular fluid. Salivary enzymes such as esterase, alpha-amylase, lipase, and anhydrase are some examples of extracellular enzymes that are involved in the initial digestion of food in the oral cavity.
  • Some other examples of extracellular enzymes are pepsin, chymotrypsin, elastases, collagenases, pancreatic amylase, pancreatic nucleases, and nucleosidases, etc. Moreover, intestinal enzymes such as peptidase, sucrase, and maltase are also extracellular enzymes.

References

  1. Denise R. Ferrier, Lippincott Illustrated Reviews, Biochemistry, Ed. 6th
  2. Rodwell, Kennelly, Harper’s Illustrated Biochemistry, Ed. 30th