Enzymes – An Introduction
Enzymes are proteins that aid in accelerating our bodies' chemical reactions or metabolism. Some compounds are created, while others are broken down. Enzymes are a part of all living things. Enzymes are created by our bodies spontaneously by lowering the threshold of the target reaction. They achieve this by adhering to a substrate, a different material.
Intracellular Enzymes
Enzymes found inside the cell membrane is referred to as intracellular enzymes. The cytoplasmic fluid of the cell may contain intracellular enzymes unbound or bound to specific organelles, such as ribosomes. The cell's membrane-bound organelles, such as the mitochondria, lysosomes and nucleus, also contain enzymes.
Cytoplasmic Enzyme
The fluid found inside the cell membrane is called cytoplasm. The cytoplasm of the cell contains all the cell's organelles. The primary centre of cellular metabolism is the cytoplasm. The enzyme that catalyses important metabolic processes within the cytoplasm is
Glycolysis is an anaerobic reaction in which one molecule of glucose is broken down into 2 molecules of Pyruvic acid. The conversion of glucose into pyruvic acid involves various intracellular enzymes such as Hexokinase, Phosphoglucose Isomerase, Phosphofructokinase (PFK), Aldolase, Isomerase, Triosephosphate dehydrogenase, Phosphoglycerokinase, Mutase, Enolase and Pyruvate kinase.
Gluconeogenesis is a process where glucose is synthesised from a non-carbohydrate source. The enzymes involved are Malate dehydrogenase (cytoplasmic), PEP carboxykinase, Fructose 1,6- bisphosphatase and Glucose 6-phosphatase, along with enzymes involved with glycolysis.
UDP-glucose phosphorylase, Glycogen synthase, Glycogenin, branching enzymes, Glycogen phosphorylase and Debranching enzymes are used during Glycogen metabolism, which is involved in the synthesis and degradation of glycogen.
Argininosuccinate Synthetase, Arginosuccinase and Arginase are the enzymes involved in the urea cycle.
Aspartate transaminase, alanine transaminase, glutamate dehydrogenase, arginase, serine dehydratase, tyrosine transaminase, glutamine synthetase, glutaminase and adenylate deaminase are some of the enzymes involved in amino acid metabolism.
Mitochondrial Enzyme
Various metabolic processes taking place inside cells are also carried out in mitochondria. Consequently, it also has a massive number of enzymes.
Citrate synthase, Aconitase, Isocitrate dehydrogenase, Alpha-ketoglutarate dehydrogenase complex, Succinate thiokinase, Succinate dehydrogenase, Fumarase and Malate dehydrogenase are the enzyme involved in the tricarboxylic acid cycle during which pyruvate and other carboxylic acid obtained are oxidized into simple carbohydrate molecule.
Pyruvate carboxylase and Malate dehydrogenase are involved in gluconeogenesis.
Carbamoyl phosphate synthetase and Ornithine transcarbamoylase are involved in the urea cycle.
Nuclear Enzyme
The nucleus contains both genetic material and enzymes which are also involved in cellular metabolisms such as DNA replication, transcription, and mitosis. The enzymes involved in the process are DNA Polymerase, RNA Polymerase, Nucleoside dehydrogenase, Nucleoside phosphorylase, Helicase, Ligase, Topoisomerase, Telomerase, DNA primase, Endonucleases, Exonucleases and DNA repair enzymes.
Extracellular Enzyme
The extracellular enzymes are the enzymes which are present in the extracellular fluid. The reaction takes place outside the cell present in tissue spaces, in body fluids like saliva and blood, and cavities of organs like the stomach and intestine.
Salivary Enzyme
Salivary gland produces saliva which contains enzymes that help in the digestion of food. The enzymes involved in the process of digestion are esterase, alpha amylase, lipase, carbonic anhydrase, and lysozymes.
Pepsin helps in partial digestion of protein particles. It is initially inactive as pepsinogen and gets activated as pepsin by hydrochloric acid present in the stomach.
Trypsin helps in the digestion of protein particles. It is produced as trypsinogen by the pancreas and is activated by enterokinase enzymes produced by the duodenum.
Chymotrypsin is produced by chymotrypsinogen. It is activated by trypsin enzymes and helps in protein digestion.
Elastases and Collagenase are proteolytic enzymes which break elastin and collagen fibres present in food.
Pancreatic amylase is an enzyme produced by the pancreas that is involved in carbohydrate digestion.
Intestinal Enzyme
Intestinal enzymes are the enzymes which help in the final digestion of food, and they are:
Peptidases are enzyme which breaks peptide bonds of protein into amino acid and
Sucrase are enzyme which breaks sucrose into glucose and fructose
Maltase are enzymes which digest maltose into two glucose molecules.
Lactase are enzymes which break lactose into glucose and galactose.
Difference Between Intracellular and Extracellular Enzyme
Interesting Facts
Several places in your digestive system produce enzymes.
Different types of enzymes are produced for digesting different foods.
Production of enzymes decreases as our body gets older.
Key Features
Both intracellular and extracellular enzymes are involved in body metabolism
The difference is that intracellular enzymes are produced within the cell and extracellular enzymes are produced outside the cell.
The enzymes present in cytoplasm and mitochondria are involved in both gluconeogenesis and in the urea cycle.
FAQs on Intracellular and Extracellular Enzyme
1. What are intracellular and extracellular enzymes?
Intracellular enzymes are biological catalysts that function inside the cell where they are produced. They are responsible for catalysing the vast majority of metabolic pathways, such as glycolysis and DNA replication. Extracellular enzymes are synthesised inside the cell but are secreted to function outside the cell, such as in the digestive tract or blood plasma.
2. What is the main difference between intracellular and extracellular enzymes?
The primary difference lies in their site of action. Intracellular enzymes catalyse reactions within the cell's cytoplasm, nucleus, or organelles like mitochondria. In contrast, extracellular enzymes are transported out of the cell to act on external substrates. For example, digestive enzymes like pepsin act in the stomach, far from the cells that produced them.
3. Can you give examples of intracellular and extracellular enzymes based on their location?
Certainly. The location of an enzyme is directly related to its function:
- Examples of Intracellular Enzymes: DNA Polymerase (found in the nucleus for DNA replication), Hexokinase (found in the cytoplasm for glycolysis), and ATP synthase (found in mitochondria for energy production).
- Examples of Extracellular Enzymes: Salivary amylase (secreted into saliva to digest starch), Pepsin (secreted into the stomach to digest protein), and Trypsin (secreted into the small intestine for further protein digestion).
4. Are endoenzymes and exoenzymes the same as intracellular and extracellular enzymes?
Yes, these terms are often used interchangeably and refer to the same concept. Endoenzymes is another name for intracellular enzymes, as they function 'endo' or within the cell. Similarly, exoenzymes is another term for extracellular enzymes, as they are secreted to function 'exo' or outside the cell. Using either terminology is generally correct.
5. Why must digestive enzymes like pepsin be extracellular?
Digestive enzymes like pepsin must be extracellular because their function is to break down large food molecules (like proteins) into smaller units that can be absorbed by cells. If pepsin were an active intracellular enzyme, it would digest the cell's own essential proteins, a process called autolysis, which would lead to cell death. Secreting it into the stomach lumen ensures it only acts on dietary proteins.
6. Where are intracellular enzymes specifically found within a cell?
Intracellular enzymes are not just floating freely; they are often localised to specific compartments to carry out specialised functions:
- In the Cytoplasm: Enzymes for metabolic pathways like glycolysis and the pentose phosphate pathway.
- In the Mitochondria: Enzymes for the Krebs cycle and electron transport chain, crucial for cellular respiration.
- In the Nucleus: Enzymes vital for genetic processes, such as DNA polymerase (for replication) and RNA polymerase (for transcription).
- In Lysosomes: A range of hydrolytic enzymes that break down cellular waste and foreign materials.
7. How does the location of an enzyme (intracellular vs. extracellular) relate to its specific function in metabolism?
The location is critically tied to function. Intracellular enzymes manage the cell’s internal economy—they build, repair, and power the cell by catalysing reactions involving molecules already inside (e.g., producing ATP). Extracellular enzymes act as external processors; they break down large, complex external substrates (like starches and proteins in food) into smaller, simpler molecules that are then small enough to be transported into the cell for use by intracellular enzymes.
8. What key factors, like temperature and pH, affect how enzymes function?
All enzymes, whether intracellular or extracellular, are sensitive to their environment. The two most critical factors are:
- Temperature: Enzyme activity generally increases with temperature up to an optimal point. Beyond this, high temperatures cause the enzyme to lose its specific three-dimensional shape (denature), drastically reducing its activity.
- pH: Each enzyme has an optimal pH at which it functions most effectively. For example, pepsin in the stomach works best in a highly acidic environment (pH 1.5-2.5), while trypsin in the small intestine requires an alkaline environment (pH ~8).
9. How are extracellular enzymes produced by the cell and then released to function outside?
Extracellular enzymes are proteins and follow a specific production and export pathway. They are first synthesized on ribosomes attached to the endoplasmic reticulum, then move into the Golgi apparatus for modification and packaging. Finally, they are enclosed in secretory vesicles, which travel to the cell membrane, fuse with it, and release the enzymes outside the cell through a process called exocytosis. This pathway ensures the active enzyme never damages internal cell structures.
10. Why can't humans digest plant fibre, and what type of enzyme is missing?
Humans cannot digest plant fibre because we lack the gene to produce the specific extracellular enzyme called cellulase. Plant fibre is primarily made of cellulose, a complex carbohydrate. Without cellulase to break it down in our digestive system, cellulose passes through undigested, acting as dietary roughage rather than an energy source. This highlights the importance of having the correct enzymes to process specific substrates.
