The coagulation cascade.

A cascade of zymogen activations occur within both the intrinsic and extrinsic blood coagulation pathways.

A zymogen is an inactive enzyme precursor, a proenzyme, that displays no catalytic activity until it is transformed into its active form by a process such as the cleavage of one or more peptide bonds. Pancreatic digestive enzymes are secreted as zymogens partly to prevent the enzymes from digesting proteins in the cells in which they are synthesized. An additional important role of zymogen activations occurs in blood clotting. For blood clotting, the response time must be fast in order to achieve clotting at the right spot and time to prevent excessive bleeding. Zymogen cascades are therefore employed to achieve that rapid response in which activation of one clotting factor is then responsible for activating another clotting factor.

WikiPremed Resources

Learning Goals


Beginning with enteropeptidase, be capable of describing the activation of pancreatic digestive enzymes.

Understand the mechanism in blood clotting of the conversion of fibrinogen into fibrin and subsequent cross-linking of fibrin to form a hard clot.

Understand the distinction between the intrinsic and extrinsic coagulation cascades.

Suggested Assignments

Conceptual Vocabulary for Zymogens

Lipids can be broadly defined as any fat-soluble (hydrophobic), naturally-occurring molecules.
Fatty acid
A fatty acid is a carboxylic acid often with a long unbranched aliphatic tail, which is either saturated or unsaturated
A wide group of molecules, fats are generally triesters of glycerol and fatty acids.
A triglyceride consists of glycerol esterified with three fatty acids, although slightly more formally, the form is known as triacylglycerol or triacylglyceride.
Saturated fat
A saturated fat is a fat that consists of triglycerides containing fatty acids with no double bonds between the carbon atoms of the fatty acid chain
Phospholipids are a class of lipids which are a major component of all biological membranes, along with glycolipids, cholesterol and proteins.
Monounsaturated fat
Monounsaturated fats are fatty acids having a single double bond present in the fatty acid chain, with all of the carbons in the chain single bonded
Polyunsaturated fat
Polyunsaturated fats are fatty acids having more than one double bond present in the fatty acid chain.
A diglyceride, more formally known as a diacylglycerol, is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages.
A steroid is a terpenoid lipid characterized by a carbon skeleton with four fused rings generally arranged in a 6-6-6-5 fashion.
Cholesterol is a sterol found in the cell membranes of all physiological tissues and transported in the blood plasma of all animals.
Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes.
Phosphatidylcholine is a phospholipid which is a major constituent of cell membranes. This lipid is such a major component of lecithin that in some contexts the terms are used as synonyms.
Including the essential oils of many plants, terpenes are a large and varied class of hydrocarbons which are derived biosynthetically from units of isoprene.
Lecithin is mostly a mixture of glycolipids, triglycerides, and phospholipids. However, in biochemistry, the term is usually used as a synonym for pure phosphatidylcholine.
Isoprene is a common synonym for the chemical compound 2-methylbuta-1,3-diene. It is an important biological material, being a precursor for many important classes such as the terpenes and steroids.
Squalene is the triterpene which is the biochemical precursor to the whole family of steroids
A prostaglandin is any member of a group of lipid compounds having 20 carbon atoms, including a 5-carbon ring which are derived enzymatically from fatty acids, having important functions in the animal body especially in immunity.
Sterols, or steroid alcohols are a subgroup of steroids with a hydroxyl group in the 3-position of the A-ring.
Omega-3 fatty acid
Omega-3 fatty acids are a family of polyunsaturated fatty acids which have in common a double bond in the third carbon-carbon bond from the terminal end.
Omega-6 fatty acid
Omega-6 fatty acids are a family of polyunsaturated fatty acids which have in common a double bond in the sixth carbon-carbon bond from the terminal end.
Omega-9 fatty acid
Omega-9 fatty acids are a family of polyunsaturated fatty acids which have in common a double bond in the ninth carbon-carbon bond from the terminal end.
Inositol is a carbocyclic polyol that plays an important role as the structural basis for a number of secondary messengers in eukaryotic cells.
Sphingolipids are a class of lipids derived from the aliphatic amino alcohol sphingosine.
The terpenoids, sometimes referred to as isoprenoids, are a large and diverse class of naturally occurring organic chemicals similar to terpenes, derived from five-carbon isoprene units assembled and modified in thousands of ways.
Eicosanoids are signaling molecules derived from omega-3 or omega-6 fats. They exert complex control over many bodily systems, mainly in inflammation or immunity, and as messengers in the central nervous system.
Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons.
Monoterpenes are a class of terpenes that consist of two isoprene units.
Advanced terms that may appear in context in MCAT passages
The terpene carotene is an orange photosynthetic pigment important for photosynthesis. It is responsible for the orange colour of the carrot and many other fruits and vegetables.
Prostanoid is the term used to describe a subclass of eicosanoids consisting of the prostaglandins, the thromboxanes and the prostacyclins.
Phosphatidylserine is a phospholipid nutrient found in fish, green leafy vegetables, soybeans and rice, and is essential for the normal functioning of neuronal cell membranes and activates protein kinase C which has been shown to be involved in memory function.
Carotenoids are organic pigments naturally occurring in plants, some algae, some types of fungus and some bacteria. They are split into two classes, xanthophylls and carotenes.
Cephalin is a phospholipid found in all living cells, although in human physiology it is found particularly in nervous tissue such as the white matter of brain, nerves, neural tissue, and in spinal cord. It is the principal phospholipid in bacteria.
Cardiolipin (bisphosphatidyl glycerol) is an important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid.
Sphingosine is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin.
Composed of sphingosine and a fatty acid, ceramides are a family of lipid molecules found in high concentrations within the cell membrane.
Sesquiterpenes are terpenes that consist of three isoprene units.
Lycopene is a bright red carotenoid pigment, a phytochemical found in tomatoes and other red fruits. It is the most common carotenoid in the human body and is one of the most potent carotenoid antioxidants.
Xanthophylls (originally phylloxanthins) are yellow pigments from the carotenoid group.
Thromboxane is a member of the family of lipids known as eicosanoids. It is named for its role in clot formation.
Leukotrienes are eicosanoid lipid mediators produced in the body from arachidonic acid by the enzyme 5-lipoxygenase. These substances may be responsible for a number of the effects of asthma and allergies.