4. Heparin and Heparan Sulfate
Previously, we examined the structural linkages involved in the formation of hyaluronic acid, chondroitin sulfate, dermatan sulfate, and keratan sulfate. These are types of glycosaminoglycans (GAGs) characterized by repeating disaccharide units composed of an amino sugar and a uronic acid. Hyaluronic acid is composed of glucuronic acid and N-acetylglucosamine (GlcA + GlcNAc); chondroitin sulfate consists of glucuronic acid and N-acetylgalactosamine (GlcA + GalNAc); dermatan sulfate is formed from iduronic acid and N-acetylgalactosamine (IdoA + GalNAc); and keratan sulfate is composed of repeating disaccharides of N-acetylglucosamine and galactose (GlcNAc + Gal) without a uronic acid. We also noted that except for hyaluronic acid, the rest can undergo sulfation at various positions.
Structure of Heparin and Heparan Sulfate (An Expanded Cast of Participants)
Heparin and heparan sulfate, which we now turn to, are slightly more complex than the the above GAGs. This is due to a slightly broader variety of monosaccharides involved in the formation of their disaccharide units. While they too follow the basic combination pattern of “uronic acid + amino sugar,” both glucuronic acid and its epimer iduronic acid can participate, and the amino sugar glucosamine (GlcN) may either be acetylated to become N-acetylglucosamine (GlcNAc) or sulfated to become N-sulfoglucosamine (GlcNS), all of which can be incorporated into disaccharide units. With more building blocks available, the number and diversity of possible combinations increases significantly.
In addition, because the sites for potential sulfation are quite varied, it is difficult to present heparin and heparan sulfate using a standard structural model like those of other GAGs. So, I couldn’t draw a detailed diagram. Furthermore, the presence of specific sequences such as the “pentasaccharide sequence,” which appears intermittently along the disaccharide chains, adds to the structural diversity of heparin and heparan sulfate, enabling them to perform a wide variety of biological functions.
The disaccharide units forming heparin and heparan sulfate are composed of β-configured uronic acids and α-configured amino sugars, connected via β(1→4) linkages between the uronic acid and amino sugar, and α(1→4) linkages between the disaccharide units
Similarities and Differences Between Heparin and Heparan Sulfate
Heparin and heparan sulfate are very similar and are often discussed together. While there are slight differences in their components, the most significant difference lies in the degree of sulfation. Heparin is considered a highly sulfated version of heparan sulfate. [1] One of the main characteristics of heparin is its strong sulfation, giving it the highest negative charge density among biological macromolecules. Heparin contains approximately 2.7 sulfate groups per disaccharide unit, whereas heparan sulfate contains one or fewer sulfate groups per disaccharide, indicating a much lower degree of sulfation.[2] The various sulfation patterns exhibited by these molecules are crucial in determining the diverse functions that heparin and heparan sulfate perform within the body.
Heparin is best known as an anticoagulant used for treating thrombosis, and heparan sulfate also has anticoagulant properties. Unlike heparan sulfate, which is always present in the cell membranes and extracellular matrices of almost all endothelial cells and exerts a mild but continuous anticoagulant effect, heparin is not normally found circulating in the bloodstream. Instead, it is stored in granules within immune cells such as mast cells and basophils, along with histamine. When these immune cells are activated during inflammatory or immune responses, the granules rupture and release heparin into the bloodstream, where it prevents clot formation and helps maintain blood flow at the site of inflammation, allowing immune cells like leukocytes to reach the affected area more easily. Thus, heparan sulfate serves a constitutive anticoagulant role, while heparin functions as a potent, rapidly acting anticoagulant in emergency situations.
Function of Heparin and Heparan sulfate
To understand their anticoagulant mechanisms, it is helpful to understand the process of hemostasis. For detailed information on this topic, please refer to a separate article. Briefly, heparin and heparan sulfate exert their effects by binding to antithrombin III (ATIII), inhibiting two key coagulation factors. Notably, when heparin binds to ATIII, it significantly enhances ATIII’s activity. The high negative charge from its extensive sulfation allows heparin to bind with high affinity to ATIII. In contrast, heparan sulfate has a much weaker anticoagulant effect because its lower sulfation level leads to a weaker interaction with ATIII.
Heparin predominantly contains highly sulfated iduronic acid (IdoA2S) and sulfated glucosamine (GlcNS6S), whereas heparan sulfate mainly consists of glucuronic acid (GlcA) and N-acetylglucosamine (GlcNAc). This is the key structural reason for their difference in anticoagulant strength
Secondary hemostasis: thrombin-mediated fibrin mesh formation(intrinsic/extrinsic pathways)
Antithrombin III (ATIII)
ATIII is a major anticoagulant protein produced in the liver and naturally present in plasma. In simple terms, thrombin is a coagulation factor that normally exists in its inactive precursor form, prothrombin. Once the coagulation cascade is triggered, thrombin is activated in the final stages and converts the soluble fibrinogen protein into insoluble fibrin strands that form a mesh to stabilize the platelet plug.
Antithrombin III belongs to the serpin (serine protease inhibitor) family and prevents the activation of thrombin. It inhibits the activation of two key coagulation factors: thrombin (factor IIa) and factor Xa(with Roman numerals II and X indicating 2 and 10, respectively). Without activation, ATIII binds slowly and weakly to these factors, but once bound to heparin, ATIII undergoes a conformational change that dramatically accelerates its inhibitory activity.
Pentasaccharide Sequence
Therefore, the effectiveness of heparin as an anticoagulant depends on how well it binds to antithrombin III—a process that directly correlates with its degree of sulfation. Sulfation enhances electrostatic interactions and enables specific sulfation patterns to selectively modulate binding to different proteins. In the picture above, we looked at the various combinations that can form heparin. Inside the heparin structure, there is a special pentasaccharide sequence that has a much higher degree of sulfation pattern than the typical heparin structure.
This unique five-sugar sequence (pentasaccharide) is the key region responsible for the strong binding to antithrombin III via its intense negative charge. Upon binding, the structure of ATIII is altered in such a way that it gains high affinity for factor Xa, leading to the formation of a covalent bond and the inactivation of the factor. This highly specific pentasaccharide sequence is found in approximately one-third(or about 30%) of the entire heparin chain. Because binding and activating antithrombin III is essential to heparin’s function as an anticoagulant, this pentasaccharide sequence is considered an essential and core functional domain for its anticoagulant activity.
UFH (Unfractionated Heparin) vs. LMWH (Low Molecular Weight Heparin)
Heparin is one of the most widely used carbohydrate-based drugs, especially in cardiovascular medicine as an anticoagulant and antithrombotic agent. It is a natural product obtained through extraction and purification from animal tissues. Based on molecular size and structural characteristics, heparin is generally classified into two major forms: unfractionated heparin (UFH) and fractionated low molecular weight heparin (LMWH). Typically, when people refer to "heparin," they mean unfractionated heparin (UFH), whereas LMWH consists of smaller molecules derived from UFH.
Essentially, the difference lies in size. Larger heparin molecules with longer chains contain more pentasaccharide sequences, which exhibit high negative charge and high affinity for ATIII, allowing them to control a broader range of coagulation factors. In contrast, LMWH mainly acts by inhibiting factor Xa. However, larger molecular weight makes it difficult to control dosing and increases the risk of bleeding, whereas smaller molecular weight makes the drug response more predictable and reduces bleeding risk, though it is relatively more expensive.
Heparin is often used in acute thrombotic conditions, and the choice between UFH and LMWH depends on the clinical context. Unlike the two types derived from animal tissues (such as porcine intestinal mucosa), there are also synthetic anticoagulant drugs that consist solely of the high-affinity pentasaccharide sequence, offering a targeted mechanism of action.
For reference, a simple comparison table has been created to summarize the three types of anticoagulant drugs.
[References]
https://www.mdpi.com/1424-8247/16/4/584
https://pmc.ncbi.nlm.nih.gov/articles/PMC3755452/
