Panoramic View of the Kallikrein Family: From Structure and Function to Disease and Drug Development

KLKs (kallikreins) are a family of tissue kallikrein serine proteases which, together with plasma kallikrein (KLKB1), constitute the Kallikrein–Kinin System (KKS). KKS’s core physiological function is to regulate vasodilation and vascular permeability. By releasing bradykinin, it induces vasodilation and lowers blood pressure. In addition, KKS participates in inflammatory and pain responses, amplifying inflammatory signals during tissue injury or infection.

KLKs are produced in multiple tissues, including blood vessels, kidneys, pancreas, and salivary glands. Through specific enzymatic reactions, KLKs cleave kininogens and other protein substrates, forming localized proteolytic cascades that regulate the microenvironment, vascular tone, inflammation, and tissue remodeling.

The Kallikrein–kinin System (KKS) in plasma and tissues [1]

Structure of KLK Proteins

KLKs comprise 15 members (KLK1–KLK15) of diverse and specialized functions. One of the most well-known members is KLK3, also known as prostate-specific antigen (PSA), a key biomarker for prostate cancer. All 15 KLKs are thought to be transcribed and synthesized as secreted precursor polypeptides. They share a high degree of homology and their three-dimensional structures and catalytic mechanisms are highly conserved.

Structurally, each KLK contains:

• a signal peptide, which directs the protein to the secretory pathway;

• a pro-peptide, which serves an autoinhibitory role to prevent premature activation during secretion;

• a catalytic domain, which constitutes the main structure of the mature enzyme.

The catalytic domain contains the conserved catalytic triad characteristic of serine proteases: His–Asp–Ser [2]. Substrate specificity and binding sites for inhibitors or antibodies are largely determined by differences in the surface loops surrounding the S1 pocket [3]. Most KLKs are secreted as trypsin-like zymogens and require removal of the N-terminal pro-peptide domain to enable enzymatic activity.

Activation can occur through autocatalytic cleavage or via cleavage by other proteases (including other KLKs), to drive proteolytic cascades. For example, KLK5 can activate KLK7 and KLK14, while KLK14 can in turn activate KLK5 and KLK7. This cascade is particularly critical in epidermal barrier formation and corneocyte desquamation, where KLK-specific protease inhibitors modulate the skin shedding process [4].

KLK Proteins and Disease

KLKs are no longer considered as mere passive biomarkers in diseases, but as active pathogenic drivers. Dysregulation of the proteolytic activity of KLKs can trigger abnormal signaling pathways, amplify inflammation, and induce tissue remodeling. This can contribute to pathologies, including skin diseases, cancer invasion, cardiovascular disease, and neurodegenerative disorders.

Physiological and pathological roles associated with KLK function [5]

Cardiovascular Diseases

Dysregulation of KLK activity is also closely linked to cardiovascular diseases. Excessive KLK activity leads to overproduction of kinins, causing excessive vasodilation and potentially contributing to thrombosis. Conversely, insufficient KLK function results in inadequate kinin generation, impaired vasodilation, and poor blood pressure regulation, which may lead to inadequate tissue perfusion.

KLK1 plays a prominent role in cardiovascular and cerebrovascular protection and its clinical use in peripheral arterial disease, chronic kidney disease, and ischemic conditions. DM199 (Rinvecalinase alfa), developed by DiaMedica, is a recombinant human KLK1 protein analogous to endogenous urokinase. It is designed to increase bradykinin production and activate bradykinin B2 receptors (B2R) in arteries affected by acute ischemic stroke (AIS), particularly in the ischemic penumbra, thereby improving collateral circulation. By delivering essential oxygen and nutrients to damaged brain tissue, DM199 may protect or restore neuronal function, reduce penumbral size, and minimize neuronal death (infarction) and brain injury. DM199 is currently in Phase II/III clinical trials for AIS. In addition, KLK1 is closely associated with inflammatory diseases of the respiratory tract [6] and gastrointestinal tract [7].

DM199 significantly improves collateral circulation in acute ischemic stroke [8]

 

Neurodegenerative Diseases

KLK6 and KLK8 are known to contribute to neurodegenerative diseases by cleaving and activating specific neuromodulatory molecules[9, 10]. Dysregulation of these enzymes may reduce neuroprotection and exacerbate neuroinflammation, accelerating disease progression. Maintaining an appropriate balance between KLK expression and activity is essential for central nervous system homeostasis.

Studies show that KLK8 cleaves the Ephrin B2 receptor, inducing increased expression of FKBP5, which is associated with anxiety-related behavior in mice [11]. Excessive KLK8 activity also affects other signaling pathways directly or indirectly related to Alzheimer’s disease (AD). In particular, KLK8 inhibits AKT and activates GSK-3β, leading to increased Tau phosphorylation and accumulation of neurofibrillary tangles, which are considered neurotoxic. KLK8 can also cleave Neuregulin-1, generating protein fragments that enhance inhibitory synaptic connections between neurons and

Molecular mechanisms involving KLK8 in AD [10]

However, the same enzyme may cleave different substrates and exert distinct effects depending on brain region, cell type, and disease stage, posing challenges for drug development. The field remains exploratory, and future strategies may rely on biomarker-based patient stratification, enzyme activity-guided decision-making, selective and controllable pharmacological approaches, and combination therapies to treat neurodegenerative diseases.

KLK Family in Other Disease Areas

While the roles of KLK6/8 and KLK1 underscore the family’s significance in neurodegenerative and cardiovascular health, the functional reach of these proteases extends across a diverse pathological landscape. The KLK family represents a sophisticated network of enzymatic regulators, and their dysregulation is a primary driver in several other high-impact clinical indications.

To further explore how specific KLK members serve as both critical disease mediators and diagnostic biomarkers, we invite you to read our focused deep-dives:

• Prostate Cancer: Analyzing the pivotal role of KLK2 in oncogenic signaling and its utility in modern diagnostics.

• Skin Diseases: Examining the KLK5/7 axis and how its disruption leads to impaired barrier function and chronic inflammatory skin conditions.

KACTUS Full Portfolio of KLK Proteins

As proteolytic enzymes, KLKs represent a class of tissue-specific protease targets that are primarily activated in specific tissues or lesions, offering an improved safety window by reducing off-target risks. This characteristic has made KLKs an emerging hotspot for antibody and large-molecule drug development. With continued advances in understanding KLK protease biology, KLKs are rapidly entering an era of targeted therapeutics. KACTUS supplies high-quality, full-series KLK family proteins, including both pro-forms and active forms, all of which have passed rigorous quality control and can be applied to KLK-related drug development.

Product Validation

Enzymatic activity measured by its ability to cleave the fluorogenic peptide substrate: Pro-Phe-Arg-7-amido-4-methylcoumarin (PFR-AMC). The specific activity is >8000 pmol/min/µg.

Purity of KLK1 is greater than 95% as determined by SEC-HPLC.

Enzymatic activity measured by its ability to cleave the fluorogenic peptide substrate: BOC-Gln-Ala-Arg-AMC. The specific activity is >1500 pmol/min/µg.

The purity of Human Kallikrein 6 is greater than 95% as determined by SEC-HPLC.

Enzymatic activity measured by its ability to cleave the fluorogenic peptide substrate: BOC-Val-Pro-Arg-AMC. The specific activity is >500 pmol/min/ug.

The purity of Human Kallikrein 8 is greater than 95% as determined by SEC-HPLC.

 

Product List

References

[1] Recent advances in the discovery and development of drugs targeting the kallikrein-kinin system. doi: 10.1186/s12967-024-05216-5.

[2] Unleashing the therapeutic potential of human kallikrein-related serine proteases. doi: 10.1038/nrd4534.

[3] Kallikreins - the melting pot of activity and function. doi: 10.1016/j.biochi.2015.09.023.

[4] Roles of Kallikrein-Related Peptidase in Epidermal Barrier Function and Related Skin Diseases. Doi: 10.1016/j.jdermsci.2019.06.007

[5] Involvement of Kallikrein-Related Peptidases in Normal and Pathologic Processes. doi: 10.1155/2015/946572. Epub 2015 Dec 9.

[6] Specific inhibition of tissue kallikrein 1 with a human monoclonal antibody reveals a potential role in airway diseases. doi: 10.1042/BJ20090010.

[7] KLK1 as an Epithelial-Specific Brake Inhibits Colorectal Tumorigenesis by Suppressing B1R-Mediated Fibroblast Phenotypic Transition. doi: 10.1002/advs.202507063.

[8] https://www.diamedica.com/pipeline/acute-ischemic-stroke

[9] Involvement of Kallikrein-Related Peptidases in Nervous System Disorders. doi: 10.3389/fncel.2020.00166.

[10] Kallikrein-related peptidase's significance in Alzheimer's disease pathogenesis: A comprehensive survey. doi: 10.1016/j.biochi.2024.04.001.

[11] Neuropsin cleaves EphB2 in the amygdala to control anxiety. doi: 10.1038/nature09938.