Introduction

Hepatitis B virus (HBV) is a small, enveloped, hepatotropic DNA virus belonging to the family Hepadnaviridae and genus Orthohepadnavirus.

Despite having a genome of only approximately 3.2 kb, HBV has a highly organized structure and an exceptionally compact genetic architecture. Its structure is closely linked to its unique replication strategy, hepatocyte tropism, persistence, immune escape, and clinical manifestations.

The infectious HBV virion is called the:

The complete virion is approximately 42 nm in diameter and consists of:

1. Lipid envelope

2. Hepatitis B surface proteins

3. Nucleocapsid/core

4. Viral genome

5. Viral polymerase

HBV also produces enormous numbers of non-infectious subviral particles, consisting predominantly of HBsAg-containing spherical and filamentous structures.

1. Classification

2. The Infectious HBV Virion

The complete infectious HBV particle is called the Dane particle, named after David Dane and colleagues, who described the complete viral particle in 1970.

It is approximately 42 nm in diameter.

Structurally, it can be divided into:

Outer layer

Lipid envelope containing HBV surface proteins

Inner layer

Icosahedral nucleocapsid/core

Inside the core

Partially double-stranded viral DNA + viral polymerase

A simplified organization is:

Lipid envelope

↓

HBs proteins

↓

Core/nucleocapsid

↓

HBV DNA + polymerase

3. HBV Virion — Structural Diagram

The surface proteins are embedded in the lipid envelope, whereas the core protein forms the nucleocapsid surrounding the viral genome.

4. HBV Envelope

HBV is an enveloped virus.

The envelope is derived largely from host cellular membranes and contains viral surface proteins.

The major surface proteins are:

1. Large surface protein — L-HBs

2. Middle surface protein — M-HBs

3. Small surface protein — S-HBs

All three share the S domain.

5. Surface Gene Organization

The HBV surface region contains:

• pre-S1

• pre-S2

• S

Different start sites produce different surface proteins.

L-HBs

Contains:

pre-S1 + pre-S2 + S

M-HBs

Contains:

pre-S2 + S

S-HBs

Contains:

S

Therefore:

6. Large Surface Protein — L-HBs

The large surface protein is composed of:

pre-S1 + pre-S2 + S

It plays an important role in:

• Hepatocyte attachment

• Viral entry

• Receptor recognition

• Virion assembly

• Hepatocyte tropism

The pre-S1 domain contains the critical region involved in interaction with the hepatocyte receptor NTCP.

A lipid modification, particularly N-terminal myristoylation, of the pre-S1 region is important for efficient receptor interaction and infectivity.

7. NTCP — The Major HBV Entry Receptor

The sodium taurocholate cotransporting polypeptide: NTCP

is encoded by the SLC10A1 gene and is expressed on hepatocytes.

The myristoylated pre-S1 region of L-HBs interacts with NTCP.

This interaction is a major determinant of HBV hepatotropism.

It also provides an important explanation for why HBV preferentially targets hepatocytes.

8. Middle Surface Protein — M-HBs

M-HBs contains:

pre-S2 + S

It contributes to the viral envelope and virion assembly.

Compared with L-HBs, it lacks the pre-S1 domain.

Its biological role is less directly associated with receptor-mediated entry than that of L-HBs.

9. Small Surface Protein — S-HBs

S-HBs is the major and most abundant envelope protein.

It consists primarily of the:

S domain

It is an important structural component of:

• Infectious virions

• Spherical subviral particles

• Filamentous subviral particles

It is also the principal antigen detected as:

HBsAg

in routine diagnostic testing.

10. HBsAg

HBsAg means:

It represents HBV surface proteins circulating in the blood.

Importantly, most circulating HBsAg is not present in complete infectious Dane particles.

Instead, huge quantities of HBsAg are released as non-infectious subviral particles.

This explains why the concentration of HBsAg in blood can be extremely high compared with the concentration of complete infectious virions.

11. Subviral Particles

HBV produces enormous numbers of particles that contain HBsAg but lack:

• Nucleocapsid

• Complete viral genome

• Viral polymerase

Therefore, they are:

There are two major morphological forms:

Spherical particles

Approximately 20–22 nm

Filamentous particles

Approximately 20 nm in diameter, but much longer and variable in length.

12. Why Does HBV Produce So Many Subviral Particles?

This massive excess of surface antigen is thought to have important biological consequences.

One proposed mechanism is:

Immune decoy effect

Large quantities of HBsAg-containing particles may interact with circulating anti-HBs antibodies and contribute to immune evasion.

They also make HBsAg an extremely useful diagnostic marker.

13. HBV Nucleocapsid

Inside the envelope is the:

Nucleocapsid

The nucleocapsid is formed primarily by:

HBcAg

or:

Hepatitis B core antigen

Core protein assembles into an approximately icosahedral capsid surrounding the viral genome and polymerase.

14. HBcAg

HBcAg is the structural core protein.

Important functions include:

• Nucleocapsid formation

• Genome packaging

• Interaction with viral polymerase

• Organization of intracellular replication intermediates

• Interaction with host immune responses

HBcAg is highly immunogenic.

However:

Instead, antibodies against it are detected.

15. Anti-HBc

Because HBcAg itself is not routinely measured in serum, its immunological significance is assessed through:

Anti-HBc

There are two major forms:

• IgM anti-HBc

• IgG/total anti-HBc

IgM anti-HBc

Suggests recent infection and is particularly important in acute HBV infection.

IgG/total anti-HBc

Persists after natural infection and indicates previous or ongoing exposure.

A person vaccinated against HBV does not develop anti-HBc solely because of vaccination.

16. Core Protein Structure

HBcAg contains functional regions that allow it to:

• Assemble into capsids

• Bind nucleic acids

• Interact with viral polymerase

• Participate in genome packaging

• Influence intracellular trafficking

The core protein has:

Assembly domain

Responsible for capsid formation.

Arginine-rich C-terminal domain

Important for:

• Nucleic acid interaction

• Genome packaging

• Phosphorylation-dependent regulation

• Intracellular trafficking

17. HBV Capsid

The HBV core protein assembles into an approximately:

Icosahedral nucleocapsid

The capsid provides a protected environment for the viral genome and polymerase.

It is not simply a passive shell.

It is an active component of the HBV replication machinery.

18. HBV Genome Inside the Core

The mature Dane particle contains:

Relaxed circular DNA — rcDNA

The genome is:

• Circular

• Partially double-stranded

• Approximately 3.2 kb

The two DNA strands are not equivalent.

The:

Minus strand

is essentially complete.

The:

Plus strand

is incomplete and variable in length.

This incomplete plus strand gives HBV its characteristic partially double-stranded genome.

19. Viral Polymerase

The HBV polymerase is packaged within the nucleocapsid.

It is a multifunctional protein with several activities/domains, including:

1. Terminal protein domain

2. Spacer region

3. Reverse transcriptase/DNA polymerase domain

4. RNase H domain

This single viral protein performs several critical steps in HBV genome replication.

20. Terminal Protein Domain

The terminal protein domain plays an unusual role in HBV DNA synthesis.

The polymerase becomes covalently linked to the nascent viral DNA during initiation of reverse transcription.

The terminal protein functions as a protein primer for DNA synthesis.

This is an unusual feature among viral DNA replication systems.

21. Reverse Transcriptase Domain

The polymerase contains a reverse transcriptase/DNA polymerase domain.

Its central function is:

RNA → DNA

The viral polymerase uses the pgRNA template to synthesize the viral DNA genome.

This makes HBV:

This property is therapeutically important because nucleos(t)ide analogues target HBV polymerase activity.

22. RNase H Domain

The RNase H domain degrades the RNA component of the RNA-DNA hybrid formed during reverse transcription.

Thus:

pgRNA

↓

DNA synthesis

↓

RNA degradation

↓

DNA genome maturation

The incomplete nature of the plus strand is related to the mechanism and timing of genome synthesis.

23. HBx Protein

HBx is encoded by the:

X gene

HBx is not a major structural component of the mature Dane particle.

It is primarily considered a regulatory protein.

HBx interacts with multiple host cellular pathways.

Functions attributed to HBx include effects on:

• Transcription

• Signal transduction

• Cell-cycle regulation

• DNA damage responses

• Mitochondrial function

• Epigenetic regulation

HBx is also implicated in HBV persistence and hepatocarcinogenesis.

24. Precore Protein and HBeAg

HBeAg is produced from the precore/core region.

It is not simply another structural component of the mature nucleocapsid.

HBeAg is a:

Secreted viral protein

and differs structurally and biologically from HBcAg.

This distinction is essential:

25. HBeAg and Infectivity

Traditionally:

HBeAg positivity → active viral replication → high infectivity

This remains useful clinically.

However, it is not absolute.

Some HBV variants have mutations affecting the precore or basal core promoter regions.

These can result in:

HBeAg-negative chronic hepatitis B

despite:

ongoing HBV replication

Therefore:

26. HBV Genome — Structure–Protein Relationship

The genome is highly compact because these regions overlap extensively.

27. HBV Structure and Antigen Distribution

A useful way to remember HBV is:

Outside

HBsAg

Inside

HBcAg

Secreted

HBeAg

Genome

HBV DNA

Replication enzyme

HBV polymerase

28. Electron Microscopy

Electron microscopy classically demonstrates three major HBV-associated particle forms:

1. Dane particles

Approximately:

42 nm

Complete infectious virions.

2. Spherical particles

Approximately:

20–22 nm

Non-infectious HBsAg particles.

3. Filamentous particles

Approximately:

20 nm diameter

Variable length.

The presence of large quantities of spherical and filamentous HBsAg particles is a classic morphological characteristic of HBV infection.

29. Dane Particle vs Subviral Particle

30. HBV Structure and Diagnosis

Structural biology directly explains several diagnostic findings.

HBsAg

Derived from viral surface proteins.

Anti-HBs

Antibody directed against surface antigen.

Anti-HBc

Antibody directed against core antigen.

HBeAg

Secreted precore-associated antigen.

HBV DNA

Measures the viral genome and therefore provides a direct measure of viral replication.

This relationship makes HBV serology much easier to understand when the virion is visualized as a structure rather than memorized as isolated markers.

31. HBV Structure and Vaccination

The HBV vaccine uses:

Recombinant HBsAg

rather than complete infectious HBV.

The vaccine therefore exposes the immune system to the major protective surface antigen without providing:

• Complete virions

• HBV polymerase

• HBV DNA capable of productive infection

Vaccination generates:

Anti-HBs

which can neutralize HBV by targeting the viral surface proteins required for hepatocyte entry.

32. Structure and Viral Entry

The basic sequence is:

HBV envelope

↓

L-HBs pre-S1 domain

↓

NTCP on hepatocyte

↓

Entry

↓

Nucleocapsid trafficking

↓

Nuclear delivery of rcDNA

↓

cccDNA formation

This connection between structure and replication is fundamental to understanding HBV biology.

33. Structure and Antiviral Therapy

The HBV polymerase is one of the most important therapeutic targets.

Nucleos(t)ide analogues inhibit viral DNA synthesis by targeting the reverse-transcription machinery.

Examples include:

• Tenofovir disoproxil fumarate

• Tenofovir alafenamide

• Entecavir

Therefore:

34. Structure and Drug Resistance

Mutations in the polymerase gene can alter susceptibility to nucleos(t)ide analogues.

Because the HBV genome is highly overlapping, polymerase mutations can sometimes simultaneously alter the corresponding overlapping surface gene.

This creates a fascinating example of how:

Genome organization → protein structure → drug resistance → antigenic phenotype

are interconnected.

35. Why HBV Has Such a Small Genome

HBV has a genome of only about:

3.2 kb

Yet it encodes multiple proteins.

This is possible because the genome contains:

Extensive overlapping open reading frames

The four major ORFs are:

S + C + P + X

The P gene overlaps substantially with other regions.

This highly economical genome organization allows HBV to encode a sophisticated replication system using a very small amount of genetic material.

36. Structural Organization — From Outside to Inside

A useful examination-oriented sequence is:

Lipid envelope

↓

L-HBs / M-HBs / S-HBs

↓

Nucleocapsid

↓

HBcAg

↓

HBV polymerase

↓

Relaxed circular partially dsDNA

Remember:

37. High-Yield Structure Facts

Fact 1

Complete infectious HBV particle:

Dane particle

Fact 2

Dane particle:

~42 nm

Fact 3

HBV subviral particles:

~20–22 nm spheres and filaments

Fact 4

HBV is:

Enveloped

Fact 5

Envelope proteins:

L, M and S

Fact 6

L protein contains:

pre-S1 + pre-S2 + S

Fact 7

M protein contains:

pre-S2 + S

Fact 8

S protein contains:

S domain

Fact 9

Important HBV entry receptor:

NTCP

Fact 10

Core protein:

HBcAg

Fact 11

HBeAg is:

Secreted, precore-derived protein

Fact 12

Viral genome:

~3.2-kb partially ds circular DNA

Fact 13

Polymerase contains:

Terminal protein + reverse transcriptase/DNA polymerase + RNase H functions

Fact 14

HBx:

Regulatory protein rather than a major structural virion component

Fact 15

HBV replication:

Reverse transcription

Fact 16

HBV vaccine antigen:

Recombinant HBsAg

38. One-Table Master Summary

39. Clinical Correlation

HBV structure explains much of its clinical microbiology:

HBsAg

Because the surface protein is abundantly produced, HBsAg becomes the major screening marker.

Anti-HBs

Protective antibody directed against surface antigen.

Anti-HBc

Evidence of natural infection because core antigen is not included in vaccination.

HBeAg

Marker associated with active replication, although HBV DNA is more direct.

HBV DNA

Direct molecular evidence of viral replication.

Polymerase

Major target of nucleos(t)ide antiviral therapy.

cccDNA

Major reason HBV is difficult to eradicate completely.

Surface mutations

Can influence immune recognition and diagnostic detection.

Precore mutations

Can produce HBeAg-negative replicating HBV.

40. Final Concept

HBV should not be remembered simply as a "42-nm DNA virus."

Its structure is a tightly integrated system:

Surface proteins→ hepatocyte attachment and entry

Core protein→ capsid formation and genome packaging

Polymerase→ reverse transcription

HBV DNA→ genetic information

cccDNA→ long-term nuclear persistence

HBx→ regulation and contribution to pathogenesis

HBeAg→ secreted viral antigen associated with replication phenotype

Together, these components explain the characteristic biology of HBV:

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