Albumin is a globular protein characterized by its highly organized and compact three-dimensional structure, which is crucial for its diverse functions within the body. Structurally, it is a single polypeptide chain composed of 585 amino acids with a molecular weight of 66.7 kDa, and it is usually nonglycosylated.
The Primary Structure: A Single Polypeptide Chain
The fundamental building block of albumin is its primary structure: a single polypeptide chain. This chain is precisely arranged from 585 amino acids linked together. This extensive amino acid sequence dictates its subsequent folding into a unique three-dimensional shape. With a molecular weight of 66.7 kilodaltons (kDa), albumin is a relatively large protein. A notable characteristic is that it is typically nonglycosylated, meaning it does not have sugar molecules attached, which differentiates it from many other proteins found in blood plasma.
Domains and Disulfide Bonds: Shaping the Tertiary Structure
Albumin's impressive three-dimensional form, known as its tertiary structure, is a compact and highly stable globular shape. This intricate folding is achieved through a hierarchical arrangement:
- Three Homologous Domains: The polypeptide chain folds into three main structurally similar domains, typically labeled Domain I, Domain II, and Domain III. Each of these domains is further subdivided into two subdomains, often referred to as A and B. This creates a six-subdomain structure (IA, IB, IIA, IIB, IIIA, IIIB) that gives albumin its characteristic heart-shaped or triangular appearance.
- Disulfide Bonds: The stability of this complex structure is significantly reinforced by 17 disulfide bonds. These covalent linkages form between cysteine residues within the polypeptide chain, acting like internal staples that lock the protein into its specific conformation. These bonds are vital for maintaining the protein's structural integrity against various stresses.
As albumin consists of a single polypeptide chain and does not associate with other protein subunits to form a larger complex, it lacks a quaternary structure. It functions as a monomer.
Key Structural Features of Albumin
The following table summarizes the essential structural characteristics of albumin:
| Feature | Description |
|---|---|
| Protein Type | Globular protein |
| Composition | Single polypeptide chain |
| Amino Acids | 585 |
| Molecular Weight | 66.7 kDa |
| Glycosylation | Usually nonglycosylated |
| Domains | Three homologous domains (I, II, III), each with two subdomains (A, B) |
| Disulfide Bonds | 17 stabilizing covalent linkages |
| Overall Shape | Heart-shaped or triangular, compact |
| Quaternary Structure | None (functions as a monomer) |
For a visual representation and detailed structural data, resources like the Protein Data Bank (PDB) offer crystallographic models of albumin.
Functional Implications of Albumin's Structure
The specific structure of albumin directly enables its wide array of physiological functions:
- Binding Pockets: The compact, multi-domain structure creates several hydrophobic and hydrophilic binding pockets. These sites allow albumin to bind and transport diverse molecules, including fatty acids, steroid hormones, bilirubin, and many drugs, facilitating their distribution throughout the body.
- Osmotic Pressure Maintenance: Its relatively large size and high concentration in plasma contribute significantly to the colloid oncotic pressure, preventing fluid from leaking out of blood vessels into tissues.
- pH Buffering: Certain amino acid residues within its structure contribute to its capacity as a plasma buffer, helping to maintain stable blood pH.
Understanding the detailed structure of albumin is crucial for comprehending its roles in health and disease, including how it interacts with drugs and its involvement in various physiological processes. More insights into its function can be found in detailed reviews on human serum albumin.
