Albumin is the most abundant protein in human blood plasma, serving a foundational role in circulatory physiology. Understanding the physical characteristics of this molecule, particularly its molecular weight (MW), is fundamental to grasping how it performs its functions. Molecular weight represents the mass of a single molecule relative to a defined unit mass. This measurement dictates a protein’s fate within the body, including where it travels, what it binds to, and how long it remains in circulation.
Defining the Molecular Weight of Human Serum Albumin
The precise molecular weight of human serum albumin (HSA) is approximately 66.5 kilodaltons (kDa). This is often expressed more exactly as 66,439 Daltons, representing the collective mass of its constituent atoms. The Dalton (Da) is the standard unit used in molecular biology, roughly equaling the mass of a single hydrogen atom. Since proteins are large molecules, the kilodalton (1,000 Daltons) provides a more practical unit for measurement.
Human serum albumin is a single-chain polypeptide composed of about 585 amino acid residues. Its globular, heart-shaped structure is defined by these amino acids and held together by numerous disulfide bridges. This configuration contributes directly to its measured weight and stability in the bloodstream.
Albumin functions as a carrier protein, transiently binding to molecules like hormones, metal ions, and fatty acids. While binding partners temporarily increase its effective size and mass in circulation, the core structure remains fixed at its characteristic 66.5 kDa weight.
How Albumin’s Size Dictates its Function in the Body
Albumin’s large size allows it to regulate fluid distribution throughout the body. It constitutes a majority of the plasma proteins that generate oncotic pressure (colloid osmotic pressure). This pressure is a pulling force that draws water back into the blood vessels from surrounding tissues.
Capillary walls, which are semi-permeable, permit water and small solutes to pass through, but exclude large molecules like albumin. Since albumin is confined within the bloodstream, it creates a concentration gradient across the capillary wall. This gradient causes water to move back into the vessel, counteracting hydrostatic pressure. This balance prevents excess fluid accumulation in the tissues, known as edema.
Albumin’s sizable structure also makes it an effective transport molecule for various substances. It possesses multiple binding sites, including hydrophobic pockets, allowing it to safely carry molecules that are not soluble in water, such as long-chain fatty acids and bilirubin. The protein’s mass provides a stable platform, ensuring these attached molecules are not prematurely metabolized or diffused away.
By binding to and transporting drugs, hormones, and other compounds, albumin regulates their distribution and availability to target cells. Its large size ensures these substances remain in circulation for a longer period. This transport role is a consequence of its molecular size, which offers stability and a large surface area for ligand attachment.
Molecular Weight and the Kidney Filtration Barrier
The clinical relevance of albumin’s molecular weight is most evident at the kidney’s filtration apparatus, the glomerulus. The glomerular filtration barrier (GFB) is a highly specialized structure composed of three layers: the fenestrated endothelial cells, the glomerular basement membrane, and the podocytes with their slit diaphragms. This barrier filters out waste and small molecules while retaining large plasma proteins like albumin.
The GFB employs both size- and charge-selectivity to prevent albumin from entering the urine. Albumin’s 66.5 kDa size is too large to pass through the barrier’s pores, which function as a molecular sieve. Furthermore, albumin carries a net negative electrical charge, which is repelled by the negative charges on the glomerular basement membrane.
The presence of albumin in the urine, known as albuminuria, indicates that the integrity of the filtration barrier has been compromised. Microalbuminuria refers to an increased, though small, excretion rate of albumin, typically 30–299 milligrams per day. This finding is often an early sign of damage to the kidney’s filtering units, commonly seen in individuals with diabetes or high blood pressure.
If damage progresses, the excretion rate increases to 300 milligrams per day or more, termed macroalbuminuria. The passage of a protein of albumin’s size and charge into the urine signals a severe breakdown of the GFB. Monitoring these albumin levels is a standard method for assessing the progression of kidney disease and the associated risk of cardiovascular complications.