Albumin is the most abundant protein found in blood plasma, making up about half of all proteins in this fluid. It functions as the circulatory system’s primary transport vehicle, carrying numerous substances throughout the body. This protein is synthesized in the liver and plays a part in maintaining the fluid balance within blood vessels.
The Amino Acid Blueprint
The foundation of albumin’s structure is its primary sequence of amino acids. Human serum albumin is built from a single, continuous chain containing 585 of these molecular building blocks. This specific linear arrangement is dictated by the genetic code and serves as the instruction manual for the protein’s final three-dimensional shape. The sequence itself is highly conserved, meaning it varies little between individuals.
This precise order of amino acids determines the chemical properties along the chain, influencing how different sections will interact with one another. Even a small change in this sequence can have considerable effects on the protein’s stability and function. The entire identity and capability of the albumin molecule are encoded within this initial sequence.
Folding into Helices and Domains
Once synthesized, the long amino acid chain of albumin begins to fold into a specific and stable three-dimensional form. A significant portion of the chain, approximately 67%, twists into structures known as alpha-helices. These helical segments then fold upon themselves, creating a compact, roughly heart-shaped globular protein.
This complex structure is further organized into three main sections called domains, labeled I, II, and III. Each of these domains is itself composed of two smaller subdomains. This modular arrangement provides structural flexibility and creates distinct regions on the protein’s surface.
To secure this structure, the protein relies on 17 disulfide bonds. These strong covalent bonds act like molecular staples, linking different parts of the amino acid chain together and locking the helices and domains into their correct positions. This network of disulfide bonds confers stability to the albumin molecule, allowing it to withstand various conditions within the bloodstream while maintaining its shape.
Specialized Binding Pockets
The folding of albumin’s domains creates specifically shaped clefts and cavities on its surface. The unique chemical environment within these pockets, determined by the specific amino acids lining them, allows them to bind to a wide variety of molecules for transport through the bloodstream.
Two of the most well-characterized of these are known as Sudlow Site I, located in subdomain IIA, and Sudlow Site II, found in subdomain IIIA. For instance, these pockets are responsible for binding and transporting fatty acids, which are insoluble in water, and steroid hormones like thyroxine.
Beyond these, albumin also transports bilirubin, a waste product from the breakdown of red blood cells, and various metal ions. Its transport services extend to pharmaceutical drugs, whose distribution and effectiveness in the body are often dependent on their ability to bind to albumin.
Structural Changes and Clinical Relevance
The structure of albumin is not static and can be altered by various physiological conditions, which in turn affects its function. One common modification is glycation, a process where sugar molecules non-enzymatically attach to the protein. In conditions like poorly controlled diabetes, elevated blood sugar levels accelerate this process, impairing albumin’s ability to bind and transport molecules effectively.
Oxidative stress, a state of molecular damage caused by reactive oxygen species, can also modify the protein’s structure. These changes can impact the integrity of its binding sites and overall stability. Because albumin is synthesized in the liver, its concentration in the blood is a direct indicator of liver function.
Measuring albumin levels in the blood is a common diagnostic tool. Low levels, a condition known as hypoalbuminemia, can signal liver disease, as a damaged liver cannot produce enough of the protein. It can also indicate kidney disease, where damaged kidneys may allow an excessive amount of albumin to be lost from the blood into the urine.