It is amazing what you’ll find by just looking around. While reviewing recent blood test results it occurred to me that I didn’t know the first thing about albumin as a protein. A Google word search led to numerous links but provided many images as well. The crystal structure is below.

It is not uncommon to describe the enzyme-substrate complex as a highly specific lock and key structure. In the earlier literature is was axiomatic that enzymes are described as being highly substrate specific and use a single binding site for a given substrate. This notion is not always correct as the above graphic shows. Albumin is produced in the liver and is sort of a molecular ox cart- it can transport many substrates in the blood.

The job of human albumin is to get various substrates mobilized in the bloodstream and offer them at a desirable location. With the high molecular weight of enzymes, and the consequent low molarity available, it is astonishing that the heat of binding of substrate to enzyme can be measured at all.

One way to determine binding enthalpy and stoichiometry of a substrate to enzyme is ITC- Isothermal Titration Calorimetry. These calorimeters are available from several manufacturers such as TA Instruments and Malvern. ITC is just a type of reaction calorimeter that allows for immediate access to the reaction mixture. It is a microscale RC1 in effect. An enzyme solution can be titrated with substrate allowing for a visual determination of an equivalence point where 1 eq of enzyme active sights just matches the titrant equivalents. From such an experiment both enthalpy and stoichiometry can be measured. The image below is from TA Instruments and nicely shows the graphic output of an ITC experiment.

Above, the background signal from the buffer represents noise in the enthalpy signal.

TA Instruments also offer equipment for so-called nano scale experiments. See below.

The TA ITC specification table. Note the minimum heat in the low volume column: 0.04 to 0.05 microJoules with a 190-microliter sample cell size.

Albumin is endowed with binding sites open to a variety of substrates. It is like a wheelbarrow or an ox cart. It can ‘carry’ numerous substrates across several categories.

The downside of such low specificity is that albumin can bind many drug compounds at the expense of dose delivery to the desired site. Doses of drugs must be adjusted to account for drug lost to blood proteins like albumin.