Human Biochemistry Volume 1 - Murray R. 1993

Structure and Functions of Proteins and Enzymes
Proteins: Structure and Properties
Protein Classification

There is no universally satisfactory protein Classification system. Only a few commonly used classification systems exist, which partially contradict one another. From the perspective of key protein properties, all of them are of limited value. However, these systems and their corresponding terminology are used in clinical laboratories, so it is worthwhile to review them briefly. We will discuss the main Features of protein classification systems based on solubility, molecular shape, function, physical properties, and three-dimensional Structure.

Solubility

The Classification of Proteins based on their solubility was introduced in 1907–1908 and is still in use today, especially in clinical biochemistry (Table 5.1). There are no strictly established boundaries between the classes. For example, a clear distinction between albumins and globulins is impossible based solely on their solubility in Water and salt solutions. Therefore, globulins are subdivided into pseudoglobulin, which are readily soluble in water, and euglobulins, which are insoluble in salt-free water.

Class="center">Table 5.1. Classification of proteins based on their solubility

Molecular Shape

Based on the axial ratio (length to width), Proteins can be divided into two broad classes. In Globular proteins, this ratio is less than 10 and in most cases does not exceed 3–4. They are characterized by a compact folding of polypeptide chains. Examples include Insulin, plasma albumins and globulins, and many Enzymes. Fibrous proteins, in which the axial ratio exceeds 10, consist of bundles of polypeptide chains wound spirally around each other and linked together by transverse covalent or Hydrogen Bonds. Examples include keratin, Myosin, Collagen, and fibrin.

Functions

Proteins can be classified according to their biological functions; for instance, they can be subdivided into structural, catalytic, and transport proteins (Table 5.2). In turn, catalytic proteins (enzymes), which comprise the majority of diverse protein types, can be subdivided According to the type of reaction they catalyze (Chap. 7).

Physical Properties

For A number of medically important proteins, specialized classification systems exist that allow differentiation within families of similar proteins. For example, two systems of plasma lipoprotein nomenclature are widely used and a third is currently under Discussion. In the first system, Lipoproteins are classified according to their behavior in an electric or gravitational field; thus, based on electrophoretic mobility at pH 8.6, starting a₁-, a₂-, ß-, and y-lipoproteins are distinguished. The second classification system for lipoproteins is based on their hydrated density; in this case, chylomicrons, VLDL, LDL, HDL, and VHDL are distinguished (Chap. 26). A third type of classification is also possible, based on the Introduction/19.html">Primary Structure of apoproteins. This system distinguishes six classes of plasma lipoproteins, characterized by the presence of apoproteins A, B, C, D, E, and F, respectively. Apoproteins can be differentiated using immunological criteria.

Table 5.2. Main Functions of Proteins

Three-Dimensional Structure

Proteins can also be distinguished based on whether they possess a quaternary structure (see below). In addition, structural similarities among certain proteins, revealed primarily through X-ray crystallography, serve as a valuable basis for protein classification. For instance, nucleotide-binding proteins are typically characterized by the presence of a "nucleotide-binding domain" In their tertiary structure and are likely evolutionarily related proteins.