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4.4.3.1. Holoproteins

Fibrous or filamentous proteins or scleroproteins

They have a filamentous conformation (tertiary structure), are resistant and insoluble in water and appear mainly in animals. They generally perform structural functions. Collagen, keratin, elastin, actin, myosin, and fibroins belong to this group.

Collagen

In connective, cartilaginous, and bone tissues.

Keratin

Rich in cysteine. In epidermal formations, such as hair, nails, wool, horns, hooves, feathers, etc.

Elastin

It has elasticity. In blood vessels, lungs, etc.

Fibroin

In the silk threads.

Globular proteins or spheroproteins

They have globular conformation (tertiary and quaternary structure, and are soluble in water or in polar solutions. Protamineshistonesprolaminesgluteninsalbumins and globulins belong to this group .

Protamines

Very basic

Soluble in water. They are associated with DNA in the sperm of all animals.

Histones

Basics

Soluble in water. They are associated with the DNA of the nucleus (except in sperm) forming chromatin .

Albumins

Soluble in water. They collaborate in the transport of other molecules and act as a reserve of amino acidsSerum albumin from blood plasma, ovalbumin from egg white, lactalbumin from milk, and globin, which is part of hemoglobin, belong to this group .

Globulins

Soluble in saline solutions. Examples are egg ooglobulin, milk lactoglobulin, and blood serum globulins; also α-globulin, which is associated with hemoglobin, and γ-globulins or immunoglobulins, which constitute antibodies.