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Proteins can be separated into broad groups
Fibrous proteins form long fibres and often have repeated sequences of amino acids. They are insoluble in water. Structural proteins, including collagen and keratin, are usually fibrous. They are found in the tough parts of organisms, such as bones, beaks, claws, skin, hair and feathers. Structural proteins knit together in stable arrays, with lots of cross-links to hold them together.
Globular proteins include haemoglobin, antibodies in mammals and most enzymes. They tend to be soluble in water because their 3D structure has the hydrophobic (water-hating) parts facing the centre and the hydrophilic (water-loving) parts facing the outside.
Conjugated proteins are those that include non-protein parts, such as haemoglobin. In the human form, this protein has four polypeptide chains, each arranged around a (non-protein) haem group. Read more in our ‘Focus Protein: Haemoglobin’ article.