Proteins constitute 50% or more of the dry weight of a cell. They include:
- Enzymes - There are a large number of enzymes involved in milk synthesis.
For example, lactose synthase is responsible for lactose synthesis, acetyl-CoA
carboxylase and fatty acid synthetase are two type of enzymes involved in
milk aft synthesis, and several proteases are involved in milk protein processing
and interactions of the mammary cels with their extraccellular environment.
- Structural proteins - There are many structural proteins involved in mammary
gland structure and function. For example, collagens, laminin and other connective
tissue proteins form the basis of the extracellular matrix of the tissue (the
noncellular components of the stroma and the basement membranes of the alveoli),
and cytoskeletal proteins are involved in cell structure, cell movement and
cell function (tubulin, actin, cytokeratin are some discussed in the Histology
and Cell Biology section of the Mammary Structure Lesson).
- Nutrients - Casein is a good example of a protein which supplies nutrients (amino acids).
- Hormones - Prolactin, growth hormone, placental lactogen, insulin, and a number of growth factors are examples of hormones and growth factors which affect mammary gland function. Oxytocin is a peptide hormone which causes myoepithelial cell contractions during milk ejection.
- Protective agents - Immunoglobulins, lactoferrin (an iron-binding protein),
lysozyme (an enzyme), and lactoperoxidase (another enzyme) are a few examples
of factors which can protect the mammary gland from infection.
- Toxins - Some bacterial toxins are peptides or derivatives of peptides. These may cause inflammation in the mammary tissue.
- Others - Transport of substrates into the mammary cell occurs by protein molecules associated with the cell plasma membrane; transcription factors in the nucleus and translation factors in the cytoplasm are proteins; hormone receptors and growth factor receptors are proteins; and there are many more.