Protein and Carbohydrate Classification and Functions

Posted on Nov 2, 2024 in Biology

Proteins and Carbohydrates

Holoproteins

Fibrous Proteins

Coiled along a single dimension. Insoluble in water, with structural or protective functions.

  • Collagen: The main component of connective tissue, and is present as one of the main components of the extracellular matrix of the skin.
  • Myosin: A fibrous protein responsible for muscle contraction.
  • Keratins: A large group of animal proteins that are synthesized and stored in the cells of the epidermis: they form horns, nails, hair, and wool in many animals.

Globular Proteins

Spherical structure. Soluble in water, responsible for the biological activities of the cell.

  • Actin: A globular protein that assembles into filaments. Together with myosin, it is responsible for muscle contraction.
  • Albumins: Proteins with transport functions of other molecules or serve as an amino acid reserve. Representatives of this group include ovalbumin from egg white, lactalbumin from milk, and blood serum albumin.
  • Histones and Protamines: Basic proteins associated with DNA.

Heteroproteins

Chromoproteins

Proteins whose prosthetic group is a colored substance called a pigment. They are classified as:

  • Porphyrin Chromoproteins: This group includes biologically important proteins such as hemoglobin and myoglobin, which have a prosthetic group called heme, characterized by carrying a ferrous cation (Fe2+).
  • Non-Porphyrin Chromoproteins: One of the most important is hemocyanin, a blue respiratory pigment that carries copper.

Nucleoproteins

Proteins whose prosthetic group is a nucleic acid, such as histones.

Glycoproteins

Their prosthetic group is a carbohydrate covalently bound to the polypeptide chain, such as immunoglobulins.

Phosphoproteins

Proteins whose prosthetic group is phosphoric acid, such as casein in milk.

Lipoproteins

The prosthetic group is a lipid. There is a particular group of lipoproteins present in blood plasma, which are responsible for transporting insoluble lipids (cholesterol, triglycerides, etc.) between the intestine, liver, adipose tissue, and other locations. The major lipoproteins are:

  • LDL (Low-Density Lipoprotein): Carry cholesterol and phospholipids from the liver to the tissues to form cell membranes.
  • HDL (High-Density Lipoprotein): Their action is contrary to LDL; they carry cholesterol to the liver to be removed from the artery walls, thus decreasing deposits.

Protein Functions

  • Storage: Some proteins store certain chemical compounds, like amino acids, for use as nutrients or to assist in embryo formation.
  • Transport: Proteins that bind to various substances and transport them through an aqueous medium, like oxygen or lipids, or through cell membranes, such as plasma lipoproteins that transport lipids.
  • Contractile: Actin and myosin conduct muscle contraction and, therefore, enable cell movement, including the movement of bacteria and sperm.
  • Protective: Involved in blood clotting by preventing its exit from the vascular system.
  • Signal Transduction: The process by which a cell produces a response to an extracellular signal.
  • Hormonal: Important regulatory proteins that control cell functions, like metabolism or reproduction, such as insulin.
  • Structural: Provide mechanical support to plant and animal cells.
  • Enzymatic: Biochemical catalysts.
  • Homeostatic: Maintain the pH balance of the internal environment.
  • Recognition of Chemical Signals: Involved in the recognition of chemical signals.

Carbohydrates

Isomerism

Isomerism is a feature of many compounds that, while being different, have the same molecular formula. Monosaccharides often exhibit this feature.

Functional Group Isomerism

Present in compounds, such as aldose and ketose, that have identical molecular formulas but differ by having different functional groups.

Stereoisomerism

Present in molecules that look similar but have different properties because their atoms have different spatial arrangements. It is due to the presence of asymmetric carbons.

  • Enantiomers: The position of all the -OH groups of the asymmetric carbons varies. D-form: -OH on the right. L-form: -OH on the left.
  • Diastereomers or Diastereoisomers: Epimers when they differ in the position of the -OH group of a single asymmetric carbon.

Optical Activity

The presence of asymmetric carbons determines an important property of monosaccharides in solution: optical activity. This is the ability of these molecules to divert the plane of polarization of a beam of polarized light passing through the solution.

  • When the rotation is clockwise, monosaccharides are called dextrorotatory (right-handed).
  • When the rotation is counterclockwise, they are called levorotatory (left-handed).