Proteins are essential components of cell membranes, classified into six distinct types, each serving various functions. These membrane proteins play critical roles as receptors in signaling pathways, channels, gates, and pumps for transporting molecules across the membrane, as well as enzymes that catalyze lipid biosynthesis and ATP synthesis.
The structure of membrane proteins is influenced by the polarity of their amino acids. Exterior amino acids are typically polar and may have carbohydrates attached, allowing them to interact with the aqueous environment. In contrast, interior amino acids are generally non-polar, which helps them remain embedded within the hydrophobic core of the membrane. This polarity trend extends to the individual alpha helices that make up these proteins. For instance, in a protein composed of six alpha helices, the exterior portions interacting with the membrane are predominantly non-polar, while the interior portions consist of polar amino acids. This alternating pattern of polar and non-polar amino acids is crucial for the protein's functionality.
Membrane phospholipids also interact with both the interior and exterior surfaces of proteins. Embedded proteins have regions associated with the hydrophobic interior of the membrane, while some phospholipids can penetrate into the protein's interior. To determine which parts of a protein are hydrophobic or hydrophilic, scientists use a tool called the hydropathy index. This index assigns a hydropathy rating to amino acids, with positive values indicating hydrophobic characteristics and negative values indicating hydrophilic properties. Generally, regions with positive values are likely to be found in the internal environment of the membrane, while those with negative values are more likely to be exposed to the aqueous surroundings.
It is noteworthy that certain amino acids, such as tryptophan and tyrosine, frequently appear together at the membrane's edge, particularly in transmembrane proteins. These amino acids are often located at the interface between the hydrophobic core and the aqueous environment. Additionally, peripheral proteins are typically attached to fatty acids embedded in the membrane, highlighting the diverse interactions that proteins have with membrane components.
