Brenda Khan
Dietary protein is essential for human health, aiding in cell repair and the creation of new cells, as well as being important for the growth and development of children, teens, and pregnant women. The human body breaks down dietary protein into amino acids, which are the building blocks of proteins and are essential to good health. These amino acids are transported by carrier proteins and channel proteins, which facilitate the movement of molecules across cell membranes. Lipoproteins, a combination of lipids and proteins, are also involved in the transport of cholesterol and other lipids. The daily recommended intake of protein for healthy adults is 10% to 35% of their total calorie needs, and it can be obtained from a variety of sources, including animal and plant-based foods.
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Transport proteins
Specific examples of transport proteins include the Na+-K+ pump (or Na+-K+ ATPase), which maintains the concentration gradients of Na+ and K+ ions across the plasma membrane, and the Ca2+ pump, which exports Ca2+ ions from cells. Additionally, the ABCG5 and ABCG8 genes encode the sterolin-1 and sterolin-2 proteins, respectively, which function to limit the absorption of dietary plant cholesterols and promote the excretion of neutral sterols.
The SLC16A1 gene encodes a proton-linked monocarboxylate transporter 1 (MCT1), which transports monocarboxylates such as lactate, pyruvate, and ketone bodies. Defects in MCT1 function due to mutations in the SLC16A1 gene can lead to transport defects and health issues. Similarly, the SLC4A11 gene encodes a voltage-regulated, electrogenic sodium-coupled borate cotransporter, and mutations in this gene have been associated with various disorders.
Lipoproteins, which are composed of lipids and proteins, also play a crucial role in the transport of lipids and cholesterol. The main lipoproteins in the blood include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Chylomicrons are synthesized in the small intestine from dietary fat, while VLDL, LDL, and HDL are synthesized in the liver and small intestine. VLDL and LDL are involved in the transport of cholesterol and triacylglycerol (TAG), while HDL is responsible for the "reverse transport" or removal of cholesterol from tissues.
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ABC transporters
The characteristic architecture of ABC transporters consists of four domains: two ABC or nucleotide-binding domains (NBFs) and two transmembrane domains (TMDs). The NBFs, located in the cytoplasm, are responsible for binding ATP and transferring the energy required for substrate translocation across the membrane. The conserved sequence motifs of Walker A and B, separated by approximately 90-120 amino acids, are essential for this process. The signature (C) motif is another critical element found just upstream of the Walker B site.
The TMDs, on the other hand, provide substrate specificity and create the translocation pathway. They are more variable in structure, with three distinct sets of folds recognized thus far. These folds interact with the helical domains of the ABCs through coupling helices, which are located in the loops between membrane domains.
ABC transporter genes are essential for many cellular processes, and mutations in these genes have been linked to various human diseases, including cystic fibrosis, adrenoleukodystrophy, and drug-resistant tumors.
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Lipoproteins
The handling of lipoprotein particles in the body is referred to as lipoprotein particle metabolism, which is divided into two pathways: exogenous and endogenous. The exogenous pathway begins with the incorporation of dietary lipids into chylomicrons in the intestine. Chylomicrons are very large particles that transport triglycerides. In the circulation, the triglycerides carried in chylomicrons are metabolised in muscle and adipose tissue by lipoprotein lipase, releasing free fatty acids and forming chylomicron remnants, which are then taken up by the liver.
The endogenous pathway begins in the liver with the formation of very low-density lipoproteins (VLDL), which are another type of "bad cholesterol". VLDLs carry triglycerides and, to a lesser degree, cholesterol, to the tissues. When VLDLs give up their fatty acids, they become intermediate-density lipoproteins (IDL), which are either removed by the liver or converted into low-density lipoprotein (LDL), also known as "bad cholesterol". LDL carries cholesterol that accumulates as plaque inside blood vessels, increasing the risk of coronary artery disease, heart attacks, and strokes.
On the other hand, high-density lipoprotein (HDL) is the "good cholesterol" that carries cholesterol back to the liver to be flushed out of the body. High levels of HDL reduce the risk of cardiovascular disease. Apolipoproteins, which are associated with HDL particles, have been found to neutralise viruses and exhibit lytic activity against certain parasites.
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Active transport
The process of active transport is particularly important in the intestinal epithelium, where it facilitates the uptake of dietary nutrients, including sugars and amino acids. The epithelial cells lining the intestine utilise active-transport systems in the apical domains of their plasma membranes to absorb these essential nutrients from the lumen of the intestine. This process is driven by the Na+ gradient established by the Na+-K+ pump.
Carrier proteins and channel proteins are essential components of active transport, mediating the selective passage of molecules across the plasma membrane. These transport proteins allow the cell to regulate its internal composition by controlling the movement of molecules into and out of the cell. The specific design of each carrier protein enables it to recognise and transport only one substance or a group of very similar substances.
An example of active transport is the Na+-Ca2+ antiporter, which simultaneously transports Na+ into the cell and Ca2+ out of the cell. Another example is the Na+-H+ exchange protein, which plays a role in regulating intracellular pH. These antiporters contribute to the overall process of active transport, enabling the movement of molecules against their concentration gradients with the help of energy derived from ATP hydrolysis.
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Passive diffusion
The process of passive diffusion can be observed in the diffusion of gases during respiration. When an individual inhales, oxygen diffuses across the membrane of the alveoli and enters the circulatory system. Simultaneously, carbon dioxide moves in the opposite direction, diffusing across the membrane of the pulmonary capillaries and exiting the body during exhalation. This exchange of gases is a biological example of passive diffusion.
At the cellular level, passive diffusion is the simplest mechanism by which molecules can cross the plasma membrane. During this process, a molecule dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution on the opposite side of the membrane. The direction of transport is determined by the relative concentrations of the molecule inside and outside the cell, with no involvement of membrane proteins.
However, it is important to note that not all molecules can undergo passive diffusion. Only small, relatively hydrophobic molecules can diffuse across a phospholipid bilayer at significant rates. These include gases (O2 and CO2), hydrophobic molecules (such as benzene), and small polar but uncharged molecules (such as H2O and ethanol). Larger uncharged polar molecules, such as glucose, and charged molecules of any size are unable to cross the plasma membrane by passive diffusion.
Facilitated diffusion, a similar process to passive diffusion, involves the assistance of transport proteins to enable the passage of molecules that cannot cross the membrane through simple passive diffusion. These transport proteins allow molecules like carbohydrates, amino acids, nucleosides, and ions to cross the plasma membrane. An example of facilitated diffusion is the absorption of glucose into cells through Glucose Transporter 2 (GLUT2) in the human body.
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Frequently asked questions
Transport proteins are membrane proteins that act as carriers for other molecules. They can be further classified into carrier proteins and channel proteins.
Transport proteins carry a wide range of molecules, including sugars, amino acids, ions, and fatty acids.
Transport proteins work by recognising and binding to specific molecules. They can also work in conjunction with other transporters to facilitate the movement of molecules against their concentration gradients.
Transport proteins are found in the plasma membrane of cells, where they help mediate the selective passage of small molecules across the membrane.
Examples of transport proteins include the Na+-K+ pump, the Ca2+ pump, and the Na+-Ca2+ antiporter. Lipoproteins, which are a combination of lipids and proteins, are another example of transport proteins involved in lipid and cholesterol transport.
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