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Glossary | Linus Pauling Institute | Oregon State University
For the most part, eukaryotes do not secrete proteins directly across the plasma membrane. Instead, the newly synthesized proteins are translocated from the cytoplasm into an intracellular organelle, the endoplasmic reticulum (ER). The ER is a series of membranous cisternae and tubules that spread throughout the cytoplasm and is continuous with nuclear membrane. As we shall see, the mechanisms for translocating proteins across ER membranes and bacterial membranes both involve a transmembrane pore. Why have an ER? A common speculation on the evolutionary advantage of the ER is that it provides a controlled milieu in which exported proteins can fold and oligomerize, without being exposed to the rigors of the extracellular world. In this regard, the lumen of the ER resembles the periplasmic space in gram-negative bacteria.
Bacteria are extremely simple from a cell biological point of view. Gram-positive bacteria have only a single membrane, the plasma membrane, separating their cytoplasm from the outside world. Gram-negative bacteria have a second outer membrane, separated from the inner membrane by periplasmic space. In this case, the newly synthesized protein that is to be exported must cross the outer membrane in addition to the inner plasma membrane.
Human Physiology - Cell structure and function - EKU
protein, any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight. Hundreds of protein molecules have been isolated in pure, homogeneous form; many have been crystallized. All contain carbon, hydrogen, and oxygen, and nearly all contain sulfur as well. Some proteins also incorporate phosphorous, iron, zinc, and copper. Proteins are large molecules with high molecular weights (from about 10,000 for small ones [of 50–100 amino acids] to more than 1,000,000 for certain forms); they are composed of varying amounts of the same 20 , which in the intact protein are united through covalent chemical linkages called bonds. The amino acids, linked together, form linear unbranched polymeric structures called polypeptide chains; such chains may contain hundreds of amino-acid residues; these are arranged in specific order for a given species of protein.
Types of Proteins
A protein molecule that consists of but a single polypeptide chain is said to be monomeric; proteins made up of more than one polypeptide chain, as many of the large ones are, are called oligomeric. Based upon chemical composition, proteins are divided into two major classes: simple proteins, which are composed of only amino acids, and conjugated proteins, which are composed of amino acids and additional organic and inorganic groupings, certain of which are called . Conjugated proteins include , which contain carbohydrates; , which contain lipids; and nucleoproteins, which contain .
Classified by biological function, proteins include the , which are responsible for catalyzing the thousands of chemical reactions of the living cell; , elastin, and , which are important types of structural, or support, proteins; and other gas transport proteins; ovalbumin, , and other nutrient molecules; , which are molecules of the immune system (see ); protein , which regulate ; and proteins that perform mechanical work, such as and , the contractile muscle proteins.
Every protein molecule has a characteristic three-dimensional shape, or conformation. Fibrous proteins, such as collagen and keratin, consist of polypeptide chains arranged in roughly parallel fashion along a single linear axis, thus forming tough, usually water-insoluble, fibers or sheets. Globular proteins, e.g., many of the known enzymes, show a tightly folded structural geometry approximating the shape of an ellipsoid or sphere.
Because the physiological activity of most proteins is closely linked to their three-dimensional architecture, specific terms are used to refer to different aspects of protein structure. The term primary structure denotes the precise linear sequence of amino acids that constitutes the polypeptide chain of the protein molecule. Automated techniques for amino-acid sequencing have made possible the determination of the primary structure of hundreds of proteins.
The physical interaction of sequential amino-acid subunits results in a so-called secondary structure, which often can either be a twisting of the polypeptide chain approximating a linear helix (α-configuration), or a zigzag pattern (β-configuration). Most globular proteins also undergo extensive folding of the chain into a complex three-dimensional geometry designated as tertiary structure. Many globular protein molecules are easily crystallized and have been examined by X-ray diffraction, a technique that allows the visualization of the precise three-dimensional positioning of atoms in relation to each other in a crystal.
The tertiary structure of several protein molecules has been determined from X-ray diffraction analysis. Two or more polypeptide chains that behave in many ways as a single structural and functional entity are said to exhibit quaternary structure. The separate chains are not linked through covalent chemical bonds but by weak forces of association.
The precise three-dimensional structure of a protein molecule is referred to as its native state and appears, in almost all cases, to be required for proper biological function (especially for the enzymes). If the tertiary or quaternary structure of a protein is altered, e.g., by such physical factors as extremes of temperature, changes in H, or variations in salt concentration, the protein is said to be denatured; it usually exhibits reduction or loss of biological activity.
The cell's ability to synthesize protein is, in essence, the expression of its genetic makeup. Protein synthesis is a sequence of chemical reactions that occur in four distinct stages, i.e., activation of the amino acids that ultimately will be joined together by peptide bonds; initiation of the polypeptide chain at a cell organelle known as the ribosome; elongation of the polypeptide by stepwise addition of single amino acids to the chain; and termination of amino-acid additions and release of the completed protein from the ribosome. The information for the synthesis of specific amino-acid sequences is carried by a nucleic acid molecule called messenger RNA (see ). Proteins are needed in the diet mainly for their amino acids, which the body uses to build new proteins (see ).
The mechanism of action of many widely used antibiotics, such as , , and , can be understood in terms of their ability to interfere with some stage of protein synthesis in bacteria.
The order of the amino acids in a protein dictates the primary structure of the protein. While other levels of structure are important, they all follow from the order of the residues. The primary structure is dictated by genetic information found in a cell; deoxyribonucleic acid (DNA ) contains the code that directs which amino acids are linked together. The processes by which the genetic code is read and proteins are synthesized are called transcription and translation.
BIOMEDICAL AND MEDICAL SCIENCES
Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.
Most of the proteins made by a cell are retained intracellularly; only a specialized subset is secreted outside the cell. Proteins secreted by cells have several major functions, including signaling to other cells, formation of an insoluble extracellular matrix surrounding the cell, and degradation of extracellular material. Molecular biology has helped explain how newly synthesized proteins are selected for secretion and transportation across the hydrophobic barrier of the plasma membrane (see Protein biosynthesis).
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Extra Cellular Matrix Is Critical to Neuroplasticity
The presence of blood protein molecules, such as and are critical factors in maintaining the proper fluid balance between cells and extracellular space. Proteins are present in the capillary beds, which are one-cell-thick vessels that connect the arterial and venous beds, and they cannot flow outside the capillary beds into the tissue because of their large size. Blood fluid is pulled into the capillary beds from the tissue through the mechanics of oncotic pressure, in which the pressure exerted by the protein molecules counteracts the blood pressure . Therefore, blood proteins are essential in maintaining and regulating fluid balance between the blood and tissue. The lack of blood proteins results in clinical edema , or tissue swelling, because there is insufficient pressure to pull fluid back into the blood from the tissues. The condition of edema is serious and can lead to many medical problems.
PCNA Gene - GeneCards | PCNA Protein | PCNA Antibody
After a carrier vesicle is formed, it must recognize and fuse with its target. Recognition and fusion (see Exocytosis) involve proteins on the vesicle (v-SNARE) and on the target membrane (t-SNARE) (13). When a carrier vesicle leaves the ER, it does not go directly to the plasma membrane but instead fuses with an organelle, the Golgi complex, which is a mandatory way station on the secretory pathway of eukaryotes. The Golgi complex is a stack of membranous cisternae, similar morphologically to a stack of pancakes. The carrier vesicles enter the cis end of a Golgi stack and exit from the trans side. A protein to be exported goes through a series of glycosylation steps in which six- or nine-carbon sugars are added to or removed from oligosaccharide chains attached to serine, threonine (O-glycosylation), or asparagine residues (N-glycosylation). The added sugars can often protect the exported protein from rapid proteolytic degradation after it is secreted into the extracellular world. A unique class of oligosaccharides is added to newly synthesized lysosomal enzymes that allows them to be diverted out of the secretory pathway to primary lysosomes.
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