Autolysis (biology)

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In biology, autolysis, more commonly known as self-digestion, refers to the destruction of a cell through the action of its own enzymes. It may also refer to the digestion of an enzyme by another molecule of the same enzyme.

Contents

The term derives from the Greek αὐτο- ("self") and λύσις ("splitting").

Biochemical mechanisms of cell destruction

Histopathology of thyroid parenchyma with autolytic changes seen at autopsy, with thyroid follicular cells sloughing off into the follicles. Histopathology of thyroid parenchyma with autolytic changes.jpg
Histopathology of thyroid parenchyma with autolytic changes seen at autopsy, with thyroid follicular cells sloughing off into the follicles.

Autolysis is uncommon in living adult organisms and usually occurs in necrotic tissue as enzymes act on components of the cell that would not normally serve as substrates. These enzymes are released due to the cessation of active processes in the cell that provide substrates in healthy, living tissue; autolysis in itself is not an active process. In other words, though autolysis resembles the active process of digestion of nutrients by live cells, the dead cells are not actively digesting themselves as is often claimed, and as the synonym self-digestion suggests. Failure of respiration and subsequent failure of oxidative phosphorylation is the trigger of the autolytic process. [1] The reduced availability and subsequent absence of high-energy molecules that are required to maintain the integrity of the cell and maintain homeostasis causes significant changes in the biochemical operation of the cell.

Molecular oxygen serves as the terminal electron acceptor in the series of biochemical reactions known as oxidative phosphorylation that are ultimately responsible for the synthesis of adenosine triphosphate, the main source of energy for otherwise thermodynamically unfavorable cellular processes. [2] Failure of delivery of molecular oxygen to cells results in a metabolic shift to anaerobic glycolysis, in which glucose is converted to pyruvate as an inefficient means of generating adenosine triphosphate. [2] Glycolysis has a lower ATP yield than oxidative phosphorylation and generates acidic byproducts that decrease the pH of the cell, which enables many of the enzymatic processes involved in autolysis.

Limited synthesis of adenosine triphosphate impairs many cellular transport mechanisms that utilize ATP to drive energetically unfavorable processes that transport ions and molecules across the cellular membrane. For example, the membrane potential of the cell is maintained by the sodium-potassium ATPase pump. Failure of the pump results in loss of membrane potential as sodium ions accumulate within the cell and potassium ions are lost through ion channels. Loss of membrane potential encourages movement of calcium ions into the cell, followed by movement of water into the cell, as driven by osmotic pressure. [3] Water retention, ionic changes, and acidification of the cell damages membrane-bound intracellular structures including the lysosome and peroxisome. [1]

Lysosomes are membrane-bound organelles that typically contain a broad spectrum of enzymes capable of hydrolytic deconstruction of polysaccharides, proteins, nucleic acids, lipids, phosphoric acyl esters, and sulfates. This process requires compartmentalization and segregation of enzymes and substrates via a single intracellular membrane that prevents unwarranted destruction of other intracellular components. Under normal conditions, the molecular machinery of the cell is further protected from lysosomal enzyme activity by regulation of cytosolic pH. The activity of lysosomal hydrolases is optimal at a moderately acidic pH of 5, which is significantly more acidic than the more basic average pH of 7.2 in the surrounding cytosol. [1] However, the accumulation of products of glycolysis decreases the pH of the cell, reducing this protective effect. Furthermore, lysosomal membranes damaged by water retention in the cell will release lysosomal enzymes into the cytosol. These enzymes are likely to be active due to the decreased cytosolic pH and are thus free to utilize cellular components as substrates. [1]

Peroxisomes typically are responsible for the breakdown of lipids, particularly long-chain fatty acids. In the absence of an active electron transport chain and associated cellular processes, there is no metabolic partner for the reducing equivalents in the breakdown of lipids. [1] In terms of autolysis, peroxisomes provide catabolic potential for fatty acids and reactive oxygen species, which are released into the cytosol as the peroxisomal membrane is damaged by water retention and digestion by other catabolic enzymes. [1]

Use

The release of catabolically active enzymes from their sub-cellular locations initiates an irreversible process that results in the complete reduction of deceased organisms. Autolysis produces an acidic, anaerobic, nutrient-rich environment that nurtures the activity of invasive and opportunistic microorganisms in a process known as putrefaction. Autolysis and putrefaction are the main processes responsible for the decomposition of remains. [1]

In the healing of wounds, autolytic debridement can be a helpful process, where the body breaks down and liquifies dead tissue so that it can be washed or carried away. Modern wound dressings that help keep the wound moist can assist in this process.[ citation needed ]

In the food industry, autolysis involves killing yeast and encouraging breakdown of its cells by various enzymes. The resulting autolyzed yeast is used as a flavoring or flavor enhancer. For yeast extract, when this process is triggered by the addition of salt, it is known as plasmolysis. [4]

In bread baking, the term (or, more commonly, its French cognate autolyse) is described as a period of rest following initial mixing of flour and water, before other ingredients (such as salt and yeast) are added to the dough. [5] The term was coined by French baking professor Raymond Calvel, who recommended the procedure as a means of reducing kneading time, thereby improving the flavor and color of bread. [6] Long kneading times subject bread dough to atmospheric oxygen, which bleaches the naturally occurring carotenoids in bread flour, robbing the flour of its natural creamy color and flavor. [6] An autolyse also makes the dough easier to shape and improves structure. [6]

In the making of fermented beverages, autolysis can occur when the must or wort is left on the lees for a long time. In beer brewing, autolysis causes undesired off-flavors. Autolysis in winemaking is often undesirable, but in the case of the best Champagnes it is a vital component in creating flavor and mouth feel. [7]

See also

Related Research Articles

Adenosine triphosphate Chemical compound

Adenosine triphosphate (ATP) is an organic compound and hydrotrope that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer. When consumed in metabolic processes, it converts either to adenosine diphosphate (ADP) or to adenosine monophosphate (AMP). Other processes regenerate ATP so that the human body recycles its own body weight equivalent in ATP each day. It is also a precursor to DNA and RNA, and is used as a coenzyme.

Glycolysis Metabolic pathway

Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvic acid, CH3COCOOH. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.

Hydrolysis is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.

Lysosome Cell organelle

A lysosome is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane proteins, and its lumenal proteins. The lumen's pH (~4.5–5.0) is optimal for the enzymes involved in hydrolysis, analogous to the activity of the stomach. Besides degradation of polymers, the lysosome is involved in various cell processes, including secretion, plasma membrane repair, apoptosis, cell signaling, and energy metabolism.

Metabolic pathway

In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reactions catalyzed by enzymes. In most cases of a metabolic pathway, the product of one enzyme acts as the substrate for the next. However, side products are considered waste and removed from the cell. These enzymes often require dietary minerals, vitamins, and other cofactors to function.

Peroxisome Type of organelle

A peroxisome (IPA: [pɛɜˈɹɒksɪˌsoʊm]) is a membrane-bound organelle (formerly known as a microbody), found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular oxygen serves as a co-substrate, from which hydrogen peroxide (H2O2) is then formed. Peroxisomes owe their name to hydrogen peroxide generating and scavenging activities. They perform key roles in lipid metabolism and the conversion of reactive oxygen species. Peroxisomes are involved in the catabolism of very long chain fatty acids, branched chain fatty acids, bile acid intermediates (in the liver), D-amino acids, and polyamines, the reduction of reactive oxygen species – specifically hydrogen peroxide – and the biosynthesis of plasmalogens, i.e., ether phospholipids critical for the normal function of mammalian brains and lungs. They also contain approximately 10% of the total activity of two enzymes (Glucose-6-phosphate dehydrogenase and 6-Phosphogluconate dehydrogenase) in the pentose phosphate pathway, which is important for energy metabolism. It is vigorously debated whether peroxisomes are involved in isoprenoid and cholesterol synthesis in animals. Other known peroxisomal functions include the glyoxylate cycle in germinating seeds ("glyoxysomes"), photorespiration in leaves, glycolysis in trypanosomes ("glycosomes"), and methanol and/or amine oxidation and assimilation in some yeasts.

Coenzyme A Coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle

Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All genomes sequenced to date encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it (or a thioester) as a substrate. In humans, CoA biosynthesis requires cysteine, pantothenate (vitamin B5), and adenosine triphosphate (ATP).

Adenosine diphosphate Chemical compound

Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbone attached to adenine and two phosphate groups bonded to the 5 carbon atom of ribose. The diphosphate group of ADP is attached to the 5’ carbon of the sugar backbone, while the adenine attaches to the 1’ carbon.

Cellular respiration Metabolic reactions in the cells of organisms converting chemical energy from oxygen molecules or nutrients into adenosine triphosphate (ATP) while releasing waste byproducts.

Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from oxygen molecules or nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy because weak high-energy bonds, in particular in molecular oxygen, are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow, controlled release of energy from the series of reactions.

Anabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breaking-down aspect. Anabolism is usually synonymous with biosynthesis.

Digestion is the breakdown of carbohydrates to yield an energy rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic. In aerobic respiration, oxygen is required. Oxygen as a high-energy molecule increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.

Maltase

Maltase is one type of alpha-glucosidase enzymes located in the brush border of the small intestine. This enzyme catalyzes the hydrolysis of disaccharide maltose into two simple sugars of glucose. Maltase is found in plants, bacteria, yeast, humans, and other vertebrates. It is thought to be synthesized by cells of the mucous membrane lining the intestinal wall.

The term amphibolic is used to describe a biochemical pathway that involves both catabolism and anabolism. Catabolism is a degradative phase of metabolism in which large molecules are converted into smaller and simpler molecules, which involves two types of reactions. First, hydrolysis reactions, in which catabolism is the breaking apart of molecules into smaller molecules to release energy. Examples of catabolic reactions are digestion and cellular respiration, where sugars and fats are broken down for energy. Breaking down a protein into amino acids, or a triglyceride into fatty acids, or a disaccharide into monosaccharides are all hydrolysis or catabolic reactions. Second, oxidation reactions involve the removal of hydrogens and electrons from an organic molecule. Anabolism is the biosynthesis phase of metabolism in which smaller simple precursors are converted to large and complex molecules of the cell. Anabolism has two classes of reactions. The first are dehydration synthesis reactions; these involve the joining of smaller molecules together to form larger, more complex molecules. These include the formation of carbohydrates, proteins, lipids and nucleic acids. The second are reduction reactions, in which hydrogens and electrons are added to a molecule. Whenever that is done, molecules gain energy.

Bioenergetics is a field in biochemistry and cell biology that concerns energy flow through living systems. This is an active area of biological research that includes the study of the transformation of energy in living organisms and the study of thousands of different cellular processes such as cellular respiration and the many other metabolic and enzymatic processes that lead to production and utilization of energy in forms such as adenosine triphosphate (ATP) molecules. That is, the goal of bioenergetics is to describe how living organisms acquire and transform energy in order to perform biological work. The study of metabolic pathways is thus essential to bioenergetics.

Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group attached or not) into a lower energy product. This process uses some of the released chemical energy, the Gibbs free energy, to transfer a phosphoryl (PO3) group to ADP or GDP from another phosphorylated compound. Occurs in glycolysis and in the citric acid cycle.

Beta oxidation Process of fatty acid breakdown

In biochemistry and metabolism, beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric acid cycle, and NADH and FADH2, which are co-enzymes used in the electron transport chain. It is named as such because the beta carbon of the fatty acid undergoes oxidation to a carbonyl group. Beta-oxidation is primarily facilitated by the mitochondrial trifunctional protein, an enzyme complex associated with the inner mitochondrial membrane, although very long chain fatty acids are oxidized in peroxisomes.

Endoplasm

Endoplasm generally refers to the inner, dense part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer (non-granulated) layer of the cytoplasm, which is typically watery and immediately adjacent to the plasma membrane. These two terms are mainly used to describe the cytoplasm of the amoeba, a protozoan, eukaryotic cell. The nucleus is separated from the endoplasm by the nuclear envelope. The different makeups/viscosities of the endoplasm and ectoplasm contribute to the amoeba's locomotion through the formation of a pseudopod. However, other types of cells have cytoplasm divided into endo- and ectoplasm. The endoplasm, along with its granules, contains water, nucleic acids, amino acids, carbohydrates, inorganic ions, lipids, enzymes, and other molecular compounds. It is the site of most cellular processes as it houses the organelles that make up the endomembrane system, as well as those that stand alone. The endoplasm is necessary for most metabolic activities, including cell division.

Glycosome Organelle containing glycolytic enzymes in some protists

The glycosome is a membrane-enclosed organelle that contains the glycolytic enzymes. The term was first used by Scott and Still in 1968 after they realized that the glycogen in the cell was not static but rather a dynamic molecule. It is found in a few species of protozoa including the Kinetoplastida which include the suborders Trypanosomatida and Bodonina, most notably in the human pathogenic trypanosomes, which can cause sleeping sickness, Chagas's disease, and leishmaniasis. The organelle is bounded by a single membrane and contains a dense proteinaceous matrix. It is believed to have evolved from the peroxisome. This has been verified by work done on Leishmania genetics.

Translocase is a general term for a protein that assists in moving another molecule, usually across a cell membrane. These enzymes catalyze the movement of ions or molecules across membranes or their separation within membranes. The reaction is designated as a transfer from “side 1” to “side 2” because the designations “in” and “out”, which had previously been used, can be ambiguous. Translocases are the most common secretion system in Gram positive bacteria.

Outline of cell biology Overview of and topical guide to cell biology

The following outline is provided as an overview of and topical guide to cell biology:

References

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