Multienzyme Complexes: Systems of enzymes which function sequentially by catalyzing consecutive reactions linked by common metabolic intermediates. They may involve simply a transfer of water molecules or hydrogen atoms and may be associated with large supramolecular structures such as MITOCHONDRIA or RIBOSOMES.Pyruvate Dehydrogenase Complex: A multienzyme complex responsible for the formation of ACETYL COENZYME A from pyruvate. The enzyme components are PYRUVATE DEHYDROGENASE (LIPOAMIDE); dihydrolipoamide acetyltransferase; and LIPOAMIDE DEHYDROGENASE. Pyruvate dehydrogenase complex is subject to three types of control: inhibited by acetyl-CoA and NADH; influenced by the energy state of the cell; and inhibited when a specific serine residue in the pyruvate decarboxylase is phosphorylated by ATP. PYRUVATE DEHYDROGENASE (LIPOAMIDE)-PHOSPHATASE catalyzes reactivation of the complex. (From Concise Encyclopedia Biochemistry and Molecular Biology, 3rd ed)Dihydrolipoyllysine-Residue Acetyltransferase: An enzyme that catalyzes the acetyltransferase reaction using ACETYL CoA as an acetyl donor and dihydrolipoamide as acceptor to produce COENZYME A (CoA) and S-acetyldihydrolipoamide. It forms the (E2) subunit of the PYRUVATE DEHYDROGENASE COMPLEX.Dihydrolipoamide Dehydrogenase: A flavoprotein containing oxidoreductase that catalyzes the reduction of lipoamide by NADH to yield dihydrolipoamide and NAD+. The enzyme is a component of several MULTIENZYME COMPLEXES.Ketone Oxidoreductases: Oxidoreductases that are specific for KETONES.Thioctic Acid: An octanoic acid bridged with two sulfurs so that it is sometimes also called a pentanoic acid in some naming schemes. It is biosynthesized by cleavage of LINOLEIC ACID and is a coenzyme of oxoglutarate dehydrogenase (KETOGLUTARATE DEHYDROGENASE COMPLEX). It is used in DIETARY SUPPLEMENTS.Geobacillus stearothermophilus: A species of GRAM-POSITIVE ENDOSPORE-FORMING BACTERIA in the family BACILLACEAE, found in soil, hot springs, Arctic waters, ocean sediments, and spoiled food products.Ketoglutarate Dehydrogenase ComplexMitochondrial Trifunctional Protein: A mitochondrial protein consisting of four alpha-subunits and four beta-subunits. It contains enoyl-CoA hydratase, long-chain-3-hydroxyacyl-CoA dehydrogenase, and acetyl-CoA C-acyltransferase activities and plays an important role in the metabolism of long chain FATTY ACIDS.Enoyl-CoA Hydratase: An enzyme that catalyzes reversibly the hydration of unsaturated fatty acyl-CoA to yield beta-hydroxyacyl-CoA. It plays a role in the oxidation of fatty acids and in mitochondrial fatty acid synthesis, has broad specificity, and is most active with crotonyl-CoA. EC 18.104.22.168.Acetyl-CoA C-Acyltransferase: Enzyme that catalyzes the final step of fatty acid oxidation in which ACETYL COA is released and the CoA ester of a fatty acid two carbons shorter is formed.Glycine Dehydrogenase (Decarboxylating): A PYRIDOXAL PHOSPHATE dependent enzyme that catalyzes the decarboxylation of GLYCINE with the transfer of an aminomethyl group to the LIPOIC ACID moiety of the GLYCINE DECARBOXYLASE COMPLEX H-PROTEIN. Defects in P-protein are the cause of non-ketotic hyperglycinemia. It is one of four subunits of the glycine decarboxylase complex.Thermoanaerobacterium: A genus of gram-positive, anaerobic bacteria in the family Thermoanaerobacteriaceae. They are thermophilic and saccharolytic.Pyruvate Dehydrogenase (Lipoamide): The E1 component of the multienzyme PYRUVATE DEHYDROGENASE COMPLEX. It is composed of 2 alpha subunits (pyruvate dehydrogenase E1 alpha subunit) and 2 beta subunits (pyruvate dehydrogenase E1 beta subunit).3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide): A ketone oxidoreductase that catalyzes the overall conversion of alpha-keto acids to ACYL-CoA and CO2. The enzyme requires THIAMINE DIPHOSPHATE as a cofactor. Defects in genes that code for subunits of the enzyme are a cause of MAPLE SYRUP URINE DISEASE. The enzyme was formerly classified as EC 22.214.171.124.Cellulases: A family of glycosidases that hydrolyse crystalline CELLULOSE into soluble sugar molecules. Within this family there are a variety of enzyme subtypes with differing substrate specificities that must work together to bring about complete cellulose hydrolysis. They are found in structures called CELLULOSOMES.Racemases and Epimerases: Enzymes that catalyze inversion of the configuration around an asymmetric carbon in a substrate having one (racemase) or more (epimerase) center(s) of asymmetry. (Dorland, 28th ed) EC 5.1.Clostridium thermocellum: A species of gram-positive, thermophilic, cellulolytic bacteria in the family Clostridaceae. It degrades and ferments CELLOBIOSE and CELLULOSE to ETHANOL in the CELLULOSOME.Cellulosomes: Extracellular structures found in a variety of microorganisms. They contain CELLULASES and play an important role in the digestion of CELLULOSE.Glycine Decarboxylase Complex H-Protein: A LIPOIC ACID-containing protein that plays the pivotal role in the transfer of methylamine groups and reducing equivalents between the three enzymatic components of the glycine decarboxylase complex.3-Hydroxyacyl CoA Dehydrogenases: Enzymes that reversibly catalyze the oxidation of a 3-hydroxyacyl CoA to 3-ketoacyl CoA in the presence of NAD. They are key enzymes in the oxidation of fatty acids and in mitochondrial fatty acid synthesis.Acetyltransferases: Enzymes catalyzing the transfer of an acetyl group, usually from acetyl coenzyme A, to another compound. EC 2.3.1.Aminomethyltransferase: A one-carbon group transferase that transfers lipoamide-linked methylamine groups to tetrahydrofolate (TETRAHYDROFOLATES) to form methylenetetrahydrofolate and AMMONIA. It is one of four components of the glycine decarboxylase complex.Escherichia coli: A species of gram-negative, facultatively anaerobic, rod-shaped bacteria (GRAM-NEGATIVE FACULTATIVELY ANAEROBIC RODS) commonly found in the lower part of the intestine of warm-blooded animals. It is usually nonpathogenic, but some strains are known to produce DIARRHEA and pyogenic infections. Pathogenic strains (virotypes) are classified by their specific pathogenic mechanisms such as toxins (ENTEROTOXIGENIC ESCHERICHIA COLI), etc.Carbon-Carbon Double Bond Isomerases: Enzymes that catalyze the shifting of a carbon-carbon double bond from one position to another within the same molecule. EC 5.3.3.Cathepsin A: A carboxypeptidase that catalyzes the release of a C-terminal amino acid with a broad specificity. It also plays a role in the LYSOSOMES by protecting BETA-GALACTOSIDASE and NEURAMINIDASE from degradation. It was formerly classified as EC 126.96.36.199 and EC 188.8.131.52.Thiamine Pyrophosphate: The coenzyme form of Vitamin B1 present in many animal tissues. It is a required intermediate in the PYRUVATE DEHYDROGENASE COMPLEX and the KETOGLUTARATE DEHYDROGENASE COMPLEX.Hydroxymethyl and Formyl Transferases: Enzymes that catalyze the transfer of hydroxymethyl or formyl groups. EC 2.1.2.Amino Acyl-tRNA Synthetases: A subclass of enzymes that aminoacylate AMINO ACID-SPECIFIC TRANSFER RNA with their corresponding AMINO ACIDS.Fatty Acid Synthases: Enzymes that catalyze the synthesis of FATTY ACIDS from acetyl-CoA and malonyl-CoA derivatives.Molecular Sequence Data: Descriptions of specific amino acid, carbohydrate, or nucleotide sequences which have appeared in the published literature and/or are deposited in and maintained by databanks such as GENBANK, European Molecular Biology Laboratory (EMBL), National Biomedical Research Foundation (NBRF), or other sequence repositories.Rhodotorula: A red yeast-like mitosporic fungal genus generally regarded as nonpathogenic. It is cultured from numerous sources in human patients.Ethylmaleimide: A sulfhydryl reagent that is widely used in experimental biochemical studies.Amino Acid Sequence: The order of amino acids as they occur in a polypeptide chain. This is referred to as the primary structure of proteins. It is of fundamental importance in determining PROTEIN CONFORMATION.Dodecenoyl-CoA Isomerase: A carbon-carbon double bond isomerase that catalyzes the movement double bond from C3 to C2 of an unsaturated acyl-CoA. The enzyme plays a key role in allowing acyl-CoA substrates to re-enter the beta-oxidation pathway.Kinetics: The rate dynamics in chemical or physical systems.Aspartate-tRNA Ligase: An enzyme that activates aspartic acid with its specific transfer RNA. EC 184.108.40.206.Cellulase: An endocellulase with specificity for the hydrolysis of 1,4-beta-glucosidic linkages in CELLULOSE, lichenin, and cereal beta-glucans.Maple Syrup Urine Disease: An autosomal recessive inherited disorder with multiple forms of phenotypic expression, caused by a defect in the oxidative decarboxylation of branched-chain amino acids (AMINO ACIDS, BRANCHED-CHAIN). These metabolites accumulate in body fluids and render a "maple syrup" odor. The disease is divided into classic, intermediate, intermittent, and thiamine responsive subtypes. The classic form presents in the first week of life with ketoacidosis, hypoglycemia, emesis, neonatal seizures, and hypertonia. The intermediate and intermittent forms present in childhood or later with acute episodes of ataxia and vomiting. (From Adams et al., Principles of Neurology, 6th ed, p936)Xylans: Polysaccharides consisting of xylose units.Acetyl Coenzyme A: Acetyl CoA participates in the biosynthesis of fatty acids and sterols, in the oxidation of fatty acids and in the metabolism of many amino acids. It also acts as a biological acetylating agent.Methanosarcina: A genus of anaerobic, irregular spheroid-shaped METHANOSARCINALES whose organisms are nonmotile. Endospores are not formed. These archaea derive energy via formation of methane from acetate, methanol, mono-, di-, and trimethylamine, and possibly, carbon monoxide. Organisms are isolated from freshwater and marine environments.Isomerases: A class of enzymes that catalyze geometric or structural changes within a molecule to form a single product. The reactions do not involve a net change in the concentrations of compounds other than the substrate and the product.(from Dorland, 28th ed) EC 5.Chromatography, Gel: Chromatography on non-ionic gels without regard to the mechanism of solute discrimination.Protein Conformation: The characteristic 3-dimensional shape of a protein, including the secondary, supersecondary (motifs), tertiary (domains) and quaternary structure of the peptide chain. PROTEIN STRUCTURE, QUATERNARY describes the conformation assumed by multimeric proteins (aggregates of more than one polypeptide chain).Molecular Weight: The sum of the weight of all the atoms in a molecule.Binding Sites: The parts of a macromolecule that directly participate in its specific combination with another molecule.Electrophoresis, Polyacrylamide Gel: Electrophoresis in which a polyacrylamide gel is used as the diffusion medium.PyruvatesModels, Molecular: Models used experimentally or theoretically to study molecular shape, electronic properties, or interactions; includes analogous molecules, computer-generated graphics, and mechanical structures.Peptide Synthases: Ligases that catalyze the joining of adjacent AMINO ACIDS by the formation of carbon-nitrogen bonds between their carboxylic acid groups and amine groups.Cellulose: A polysaccharide with glucose units linked as in CELLOBIOSE. It is the chief constituent of plant fibers, cotton being the purest natural form of the substance. As a raw material, it forms the basis for many derivatives used in chromatography, ion exchange materials, explosives manufacturing, and pharmaceutical preparations.Amino Acid Oxidoreductases: A class of enzymes that catalyze oxidation-reduction reactions of amino acids.Acyltransferases: Enzymes from the transferase class that catalyze the transfer of acyl groups from donor to acceptor, forming either esters or amides. (From Enzyme Nomenclature 1992) EC 2.3.Malate Dehydrogenase: An enzyme that catalyzes the conversion of (S)-malate and NAD+ to oxaloacetate and NADH. EC 220.127.116.11.NAD: A coenzyme composed of ribosylnicotinamide 5'-diphosphate coupled to adenosine 5'-phosphate by pyrophosphate linkage. It is found widely in nature and is involved in numerous enzymatic reactions in which it serves as an electron carrier by being alternately oxidized (NAD+) and reduced (NADH). (Dorland, 27th ed)Saccharopolyspora: A genus of gram-positive bacteria whose spores are round to oval and covered by a sheath.Catalysis: The facilitation of a chemical reaction by material (catalyst) that is not consumed by the reaction.Coenzyme ASubstrate Specificity: A characteristic feature of enzyme activity in relation to the kind of substrate on which the enzyme or catalytic molecule reacts.Catalytic Domain: The region of an enzyme that interacts with its substrate to cause the enzymatic reaction.Base Sequence: The sequence of PURINES and PYRIMIDINES in nucleic acids and polynucleotides. It is also called nucleotide sequence.Pyruvic Acid: An intermediate compound in the metabolism of carbohydrates, proteins, and fats. In thiamine deficiency, its oxidation is retarded and it accumulates in the tissues, especially in nervous structures. (From Stedman, 26th ed)Transferases: Transferases are enzymes transferring a group, for example, the methyl group or a glycosyl group, from one compound (generally regarded as donor) to another compound (generally regarded as acceptor). The classification is based on the scheme "donor:acceptor group transferase". (Enzyme Nomenclature, 1992) EC 2.Cloning, Molecular: The insertion of recombinant DNA molecules from prokaryotic and/or eukaryotic sources into a replicating vehicle, such as a plasmid or virus vector, and the introduction of the resultant hybrid molecules into recipient cells without altering the viability of those cells.Ultracentrifugation: Centrifugation with a centrifuge that develops centrifugal fields of more than 100,000 times gravity. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)Glycine Decarboxylase Complex: A enzyme complex that catalyzes the oxidative DECARBOXYLATION and DEAMINATION of GLYCINE into CARBON DIOXIDE; AMMONIA; NADH; and N5N10-methylenetetrahydrofolate. It is composed of four different component protein components referred to as H, P, L, and T.Pseudomonas: A genus of gram-negative, aerobic, rod-shaped bacteria widely distributed in nature. Some species are pathogenic for humans, animals, and plants.Sequence Homology, Amino Acid: The degree of similarity between sequences of amino acids. This information is useful for the analyzing genetic relatedness of proteins and species.Protein Structure, Quaternary: The characteristic 3-dimensional shape and arrangement of multimeric proteins (aggregates of more than one polypeptide chain).Fatty Acids: Organic, monobasic acids derived from hydrocarbons by the equivalent of oxidation of a methyl group to an alcohol, aldehyde, and then acid. Fatty acids are saturated and unsaturated (FATTY ACIDS, UNSATURATED). (Grant & Hackh's Chemical Dictionary, 5th ed)Crystallography, X-Ray: The study of crystal structure using X-RAY DIFFRACTION techniques. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)Bacterial Proteins: Proteins found in any species of bacterium.Ligases: A class of enzymes that catalyze the formation of a bond between two substrate molecules, coupled with the hydrolysis of a pyrophosphate bond in ATP or a similar energy donor. (Dorland, 28th ed) EC 6.Operon: In bacteria, a group of metabolically related genes, with a common promoter, whose transcription into a single polycistronic MESSENGER RNA is under the control of an OPERATOR REGION.Protein Binding: The process in which substances, either endogenous or exogenous, bind to proteins, peptides, enzymes, protein precursors, or allied compounds. Specific protein-binding measures are often used as assays in diagnostic assessments.Oxidation-Reduction: A chemical reaction in which an electron is transferred from one molecule to another. The electron-donating molecule is the reducing agent or reductant; the electron-accepting molecule is the oxidizing agent or oxidant. Reducing and oxidizing agents function as conjugate reductant-oxidant pairs or redox pairs (Lehninger, Principles of Biochemistry, 1982, p471).Mutation: Any detectable and heritable change in the genetic material that causes a change in the GENOTYPE and which is transmitted to daughter cells and to succeeding generations.Protein Structure, Secondary: The level of protein structure in which regular hydrogen-bond interactions within contiguous stretches of polypeptide chain give rise to alpha helices, beta strands (which align to form beta sheets) or other types of coils. This is the first folding level of protein conformation.Genes, Bacterial: The functional hereditary units of BACTERIA.Trypsin: A serine endopeptidase that is formed from TRYPSINOGEN in the pancreas. It is converted into its active form by ENTEROPEPTIDASE in the small intestine. It catalyzes hydrolysis of the carboxyl group of either arginine or lysine. EC 18.104.22.168.Magnetic Resonance Spectroscopy: Spectroscopic method of measuring the magnetic moment of elementary particles such as atomic nuclei, protons or electrons. It is employed in clinical applications such as NMR Tomography (MAGNETIC RESONANCE IMAGING).Protein Structure, Tertiary: The level of protein structure in which combinations of secondary protein structures (alpha helices, beta sheets, loop regions, and motifs) pack together to form folded shapes called domains. Disulfide bridges between cysteines in two different parts of the polypeptide chain along with other interactions between the chains play a role in the formation and stabilization of tertiary structure. Small proteins usually consist of only one domain but larger proteins may contain a number of domains connected by segments of polypeptide chain which lack regular secondary structure.Dihydroorotase: An enzyme that, in the course of pyrimidine biosynthesis, catalyzes ring closure by removal of water from N-carbamoylaspartate to yield dihydro-orotic acid. EC 22.214.171.124.Cattle: Domesticated bovine animals of the genus Bos, usually kept on a farm or ranch and used for the production of meat or dairy products or for heavy labor.Macromolecular Substances: Compounds and molecular complexes that consist of very large numbers of atoms and are generally over 500 kDa in size. In biological systems macromolecular substances usually can be visualized using ELECTRON MICROSCOPY and are distinguished from ORGANELLES by the lack of a membrane structure.Protein Subunits: Single chains of amino acids that are the units of multimeric PROTEINS. Multimeric proteins can be composed of identical or non-identical subunits. One or more monomeric subunits may compose a protomer which itself is a subunit structure of a larger assembly.Plasmids: Extrachromosomal, usually CIRCULAR DNA molecules that are self-replicating and transferable from one organism to another. They are found in a variety of bacterial, archaeal, fungal, algal, and plant species. They are used in GENETIC ENGINEERING as CLONING VECTORS.Recombinant Proteins: Proteins prepared by recombinant DNA technology.Saccharomyces cerevisiae: A species of the genus SACCHAROMYCES, family Saccharomycetaceae, order Saccharomycetales, known as "baker's" or "brewer's" yeast. The dried form is used as a dietary supplement.Sequence Alignment: The arrangement of two or more amino acid or base sequences from an organism or organisms in such a way as to align areas of the sequences sharing common properties. The degree of relatedness or homology between the sequences is predicted computationally or statistically based on weights assigned to the elements aligned between the sequences. This in turn can serve as a potential indicator of the genetic relatedness between the organisms.Dimerization: The process by which two molecules of the same chemical composition form a condensation product or polymer.Mitochondria, Liver: Mitochondria in hepatocytes. As in all mitochondria, there are an outer membrane and an inner membrane, together creating two separate mitochondrial compartments: the internal matrix space and a much narrower intermembrane space. In the liver mitochondrion, an estimated 67% of the total mitochondrial proteins is located in the matrix. (From Alberts et al., Molecular Biology of the Cell, 2d ed, p343-4)Thermodynamics: A rigorously mathematical analysis of energy relationships (heat, work, temperature, and equilibrium). It describes systems whose states are determined by thermal parameters, such as temperature, in addition to mechanical and electromagnetic parameters. (From Hawley's Condensed Chemical Dictionary, 12th ed)Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing): An enzyme that catalyzes the formation of carbamoyl phosphate from ATP, carbon dioxide, and glutamine. This enzyme is important in the de novo biosynthesis of pyrimidines. EC 126.96.36.199.Mutagenesis, Site-Directed: Genetically engineered MUTAGENESIS at a specific site in the DNA molecule that introduces a base substitution, or an insertion or deletion.Microscopy, Electron: Microscopy using an electron beam, instead of light, to visualize the sample, thereby allowing much greater magnification. The interactions of ELECTRONS with specimens are used to provide information about the fine structure of that specimen. In TRANSMISSION ELECTRON MICROSCOPY the reactions of the electrons that are transmitted through the specimen are imaged. In SCANNING ELECTRON MICROSCOPY an electron beam falls at a non-normal angle on the specimen and the image is derived from the reactions occurring above the plane of the specimen.Carboxyl and Carbamoyl Transferases: A group of enzymes that catalyze the transfer of carboxyl- or carbamoyl- groups. EC 2.1.3.Peptides: Members of the class of compounds composed of AMINO ACIDS joined together by peptide bonds between adjacent amino acids into linear, branched or cyclical structures. OLIGOPEPTIDES are composed of approximately 2-12 amino acids. Polypeptides are composed of approximately 13 or more amino acids. PROTEINS are linear polypeptides that are normally synthesized on RIBOSOMES.DNA, Bacterial: Deoxyribonucleic acid that makes up the genetic material of bacteria.Hydrogen-Ion Concentration: The normality of a solution with respect to HYDROGEN ions; H+. It is related to acidity measurements in most cases by pH = log 1/2[1/(H+)], where (H+) is the hydrogen ion concentration in gram equivalents per liter of solution. (McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed)Sequence Homology, Nucleic Acid: The sequential correspondence of nucleotides in one nucleic acid molecule with those of another nucleic acid molecule. Sequence homology is an indication of the genetic relatedness of different organisms and gene function.Mitochondria: Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive RIBOSOMES, transfer RNAs (RNA, TRANSFER); AMINO ACYL T RNA SYNTHETASES; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs (RNA, MESSENGER). Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. (King & Stansfield, A Dictionary of Genetics, 4th ed)Aspartate Carbamoyltransferase: An enzyme that catalyzes the conversion of carbamoyl phosphate and L-aspartate to yield orthophosphate and N-carbamoyl-L-aspartate. (From Enzyme Nomenclature, 1992) EC 188.8.131.52.Liver: A large lobed glandular organ in the abdomen of vertebrates that is responsible for detoxification, metabolism, synthesis and storage of various substances.Citrate (si)-Synthase: Enzyme that catalyzes the first step of the tricarboxylic acid cycle (CITRIC ACID CYCLE). It catalyzes the reaction of oxaloacetate and acetyl CoA to form citrate and coenzyme A. This enzyme was formerly listed as EC 184.108.40.206.Amino Acid Isomerases: Enzymes that catalyze either the racemization or epimerization of chiral centers within amino acids or derivatives. EC 5.1.1.Acyl Carrier Protein: Consists of a polypeptide chain and 4'-phosphopantetheine linked to a serine residue by a phosphodiester bond. Acyl groups are bound as thiol esters to the pantothenyl group. Acyl carrier protein is involved in every step of fatty acid synthesis by the cytoplasmic system.Glyceraldehyde-3-Phosphate Dehydrogenases: Enzymes that catalyze the dehydrogenation of GLYCERALDEHYDE 3-PHOSPHATE. Several types of glyceraldehyde-3-phosphate-dehydrogenase exist including phosphorylating and non-phosphorylating varieties and ones that transfer hydrogen to NADP and ones that transfer hydrogen to NAD.Aldehyde Oxidoreductases: Oxidoreductases that are specific for ALDEHYDES.Thiolester HydrolasesEnzymes, Immobilized: Enzymes which are immobilized on or in a variety of water-soluble or water-insoluble matrices with little or no loss of their catalytic activity. Since they can be reused continuously, immobilized enzymes have found wide application in the industrial, medical and research fields.Ketoglutaric Acids: A family of compounds containing an oxo group with the general structure of 1,5-pentanedioic acid. (From Lehninger, Principles of Biochemistry, 1982, p442)Enzymes: Biological molecules that possess catalytic activity. They may occur naturally or be synthetically created. Enzymes are usually proteins, however CATALYTIC RNA and CATALYTIC DNA molecules have also been identified.Centrifugation, Density Gradient: Separation of particles according to density by employing a gradient of varying densities. At equilibrium each particle settles in the gradient at a point equal to its density. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)Bacillus: A genus of BACILLACEAE that are spore-forming, rod-shaped cells. Most species are saprophytic soil forms with only a few species being pathogenic.Models, Chemical: Theoretical representations that simulate the behavior or activity of chemical processes or phenomena; includes the use of mathematical equations, computers, and other electronic equipment.Streptomyces: A genus of bacteria that form a nonfragmented aerial mycelium. Many species have been identified with some being pathogenic. This genus is responsible for producing a majority of the ANTI-BACTERIAL AGENTS of practical value.Cross-Linking Reagents: Reagents with two reactive groups, usually at opposite ends of the molecule, that are capable of reacting with and thereby forming bridges between side chains of amino acids in proteins; the locations of naturally reactive areas within proteins can thereby be identified; may also be used for other macromolecules, like glycoproteins, nucleic acids, or other.