∞ generated and posted on 2016.12.15 ∞
| An Enzyme is a protein that manipulates specific substances (substrates) so as to generate modified products – especially chemically but also physically – and to cause these modifications to occur faster (typically much faster) than they would occur in the absence of enzymatic action, but the enzyme itself is not used up in the process. |
Cells function because specific enzymes, as encoded by specific genes, determine what chemical reactions can occur. Enzymes lower the activation energy of these reactions by subtly manipulating as well as stressing substrates to form into "transition states" that are intermediate between the structure of the substrate and that of the product.
Enzymes are the primary means by which organisms control which chemical reactions occur along with when and where they occur, giving rise to numerous aspects of phenotype. This control is achieved in part through substantial enzyme specificity such that only one or a few substrates typically can be acted upon by individual enzymes—that is, by specific kinds of individual enzymes. In addition, the catalytic activity of enzymes can be controlled, particularly by allosteric regulatory molecules, so that the activity of specific enzymes is present only as needed.
The basic functioning of an enzyme involves the binding of one or more substrates to the enzyme's active site. The active site acts upon substrates in a manner that involves both physical and chemical manipulation, resulting in conversion of substrate(s) into what is known as the transition state. The transition state then spontaneously progresses into one or more products. Involved in this progression are not just the amino acid moieties that make up polypeptides but also cofactors (including coenzymes) that contribute to enzyme catalytic functioning.
Figure legend: Enzyme-mediated catalytic cycle. One or more substrates diffuses to within the vicinity of an enzyme and then collides with the enzyme. If that collision is with proper orientation to the enzyme's active site then binding occurs, forming the enzyme-substrate complex. The enzyme then acts upon the substrate, converting it to a transition state, which is simultaneously similar in chemistry and structure to both substrate and product. The transition state spontaneously – energetically – 'falls' into the conformation of one or more products. These products then diffuse out of the active site, leaving the enzyme ready for a further round of catalysis.
The rate of enzymatic reactions is a function first of how fast substrates bind to the active site – which is a function of enzyme affinity for substrate along with substrate concentration – and second how fast substrate once bound to an enzyme (enzyme-substrate complex) is converted to product and then released (enzyme turnover rate). These enzyme-kinetic considerations are complicated, however, by the fact that many enzymatically catalyzed reactions are reversible, meaning that the reverse reaction, that is, conversion of product back to substrate, can compete with the forward reaction.
Enzymes are involved in anabolic as well as catabolic reactions. There are enzymes that function outside of cells as well as the more typical intracellular activities. There are also enzymes that function outside of cells. The latter are called exoenzymes, often are involved in the digestion of extraorganismal substrates, and give rise to what can be described as extended phenotypes.
Typically enzymes as well as catalysts are much more efficient at generating these transition states than is the application of heat alone to speed up the same reactions, as one might do in a chemistry lab This greater efficiency occurs because without catalysts the formation of specific transition states is a much more random and therefore less likely process.
The following video provides a short introduction to enzyme basics:
The following video provides a visual schematic of functioning of a key enzyme in catabolic processes:
Video (Short and reasonably well done introduction to enzymes and their inhibition; uses "competitive blocker" rather than "competitive inhibitor")
The following is a list of terms associated with enzymes:
Active site, Allosteric inhibition, Apoenzyme, Coenzyme, Cofactor, Competitive inhibition, Cooperativity, Digestive enzyme, Ecology, Enzyme, Enzyme activator, Enzyme activity, Enzyme inhibitor, Enzyme saturation, Enzyme specificity, Enzyme-substrate complex, Feedback inhibition, Holoenzyme, Induced fit, Integrase inhibitor, Lumen (endomembrane), Metabolic pathway, Product, Protease inhibitor, Protein, Protein activation, Reaction rate, Saturation, Sodium-potassium pump, Substrate, Turnover rate
The following are examples of different types of enzymes:
Aminoacyl tRNA synthetase, ATP synthase, Beta lactamase, Coagulase, Dehydrogenase, DNA ligase, DNA polymerase, Helicase, Lysozyme, Protein kinase, Primase, Protein Phosphatase, Restriction endonuclease, Reverse transcriptase, RNA polymerase, Telomerase, Tyrosine kinase receptor
The following video provides a great, musical introduction to enzymes: