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If a colony of bacteria is infected with bacteriophages, the bacteria undergo chance collision and ultimately the phages attach to the bacterial cell wall through tail fibres. The DNA of the phage is injected into the host cell by dissolving the cell wall at the point of attachment, and the protein shell remains outside. The phage DNA takes up the control of the bacterial cell and replicates several times. The DNA and RNA of the bacterial cell are depolymerised and the nitrogen bases are utilised during the phage DNA replication. Similarly, the proteins of the bacterial cell are hydrolysed and the amino acids, thus formed, are used for building up of the protein shells of the virus. The newly-formed phage particles are released from the infected bacterial cells after lysis within 20min. to 1 hr. The new viruses infect other bacteria and a fresh cycle of virus replication is repeated.

Biologists occasionally need to examine some microscopic animals or plants. Microscopes magnify the specimens 1000 times or more larger than normal. So, understanding about the measurements is a must. In biology, the units of length commonly employed include the micron (abbreviated by µ) and the Angstrom (abbreviated by Å). A micron is equivalent to 10 ^-3 millimeters (mm). An Angstrom is equivalent to 10^-4 µ or    10 ^-7 mm.

Weights are expressed in milligrams (10^-3 grams) micrograms (10^-6 grams), and nanograms (10^-9 grams).

The unit of molecular weight employed is the dalton. A dalton is defined as the weight of a hydrogen atom. For example, one molecule of water (H₂0) weighs about 18 daltons . One dalton weighs 1.674 x 10^-24 grams.

An enzyme (E) combines with its substrate (S) to form an intermediate enzyme-substrate complex (ES) , which then decomposes into reaction products(P) and the free enzyme, as seen in the equation below.

E S --------------- > ES ---------- > E + P

                enzyme-substrate complex

 An enzyme causes the substrate upon which it is acting to be much more reactive than when it is free. One postulate accounting for this is that the enzyme holds the substrate in a position which strains and weakens the substrate's molecular bonds. This weakening of the bonds within the substrate makes them easier to cleave and results in a general lowering of the energy of activation of the reaction. This postulate is extremely simplistic - the actual forces at work are much more numerous and complex.

 When the substrate binds to the enzyme, it combines with only a relatively small part of the enzyme molecule the active site. Information about the active site, such as its location and the nature and sequence of amino acids in it,  provides an indication of the mechanism of binding and catalysis. The binding of the substrate to the enzyme's active, site depends on many forces: hydrogen bonding, the interaction of hydrophobic (water-repelling) groups, and the electrostatic interaction between charged groups on the amino acids. Many active sites also contain metal ions which aid in binding the substrate or expediting the catalytic reaction by withdrawing or stabilizing electrons. For example, the enzyme carboxypeptidase, which hydrolyzes polypeptide bonds of proteins in food, contains a zinc atom in its active site. The electrophilic (electron-attracting) zinc atom coordinates electrons from the carbonyl of the peptide bond, weakening the bond for attack by a specific amino acid of the enzyme at the active site. Such a mechanism, however, is beyond the scope of elementary biology and one would require a good course in biochemistry to understand fully.

 Some enzymes, the regulatory or allosteric enzymes, have two binding sites: an active site and a regulatory site. Regulatory enzymes are a key controlling factor in metabolic pathways. If the end product of a pathway is in excess, it inhibits the action of the regulatory enzyme by binding to its regulatory site. The end product shuts off the catalytic activity of the active site by altering the arrangement of the enzyme's polypeptide chains, thus deforming and inactivating the enzyme (see diagram below).

Fig. Schematic diagram showing binding at the active site (a)
and regulatory site (b) of an enzyme. Note the change in
enzyme conformation accompanying binding of product to the
regulatory site.

Allosteric enzymes have, in addition to an active site, another stereo-specific site to which an effector or modulator molecule can bind. When it does, the shape of the active site is altered so that it can or cannot bind substrate (allosteric stimulation or inhibition respectively). In this way the enzyme can be part of a fine control circuit, requiring the presence or absence of a substrate—in addition to substrate presence— before enzyme activity proceeds. Some allosteric enzymes respond to two or more such modulators, permitting still liner control over timing of enzymes activity.

• Cofactors are metal ions.
• These ions commonly provide a needed change within an active site.
•  Many enzymes use metal ions to change a non-functioning active site to a functioning one. In these enzymes, the attachment of a cofactor causes a shape change in the protein that allows it to combine with its substrate.
• The cofactors of other enzyme participate in a temporary bonds between the enzyme and its substrate when the enzyme-substrate (ES) complex is formed.
• Coenzymes are non-protein, organic molecules that participate in enzyme-catalytic reactions, often by transporting electrons in the form of hydrogen atoms, from one enzyme to another.
• Many vitamins function as coenzymes or are said to make coenzymes (e.g., Niacin and Riboflavin).
• One of the most important, coenzymes in the cell is the hydrogen acceptor Nicotine Adenine Dinucleotide (NAD⁺) is made from a B-Vitamin.
• Some enzyme (e.g.. Aspartase) bind just one very specific substrate molecule; others bind a variety of the same kind (o.g., all terminal peptide bonds in the case of exopeptidases).
• The difference arises from the degree of stereospecificity of the enzyme.
• Many need an attached prosthetic group or a diffusible coenzyme for activity. In such enzymes the protein component is termed the apoenzyme and the whole functional enzyme-cofactor complex is termed the holoenzyme.