Enzymes

- Some enzymes do not need additional components to show full activity - Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity. Cofactors can be either inorganic (metal ions Zn, Mg, Fe) or organic compounds( Co-enzymes : NADH, FADH2). Coenzymes which are released from the enzyme's active site during the reaction, or prosthetic groups, which are tightly bound to an enzyme. - The protein part in non-active enzymes are called apoenzymes. An enzyme together with the cofactors required for activity is called a holoenzyme.

Types of enzymes 1- Monomeric = one peptide series (trypsine, ribonuclease) 2- Oligomeric = 2-10 peptide series (hexokinase) 3- Multienzyme complex = multienzume or numbers of enzyme (pyruvate dehydrogenase)

Factors affecting Enzyme Activity 1- Temperature 2-pH - Acidity and Basicity 3-Enzyme Concentration 4- Substrate Concentration 5- Inhibitors and Activators

pH The p H scale measures how acidic or alkaline a substance is: The chemical properties of many solutions enable them to be divided into three groups: 1- Neutral: solutions a p H of 7. 2- Alkaline: solutions with a p H greater than 7. 3- Acidic: solutions with a p H less than 7.

In biochemistry, Michaelis–Menten kinetics is one of the best-known models of enzyme kinetics. It is named after German biochemist Michaelis and Canadian physician  Menten. The model takes the form of an equation describing the rate of enzymatic reactions, by relating reaction rate  to , the concentration of a substrate S. Its formula is given by Here,  represents the maximum rate achieved by the system, at maximum (saturating) substrate concentrations. The Michaelis constant  is the substrate concentration at which the reaction rate is half of .  Biochemical reactions involving a single substrate are often assumed to follow Michaelis–Menten kinetics, without regard to the model's underlying assumptions.

The Michaelis constant  is the substrate concentration at which the reaction rate is half of  Vmax. Michaelis–Menten saturation curve for an enzyme reaction showing the relation between the substrate concentration and reaction rate.

Lineweaver–Burk plot The plot provides a useful graphical method for analysis of the Michaelis–Menten equation: Taking the reciprocal gives where V is the reaction velocity (the reaction rate), Km is the Michaelis–Menten constant, Vmax is the maximum reaction velocity, and [S] is the substrate concentration.

An example of a Lineweaver-Burke plot.

Use: The Lineweaver–Burk plot was widely used to determine important terms in enzyme kinetics, such as Km and Vmax, before the wide availability of powerful computers and non-linear regression software. The y-intercept of such a graph is equivalent to the inverse of Vmax; the x-intercept of the graph represents −1/Km. It also gives a quick, visual impression of the different forms of enzyme inhibition.

In the competitive inhibition the maximum velocity (Vmax) of the reaction is unchanged, while the apparent affinity of the substrate to the binding site is decreased (the  dissociation constant is apparently increased). The change in (Michaelis-Menten constant) is parallel to the alteration in . Any given competitive inhibitor concentration can be overcome by increasing the substrate concentration in which case the substrate will outcompete the inhibitor in binding to the enzyme.

This reduction in the effective concentration of the E-S complex increases the enzyme's apparent affinity for the substrate (Km is lowered) and decreases the maximum enzyme activity (Vmax), as it takes longer for the substrate or product to leave the active site. Uncompetitive inhibition works best when substrate concentration is high. An uncompetitive inhibitor need not resemble the substrate of the reaction it is inhibiting.

Noncompetitive inhibitor  can bind to an enzyme with or without a substrate at different places at the same time. It changes the conformation of an enzyme as well as its active site, which makes the substrate unable to bind to the enzyme effectively so that the efficiency decreases. A noncompetitive inhibitor binds to the enzyme away from the active site, altering the shape of the enzyme so that even if the substrate can bind, the active site functions less effectively. Most of the time, the inhibitor is reversible. As the inhibitor binds to the enzyme and the enzyme-substrate complex, it reduces the concentration of enzyme available for proper catalysis. Fewer functional enzymes leads to fewer available active sites and thus a smaller Vmax. Unlike competitive inhibition, raising [S] (substrate concentration)is pointless with noncompetitive inhibition.

A noncompetitive inhibitor binds to a different site that is not the active site of the enzyme and changes the structure of the enzyme; therefore, it blocks the enzyme from binding to substrate, which stops enzyme activity. Thus, it decreases the rate of the chemical reaction of enzyme and substrate, which can not be changed by increasing concentration of substrate E + I − (through a substrate) → ES + I → E + P ES + I ⇌ ESI → NR (no reaction) where E is enzyme, I is inhibitor, ES is enzyme-substrate complex, P is product. ESI is the molecule after the inhibitor is bound to the enzyme-substrate complex. ESI cannot form any products, so the later reaction is not allowed (or, no reaction).

Inhibitors b. Noncompetitive inhibitors: Inhibitors that do not enter the active site, but bind to another part of the enzyme causing the enzyme to change its shape, which in turn alters the active site. Enzyme Noncompetitive Inhibitor Substrate active site altered

Based on the Michaelis-Menten Model, (KM ), the concentration of the substrate when the velocity is the half of the maximum velocity (or half of the substrates at maximum velocity), remains same, but the maximum velocity (Km) is decreased.

When used for determining the type of enzyme inhibition, the Lineweaver–Burk plot can distinguish competitive, non-competitive and uncompetitive inhibitors. Competitive inhibitors have the same y-intercept as uninhibited enzyme (since Vmax is unaffected by competitive inhibitors the inverse of Vmax also doesn't change) but there are different slopes and x-intercepts between the two data sets. Non-competitive inhibition produces plots with the same x-intercept as uninhibited enzyme (Km is unaffected) but different slopes and y-intercepts. Uncompetitive inhibition causes different intercepts on both the y- and x-axes .

Two theories includes the reaction between the substrate and active site to form the enzyme-substrate complex: 1- Lock and key model. 2- Induced fit model.

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