Enzymes

Definition of an enzyme Enzymeprotein Enzyme is protein catalystincrease the rate of reactions catalyst (i.e. increase the rate of reactions) NOT changed during the reaction Enzymes Enzymes direct all metabolic reactions occurring in the cells

Classification of enzymes 1- Oxidoreductase: oxidation-reduction catalyses oxidation-reduction reactions 2-Transferases: transfer catalyses transfer of C-, N- or P- containing groups 3-Hydrolases 3-Hydrolases: breakdown of bonds by addition of water catalyses breakdown of bonds by addition of water (hydrolysis) 4- Lyases: breakdown catalyses breakdown of C – C, C-S & C-N bonds 5- Isomerase 5- Isomerase: racemization of isomers catalyses racemization of isomers (D- to L- or L- to D-) 6- Ligases 6- Ligases: formation of bonds catalyses formation of bonds between carbon and O, S, N

+ E + S ES complex E + P S P P S Mechanism of action of enzymes: lock & key model enzymesubstrateactive site An enzyme binds a substrate in a region called the active site Enzymerigid Enzyme active site is rigid (stable, not malleable) Only certain substrate can fit the active site of the enzyme i.e. the enzyme can act on only one or few substrates. i.e. specific (i.e. the enzyme can act on only one or few substrates. i.e. specific)

Mechanism of enzyme action: induced fit model E + S ES complex E + P E + S ES complex E + P S P P S S adjust Active site of the enzyme adjust s its shape to bind the substrate. flexible Enzyme structure is flexible (not rigid) This mechanism gives increased range of substrate specificity (many types of substrates can bind the same enzyme)

P roperties of enzymes 1- All Enzymes are Proteins 1- All Enzymes are Proteins (except ribozymes that are RNA) 2- Active Site Each enzyme molecules contain a special pocket called the active site. The substrate bind with the active site to form enzyme substrate-complex (ES) which is converted to enzyme-product complex (EP). EP dissociates to enzyme and product 3-Catalytic Efficiency 3-Catalytic Efficiency : Each enzyme molecule is capable of transforming 100 – 1000 substrate molecules into product each second

4- Specificity 4- Specificity: Enzymes are highly specific i.e. each enzyme interacts with one or few type of substrates and catalyzes one type of reactions 5- Holoenzymes 5- Holoenzymes: Some enzymes require molecules other than proteins for enzymic activity. Holoenzymes Holoenzymes: apoenzyme (protein part) + nonprotein component Apoenzymes Apoenzymes are inactive without the nonprotein component non-protein part non-protein part: cofactors 1- a metal ion as zinc or ferrous ions (called cofactors) coenzymes 2- small organic molecules (called coenzymes) Coenzymes Coenzymes : may be only transiently associated with the enzyme prosthetic group or may be permanently associated with the enzyme (prosthetic group) Coenzymes may be derived from vitamins. (e.g. NAD+ contains niacin) Properties of enzymes Properties of enzymes (cont.)

6-Regulation 6- Regulation: Enzyme activity can be regulated by activation or inhibition. 7-Location within the cell 7- Location within the cell: Many enzymes are localized in specific organelles within the cell. This serves to isolate the reaction substrate or product from other opposite reactions. Properties of enzymes Properties of enzymes (cont.)

Optimum temperature Reaction Reaction Rate Rate Low High Low High Temperature 1- Little activity at low temperature 2- Rate of activity increases with temperature optimum temperatures 3- Most active at optimum temperatures (usually 37°C in all enzymes of humans) lostdenaturation 4- Activity is lost with denaturation at high temperatures Factors affecting enzyme activity: 1- Temperature

Reaction Reaction Rate Rate pH pH optimum pH Maximum activity at optimum pH where: R groups of amino acids have proper charge Tertiary structure of enzyme is intact Most enzymes lose activity in very low or high pH (due to loss of tertiary structure i.e. denatured) Each enzyme has its own optimum pH Optimum pH Each enzyme has its own optimum pH Factors affecting enzyme activity: 2- pH

level of blood Each enzyme has its own optimum pH

Maximum Activity Reaction Reaction Rate Rate Substrate Concentration Substrate Concentration Increasing substrate concentration increases the rate of reaction (with enzyme concentration is constant) Maximum activity reached when all of enzyme molecules combine with substrate F actors affecting enzyme activity: 3- Substrate concentration

Reaction Reaction Rate Rate Enzyme Concentration Enzyme Concentration Increasing enzyme concentration increases the rate of reaction (with substrate concentration is constant) Factors affecting enzyme activity: 4- Enzyme concentration

Factors affecting enzyme activity: 5- Inhibition of enzyme activity Inhibitor Inhibitor: is an any substance that can diminish the velocity of a reaction which is catalyzed by an enzyme. Reversible Inhibitors non-covalent Reversible Inhibitors : bind to enzymes through non-covalent bonds Irreversible Inhibitor : covalent Irreversible Inhibitor : bind to enzymes through covalent bonds Types of inhibitors according to site of binding of inhibitor: Competitive Inhibitor active Competitive Inhibitor : inhibitor binds at the active site of the enzyme Noncompetitive Inhibitor other Noncompetitive Inhibitor : inhibitor binds to a site other than active site

K m of enzyme K m (Michaelis Constant) of an enzyme is numerically equal to the substrate concentration at which the velocity of reaction is equal to 1/2 V max K m is the substrate concentration at which 1/2 maximal velocity is reached If K m is small If K m is small, the substrate concentration required for the reaction to reach 1/2 maximal velocity is small. high affinity i.e. the enzyme has a high affinity for the substrate. If K m is large If K m is large, the substrate concentration required for the reaction to reach 1/2 maximal velocity is large. low affinity i.e. the enzyme has a low affinity for the substrate

Competitive inhibitors A competitive inhibitor: similar has a structure similar to substrate of the enzyme active site of the enzyme occupies active site of the enzyme competes competes with substrate for occupying the active site of the enzyme increases Km increases Km of the enzyme to its substrate. i.e. more substrate is required to reach ½ Vmax reversed effect can be reversed by increasing substrate concentration.

A noncompetitive inhibitor : different Has a structure different from the substrate different Binds to the enzyme at a site different from active site i.e. does not compete with the substrate of the enzyme for the active site of the enzyme Changes the shape of enzyme and thus active site shape is changed. Accordingly, substrate cannot fit in the altered active site So, no reaction occurs not reversed Effect is not reversed by adding more substrate Does not change Km of the enzyme Does not change Km of the enzyme (i.e. increase of substrate concentration will not lead to reaching of ½ Vmax). Noncompetitive inhibitors

Regulation of enzyme activity The regulation of the activity of enzyme is essential for coordinating the metabolic processes. Types of regulation: 1- General: (occurs in all types of enzymes in the body) increasing substrate concentration will lead to increase activity of the enzyme 2-Special regulatory mechanisms: (not all enzymes of the body) i- Allosteric effectors ii- Covalent modification iii. Increase or decrease rate of enzyme synthesis

Allosteric effectors 1- Allosteric effectors Allosteric binding sites are sites on the enzyme different from the active site. Binding of an allosteric effectors to this site will make changes in shape of the whole enzyme with an effect on activity. So, the activity may be decreased (positive allosteric effector) or decreased (negative allosteric effector).

Covalent modification 2- Covalent modification addition of phosphate groups 1- Many enzymes may be regulated by addition of phosphate groups to the modification by phosphorylation enzyme (modification by phosphorylation) Addition of phosohate group may cause activation or inactivation of the enzyme 2- Some enzymes are released as an inactive form (zymogen) removal of a part of the enzyme modification By removal of a part of the enzyme (modification), it will be active.

Increasing or decreasing rate of enzyme synthesis 3- Increasing or decreasing rate of enzyme synthesis By this mechanism cells regulate the amount of enzyme by changing the rate of enzyme synthesis The increase (induction) or decrease (repression) of enzyme synthesis leads to change total amount of active site. slow This mechanism is slow (takes from hours or days)

Medical importance of enzymes of blood Medical importance of enzymes of blood B lood enzymes can be classified into two major groups: 1-Enzymes that have functions in blood 1- Enzymes that have functions in blood (a smaller group): They are present in blood in high amounts Example: liver secretes zymogens (inactive precursors) involved in blood coagulation. 2-Enzymes that do not have functions in blood 2-Enzymes that do not have functions in blood (a large number) These enzymes are released from cells during normal cell turn over. They have functions in cells (intracellular) BUT: they do not have a function in blood In healthy individuals, levels of these enzymes are constant. So, the presence of elevated enzyme level in blood may indicate tissue damage that leads to increase in release of these enzymes.

Many diseases that cause tissue damage results in increased release of intracellular enzymes into plasma (blood) So, the enzyme levels in blood are measured for diagnosis of these diseases Diseases of the heart, liver, skeletal muscles and other tissues are diagnosed by an elevation of a blood enzymes. The level of elevation of an enzyme correlates with the extent of tissue damage in any of these organs. Some enzymes may be available in high amount in only one organ So, the elevation of blood levels of these enzymes are diagnostic for diseases of this organ only. (specific) Example: Alanine aminotransferase (ALT) enzyme elevation in blood indicates disease of the liver (specific for liver cells). Medical importance of enzymes of blood (cont.) Medical importance of enzymes of blood (cont.)