Co-enzymes and cofactors activity in enzymes

Many Enzymes Require Cofactors for Activity A cofactor is a small non-protein molecules that is bound (either tightly or loosely) to an enzyme and is required for catalysis. Catalytic activity of many enzymes depends on the presence of cofactors. 2

Many Enzymes Require Cofactors for Activity

Types of cofactors 4 4

Essential Ion Cofactors Activator ions – bind reversibly to enzyme and often participate in substrate binding. Metal ions of metalloenzymes – cations that are tightly bound to enzyme and participate directly in catalysis (Fe, Zn, Cu, Co). Metal activated enzymes – require or are stimulated by addition of metal ions (i.e. Mg2+, is required by many ATP requiring enzymes)

inorganic cofactors Functions of iron Iron must be present in hemoglobin in order to pick oxygen Electron transport Oxygen binding Oxygen carrier

Function Oxygen Transport & Storage Hemoglobin Myoglobin Electron Transport & Energy Metabolism Cytochromes Fe-S proteins Substrate Oxidation & Reduction – Iron dependent enzyme- Ribonucleotide reductase Amino acid oxidases Fatty acid desaturases Nitric oxide synthetase Peroxidases HEME is synthesized in the liver from succinyl-CoA and Glycine. Hemoglobin represents more than 95% of the proteins of the red blood cell, and contains about 60% of the body iron. Function1) transport O2from the respiratory organ to peripheral tissue 2)Transport of CO2 and proteins from peripheral tissues t the lung for subsequent excretion 3)transport NO. 2,3-bisphosphoglycerate, an intermediate in glycolysis, stabilizes the tettrameric structure of hemoglobin, indicateing another important function of glycolysis in the functions of red blood cells. (Iron dificient, heme decreases leading O2 transport and NO transport decrease--- Vascular systeem decreases and immune system decrease) Myoglobin is a single polypeptide chain (MW16900). Contains about4% of the body’s iron. Myoglobin is the minor protein of muscle cells and is used for the short term storage of oxygen. Electron transport: In cytochromes, the oxidation and reduction of the iron atom are essential to their biological function. By contrast, oxidation of the Fe2+ to Fe3+ of hemoglobin or myoglobin destroys biological activity. All use O2 as a substrate

Examples of Iron-dependent Enzymes Aldehyde Oxidase R-CHO + O2  RCOOH + H-O-O-H Tryptophan 5-monooxygenase L-tyrptophan + BH4 + O2  5 OH L-tryptophan + BH2 + H2O Fatty Acid desaturase Stearoyl-CoA + NADH + H+ + O2  Oleoyl-CoA + NAD+ + 2H2O Peroxidase 2H2O2  2H2O + O2 (O2 is either incorporated into the product or reduced by electrons)

Inorganic cofactors Mg2 is used in glycolysis. In the first step of converting glucose to glucose 6-phosphate before ATP is used to give ADP and one phosphate group, ATP is bonded to Mg2 which stabilizing the other two phosphate groups so it is easier to release only one phosphate group without resonate with other two.

Zinc Function 300 enzymes require zinc numerous hormones require zinc DNA, RNA polymerases numerous hormones require zinc insulin transcription factors (zinc finger proteins) membrane stability myelination skeletal development

Example of prosthetic group Example of metalloenzyme: carbonic anhydrase contains zinc Metalloenzymes contain firmly bound metal ions at the enzyme active sites (examples: iron, zinc, copper, cobalt). One of the functions of the enzyme interconvert carbon dioxide and bicarbonate to maintain acid-base balance in blood and other tissues, and to help transport carbon dioxide out of tissues. A zinc prosthetic group in the enzyme is coordinated in three positions by histidineside-chains. The fourth coordination position is occupied by water This causes polarisation of the hydrogen-oxygen bond, making the oxygen slightly more negative, thereby weakening the bond.

Examples of enzymes containing this ion Cupric Cytochrome oxidase Ferrous or Ferric Cytochrome (via Heme) Hydrogenase Magnesium Glucose 6-phosphatase Hexokinase Manganese Arginase Molybdenum Nitrate reductase Nitrogenase Nickel Urease Zinc Alcohol dehydrogenase Carbonic anhydrase

Coenzyme Coenzymes are small organic non-protein molecules. Loosely attached to apoenzymes, seperated easily by dialysis they are often called cosubstrate or secondary substrate. Reaction involving: oxidoreduction, group. transfer, Isomerization and covalent bond formation req. coenzyme.

Coenzymes + apoenzyme coenzyme holoenzyme Non-Protein Total Protein Organic molecule that temporarily binds to apoenzyme in order for it to work + apoenzyme coenzyme holoenzyme Non-Protein Total Protein

The functional role of Coenzymes is to act as transporters of chemical group A coenzyme is a necessary helper for enzymes that assist in biochemical transformations. These molecules act to transfer chemical groups between enzymes or from Enzyme to substrate or product. A coenzyme Transport a variety of chemical groups (Such as Hydride, Acetyl, Formyl, Methenyl or methyl).

Vitamin Insufficiency Generally Result in malfunction of enzymes Main clinical symptoms of dietary vitamin insufficiency generally arise due the malfunction of enzymes. Dietary vitamin insufficiency leads to a lack of sufficient cofactors derived from vitamins to maintain homeostasis.

Vitamins of B complex group acting as co-enzymes active form (co-enzyme) Thiamine Vitamin B 1 TPP (thiamine pyrophosphate) Riboflavin Vitamin B 2 FMN, FAD Niacin Vitamin B 3 NAD,NADH Pantothenic acid Vitamin B 5 component of coenzyme A Pyridoxine Vitamin B 6 PLP (pyridoxal phosphate) Biotin Folic acid THF (Tetrahydrofolate) Cobalamine Vitamin B 12 cobamide

Vitamin B1 - Thiamine Entity Transferred; Aldehydes The active form is thiamin pyrophosphate (TPP) Thiamin is rapidly converted to thiamin pyrophosphate (TPP) in small intestine, brain and liver. TPP is formed from thiamin by the action of thiamine diphosphotransferase. TPP coenzyme is required by enzymes in the decarboxylation of -keto acids. Entity Transferred; Aldehydes TPP coenzyme is required by enzymes in the decarboxylation of -keto carboxylic acids.

TPP as co-enzymes (TPP) is a derivative of  thiamine (vitamin B1). Nutritional deficiency of thiamine leads to the disease beriberi. Beriberi affects especially the brain, because TPP is required for carbohydrate metabolism, and the brain depends on glucose metabolism for energy

Riboflavin functions, vit B2 Active forms are Flavin adenine dinucleotide (FAD) Flavin mononucleotide (FMN) These play key roles in hydrogen transfer reactions associated with Glycolysis TCA cycle Oxidative phosphorylation. 21

Regulated by ACTH, aldosterone, and thyroid hormone p. 283b

In tissues, riboflavin is typically converted to its coenzyme forms by flavokinase and FAD synthetase. Synthesis is under hormonal control. ACTH, aldosterone, and thyroid hormones all accelerate conversion of riboflavin to its coenzyme forms. p. 283c 23

FAD As co-enzyme

Vitamins and Derivatives Involved in Group Transfer Reactions Nicotine Adenine Dinucleotide (NAD) & Nicotine Adenine Dinucleotide Phosphate (NADP) Derivative of Niacin (B3) Serve as cofactors in oxidation / reduction reactions Act as co-substrates for dehydrogenases Entity Transferred; Hydride ion (H+ + 2e-) Electron (Hydrogen atom)

H+ (a reaction of glycolysis) glyceraldehyde 3P + NAD (substrate) (co substrate) dehydrogenase (enzyme ) DiPhosphoglyceric acid + NADH (product) (NAD act as co-enzyme & H acceptor) H+

Entity Transferred; Acetyl group and other acyl groups Coenzyme A (CoA) Derivative of Pantothenic acid (B5) Entity Transferred; Acetyl group and other acyl groups needed for energy production as well as glucose and cholesterol synthesis Deficiency can result in fatigue, retarded growth, cramps, and anemia

Pyridoxal Phosphate PLP is Derivative of Pyridoxine (Vit. B6) involved in : Transamination reactions required for the synthesis and catabolism of the amino acids. Decarboxylation reactions. Entity Transferred; Amino Groups(-NH2)

Example of co-enzyme in amino acid metabolism NH2 Glutamate + pyruvate + pyrodoxal P (co-substrate,acceptor Transaminase & donor of amino group) (enzyme) α-Ketoglutaric acid + Alanin

is a coenzyme for enzymes that transfer carboxyl groups Biotin is a coenzyme for enzymes that transfer carboxyl groups Entity Transferred; Carbon Dioxide is a coenzyme for enzymes that transfer carboxyl groups