Chapter 20 Introduction to Biochemistry

Transcription

1 Chapter 20 Introduction to Biochemistry 1

2 20.1 Chemistry in living organisms 20.1 Chemistry in living organisms chemical substances present in living organisms four elements (C, H, O, N) make up approximately 95% of the mass of living matter small amounts of S, P, Ca, Na, K, Cl, Mg, Fe trace amounts of other elements such as Cu, Mn, Zn, Co, I human body consists of ~60% water biochemistry a branch of chemistry concerned with the chemical reactions occurring in living organism four major classes of biomolecules: carbohydrates lipids proteins nucleic acids 2

3 20.2 Carbohydrates carbohydrates polyhydroxy aldehydes or polyhydroxy ketones or substances that yield these compounds when hydrolyzed empirical formulas approximated C x (H 2 O) y carbohydrates also known as saccharides plants are able to synthesize carbohydrates by the photosynthetic process the amount of energy available from carbohydrates is about 17 kj/g carbohydrates exist as sugars, starches, cellulose carbohydrates are classified as monosaccharides, disaccharides, oligosacchrides, polysaccharides monosaccharides simple sugars containing three-, four-, five-, and six-c atoms are called trioses, tetroses, pentoses, and hexoses aldose monosaccharide containing an aldehyde group on one C atom and a hydroxy group on each of the other C atoms ketose monosaccharide containing a ketone group on one C atom and a hydroxy group on each of the other C atoms ex. erythrose an aldotetrose 3

4 16 isomeric aldohexoses of C 6 H 12 O 6 are known glucose and galactose are the most important most sugars exist in a cyclic structure, forming a five- or six-membered ring ex. glucose open-chain form cyclic form glucose (C 6 H 12 O 6 ) commonly known as dextrose or grape sugar 葡萄糖 component of the disaccharides sucrose, maltose, lactose monomer of the polysaccharides sucrose, cellulose, glycogen the concentration in blood is normally mg per 100 ml also known as blood sugar galactose (C 6 H 12 O 6 ) 乳糖 galactosemia inability of infants to metabolize galactose 4

5 fructose levulose (C 6 H 12 O 6 ) 果糖 a ketohexose a constituent of the disaccharide sucrose the sweetest of all the sugars ribose (C 5 H 10 O 5 ) 核糖 an aldopentose present in adenosine triphosphate (ATP) ribose and deoxyribose are important components of the nucleic acid RNA and DNA 5

6 disaccharides whose molecules yield two molecules of monosaccharides when hydrolyzed sucrose (C 12 H 22 O 11 ) table sugar 乳糖 lactose (C 12 H 22 O 11 ) milk sugar 乳糖 maltose (C 12 H 22 O 11 ) sucrose + water H+ or sucrase glucose + fructose lactose + water maltose + water + H or lactase galactose + H or maltase glucose + glucose + glucose 6

7 polysaccharides also called complex carbohydrates can be hydrolyzed to a large number of monosaccharide units molar masses range up to 1 million or more 3 important polysaccharide: starch polymer of glucose glycogen reserve carbohydrate of the animal 肝糖 often called animal starch it is stored mainly in liver and muscle tissue cellulose polymer of glucose chief structural component of plant and wood digestion and metabolism of carbohydrates is a complex biochemical process ex. amylase amylase maltose starch dextrins maltose glucose 7

8 20.3 Lipids lipids a group of oily, greasy organic substances water insoluble but soluble in organic solvents the most abundant lipids are the fats and oils fats and oils are esters of glycerol and long-chain fatty acids (carboxylic acids) they are also called triacylglycerols or triglycerols the formulas of triglycerol molecules vary for the following reasons: 1. the length of the fatty acid chain vary from 4 to 20 carbons, the number is always even 2. each fatty acid may be saturated, or may be unsaturated and contain 1~3 C=C double bonds 3. an individual triglycerol may contain three different fatty acids the most abundant saturated fatty acids are lauric, myristic, palmitic, and stearic acids 8 (12-C) (14-C) (16-C) (18-C)

9 the most abundant unsaturated fatty acids contain 18 C atoms and have 1~3 C=C bonds oleic acid CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH linoleic acid CH 3 (CH 2 ) 4 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH linolenic acid CH 3 CH 2 CH=CHCH 2 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH others: palmitoleic acid CH 3 (CH 2 ) 5 CH=CH(CH 2 ) 7 COOH arachidonic acid CH 3 (CH 2 ) 4 (CH=CHCH 2 ) 4 CH 2 CH 2 COOH fats are solid at room temperature and generally come from animal source oils are liquid at room temperature and come from vegetable source 9

10 fats are an important energy source when oxidized to CO 2 and H 2 O, fats supply about 39 kj/g soap is made by hydrolyzing fats or oils with aqueous NaOH, this process saponification requires 3 moles NaOH per mole of fat: the most common soaps are the sodium salts of long-chain fatty acids: sodium stearate (NaOOCC 17 H 35 ) sodium palmitate (NaOOCC 15 H 31 ) sodium oleate (NaOOCC 17 H 33 ) other lipids phospholipids abundant in the brian, the spinal cord, the egg yolk, and the liver glycolipids (cerebrosides) contain a long-chain alcohol sphingosine found in large quantities in brain tissue 10

11 steroids contain four-fused carbocyclicring system cholesterol is the most abundant steroid in the body, occurs in the brain, the spinal column, and the nervous tissue 11

12 20.4 Amino acids and proteins proteins are present in all body tissue can form structural elements: hair, fingernail, wool, silk function as enzymes about 15% of the human body weight is protein proteins are polymers of amino acids with molar masses up to more than 50 million α-amino acids carboxylic acids containing an amino ( NH 2 ) group attached to C-2 R: any of the various groups there are about 200 different known amino acids in nature 20 of these amino acids are found in almost all proteins, 8 amino acids are considered essential in human nutrition proteins are polymeric substances that yielded primarily amino acids on hydrolysis the bond connecting amino acids in a protein is 12 called a peptide linkage or peptide bond

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15 ex. combine two glycine molecules to form dipeptide ex. tripeptide polypeptides contain up to about acids the amino acid units in a peptide are called amino acid residues, the amino group end is called the N-terminal residue, the other end is called C-terminal residue ex. Ala-Pro-Tyr-Met-Gly-Lys-Gly N-terminal residue C-terminal residue 15

16 naming the peptides keeping C-terminal unit s complete name the ine ending of all but the C-terminal unit is changed to -yl ex. alanyltyrosylglycine many small naturally occurring ploypeptides have significant biochemical functions ex. oxytocin controls uterine contraction during labor in childbirth Cy-Tyr-Ile-Gln-Asn-Cy-Pro-Leu-Gly-NH 2 S S vasopressin in high concentration raises the blood pressure, regulating the excretion of fluid by kidney Cy-Tyr-Phe-Gln-Asn-Cy-Pro-Arg-Gly-NH 16 2 S S

17 determining the sequence of the amino acids in polypeptide 1955 Sanger announced the structure of beef insulin, spending several years to find automated amino acid sequencer can determine the structure in few days carbohydrates and fats are used primarily to supply heat and energy to the body proteins are used mainly to repair and replace worn-out tissue proteins must be continually supplied to the body in the diet 17

18 20.5 Enzymes enzymes are the catalysts of biochemical reactions most enzymes are proteins Pasteur believed that living yeasts or bacteria were required for the catalyzed biochemical reactions he called these processes fermentation 1897 Büchner proved that the presence of living cell is not required for enzyme activity some enzymes are simple proteins consisting only of amino acid units others are conjugated and consist of a protein part apoenzyme, and a nonprotein part coenzyme a functioning enzyme consisting of both protein part and nonprotein part is called holoenzyme for some enzymes, an inorganic component, such as metal ions (Ca 2+, Mg 2+, Zn 2+ ) is required this inorganic component is an activator enzyme specificity a certain enzyme will catalyze the reaction of a specific type of substance ex. each of the sugars require a specific enzyme to hydrolyze sucrase sucrose lactase lactose substrate the substance acted on by an enzyme ex. sucrose is the substrate of the enzyme sucrase 18

19 enzymes act according to the following general sequence: E + S E-S E + P enzyme substrate intermediate product ex. hydrolysis of maltose maltase + maltose maltase-maltose maltase-maltose + H 2 O maltase + 2glucose enzyme specificity is believed to be due to the particular shape of a small part of the enzyme active site lock-and-key model recent model induce-fit model 19

20 20.6 Nucleic acids chromosomes largely consist of proteins and nucleic acids which are intimately associated in complexes called nucleoproteins nucleic acids deoxyribonucleic acid (DNA) ribonucleic acid (RNA) DNA a polymeric substance made up of thousands of units called nucleotides each nucleotide has the following sequence: there are 4 nitrogen-containing bases 20

21 the structure for a single nucleotide 1953 Watson, Crick announced the doublestrained helix structure for DNA the double helix is held together by hydrogen bonds adenine (A) ::::: thymine (T) cytosine (C) ::::: guanine (G) 21

22 the sequence of base pairs and the length of the nucleotide chains in DNA contain the genetic code of life which is passed on from one generation to another 22

23 RNA a polymer of nucleotides, but different from DNA in that 1. it is single-strained 2. it contains the pentose sugar ribose 3. it contains uracil (U) instead of thymine (T) transcription the process by which DNA directs the synthesis of RNA G :::: C A :::: U main function of RNA is to direct the synthesis of proteins three kinds of RNA are produced from DNA: messenger RNA mrna transfer RNA trna ribosomal RNA rrna mrna establishes the sequence of amino acid that comprise a specific protein trna molecules bring specific amino acids to the site of protein synthesis ribosome the actual site of protein synthesis 23

24 ribosome is composed of rrna and protein the process of cellular genetic information 24

25 20.7 DNA and genetics 20.7 DNA and genetics for heredity process to occur, the material responsible for genetic transfer must be able to make exact copies of itself the design for replication is built into Watson and Crick s DNA structure first by the nature of DNA double helical structure and second by the complementary nature of its nitrogen 25