Biomolecules

I. Introduction.

- biochemistry: study of chemical reactions occurring in living systems.

- organic compounds: molecules unique to living systems--carbohydrates, lipids, proteins, nucleic acids (all contain carbon).

II. Carbohydrates.

- contain C, H, O -- H:O ratio = 2:1.

- classified according to size/solubility as a monosaccharides, disaccharides, or polysaccharides.

A. Monosaccharides: simple sugars.

- single chain or ring structures containing 3-7 C.

- C:H:O ratio = 1:2:1.

- examples are glucose (C₆H₁₂O₆), and ribose (C₅H₁₀O₅).

- named according to the number of C they contain; most important in body are hexoses and pentoses.

1. Hexoses (6C).

a. glucose: most important CH₂O in body; all ingested CH₂Os are broken down into glucose.

b. & c. fructose and galactose: isomers of glucose, same number of C, arranged differently.

2. Pentoses: (5C).

a. deoxyribose - DNA component.

b. ribose - RNA component.

B. Disaccharides: double sugars.

- formed when 2 monosaccharides are joined by dehydration synthesis (loss H₂0).

1. sucrose.

2. maltose.

3. lactose.

- disaccharides are too large to pass through cell membranes, must be digested to monosaccharide subunits prior to absorption.

C. Polysaccharides: long chains of simple sugars linked together by dehydration synthesis.

- due to size, they are water insoluble.

- great storage products; also have structural roles.

- polysaccharides of importance to body: starch & glycogen, both glucose polymers.

1. starch: storage CH₂O formed by plants.

2. glycogen: storage carbohydrate in animal tissue, found primarily in skeletal muscle & liver cells.

III. Lipids.

- organic compounds insoluble in water but readily soluble in other lipids and organic solvents such as alcohol and ether.

- contain C, H, O; however oxygen proportions in lipids are much lower than in CH₂O.

- phosphorous found in some more complex lipids.

- lipids are diverse: triglycerides (neutral fats), phospholipids, steroids.

A. Triglycerides (TGs): neutral fats.

1. Structure.

a. glycerol: modified simple sugar.

b. three fatty acids: long chains of C and H with organic acid groups at one end.

2. Synthesis

- fatty acids attached to glycerol backbone by dehydration synthesis; glycerol backbone is identical in all TGs, fatty acid chains vary in length and saturation.

- concept of saturation: fatty acid chains with only single covalent bonds between carbons are saturated hydrocarbon chains; fatty acid chains with one or more double covalent bonds between carbons are unsaturated hydrocarbon chains.

- saturation and length of fatty acid hydrocarbon chains determines how solid a TG is at a given temperature; TGs with short and/or unsaturated fatty acid are liquid at the right temperature (plant lipids such as vegetable oils); TG with longer and/or saturated fatty acids are solid at room temperature (animal lipids such as lard, butter).

3. Functions:

- major source of stored energy in body; insulation and protection (in fat deposits).

B. Phospholipids.

1. Structure.

- modified TGs with phosphorous containing group and two fatty acid chains; phosphate group gives phospholipids characteristic properties - polar head; TGs form two non polar tails.

2. Functions:

- chief component of biological membranes.

C. Steroids:

- flat molecules formed by 4 interlocking hydrocarbon rings.

- in body, most important steroid is cholesterol - structure is the basis for all other body steroids - bile salts, vitamin D, sex hormones, adrenal cortical hormones.

IV. Proteins

- basic structural material of body.

- also play vital roles in cell function; include enzymes, hemoglobin, contractile proteins, some hormones, etc.

- most varied function of any molecule in the body.

- may contain C, O, H, N, S, P.

A. Amino acids and peptide bonds.

- building block of proteins are amino acids (aa); structure with >10 a.a. is a polypeptide; molecule with >50 a.a. is a protein.

B. Levels of protein structure.

1. Primary structure: linear sequence of aa, the polypeptide chain; determines all other levels of structure.

2. Secondary structure: conformation of the polypeptide chain.

a. alpha helix: "slinky-like"; formed by coiling of polypeptide chain, stabilized by hydrogen bonds.

b. beta pleated sheet: polypeptide chains do not coil, linked side-by-side by hydrogen bonds to form accordion-like structure

Note: a polypeptide chain can have both alpha helix regions and areas of beta sheet.

3. Tertiary structure: alpha-helical or beta-pleated regions of the polypeptide chain fold onto one another to form compact ball-like molecule; the 3-D shape assumed by various areas of secondary structure.

4. Quaternary structure: tertiary structures of two or more polypeptide chains aggregate to form a complex protein (hemoglobin).

C. Types of proteins.

1. Fibrous protein: extended, strand-like appearance; usually displays only one form of secondary structure.

- linear, insoluble in water, very stable, provide tensile strength; usually are structural proteins.

2. Globular proteins: display multiple forms of secondary structure contributing to a specific tertiary structure; some also display quaternary structure.

- usually water soluble, mobile, chemically active; crucial in all biological processes, most are functional proteins.

D. Enzymes and enzyme activity.

1. General comments:

- enzymes are globular proteins, act as biological catalysts; they cannot force a reaction to occur, only accelerate rate at which it proceeds.

- some enzymes are just globular proteins, others consist of proteins and cofactors.

- enzymes are highly specific, usually involved in control of one chemical reaction.

- enzymes are either produced in an active form or in an inactive form.

2. Mechanisms of enzyme activity:

- chemical reactions cannot occur unless participating molecules reach certain energy states.

- every reaction requires an input of energy to prime the system, the activation energy.

- enzymes lower the amount of activation energy required for a reaction to occur.

- the induced fit model.

E. Protein denaturation.

- really a loss of tertiary structure.

- destruction of tertiary structure-stabilizing bonds will alter structure and change function.

V. Nucleic acids.

A. General comments.

- composed of C, O, H, N, P.

- largest molecules in the body.

- store genetic information.

- template for production of all body proteins.

- structural units are nucleotides; nucleotides have three components joined together by dehydration synthesis: a phosphate group, pentose sugar, and a nitrogen containing base.

- in DNA sugar is deoxyribose; in RNA, ribose.

- 5 nitrogenous bases involved: adenine and guanine (purines), cytosine, thymine, and uracil (pyramidines).

B. DNA

- large double stranded polymer -- "spiral ladder", two interwoven chains of nucleotides.

- the backbone or sides of ladder are formed by alternating sugar and phosphate molecules, the rungs are formed by bases.

- the two nucleotide chains are held together by hydrogen bonds between adjacent bases.

- concept of complementarity

- complimentarity is the basis for DNA replication and translation of DNA to RNA.

C. RNA

- single strand of nucleotides.

- phosphate/ribose/nitrogenous base.

- produced from a DNA template.

- three types are mRNA, rRNA, tRNA.

VI. Adenosine trisphosphate (ATP).

A. General comments.

- while glucose is most important cellular fuel, none of the chemical energy contained in its bonds is used directly to fuel cell reactions.

- ATP provides a form of chemical energy usable by all body cells.

2. Structure.

- an adenine containing nucleotide.

- two extra phosphate groups attached to AMP (adenine monophosphate) backbone by high energy bonds; breaking of these high energy bonds by hydrolysis liberates energy used to drive cellular processes.