#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11115501
use warnings;

$_ = do { local $/; <DATA> }; # or however you want to read the file

my $pattern = 'CDECGKEFSQGAHLQTHQKVH' =~ s/\B/\n?/gr;

print lc($`) . $1 . lc(substr $', length $1) . "\n" while /(?=($pattern))/g;

__DATA__
>AAF88103.1 zinc finger protein 226 [Homo sapiens]
MNMFKEAVTFKDVAVAFTEEELGLLGPAXRKLYRDVMVENFRNLLSVGHPPFKQDVSPIERNEQLWIMTT
ATRRQGNLGEKNQSKLITVQDRESEEELSCWQIWQQIANDLTRCQDSMINNSQCHKQGDFPYQVGTELSI
QISEDENYIVNKADGPNNTGNPEFPILRTQDSWRKTFLTESQRLNRDQQISIKNKLCQCKKGVDPIGWIS
HHDGHRVHKSEKSYRPNDYEKDNMKILTFDHNSMIHTGQKSYQCNECKKPFSDLSSFDLHQQLQSGEKSL
TCVERGKGFCYSPVLPVHQKVHVGEKLKCDECGKEFSQGAHLQTHQKVHVIEKPYKCKQCGKGFSRRSAL
NVHCKVHTAEKPYNCEECGRAFSQASHLQDHQRLHTGEKPFKCDACGKSFSRNSHLQSHQRVHTGEKPYK
CEECGKGFICSSNLYIHQRVHTGEKPYKCEECGKGFSRPSSLQAHQGVHTGEKSYICTVCGKGFTLSSNL
QAHQRVHTGEKPYKCNECGKSFRRNSHYQVHLVVHTGEKPYKCEICGKGFSQSSYLQIHQKAHSIEKPFK
CEECGQGFNQSSRLQIHQLIHTGEKPYKCEECGKGFSRRADLKIHCRIHTGEKPYNCEECGKVFRQASNL
LAHQRVHSGEKPFKCEECGKSFGRSAHLQAHQKVHTGDKPYKCDECGKGFKWSLNLDMHQRVHTGEKPYK
CGECGKYFSQASSLQLHQSVHTGEKPYKCDVCGKVFSRSSQLQSHQRVHTGEKPYKCEICGKSFSWRSNL
TVHHRIHVGDKSYKSNRGGKNIRESTQEKKSIK.
AAAAAAAAAAAAAACDECGKEFSQ
GAHLQTHQKVHZZZZZZZZZZZ